CPSC Staff Statement on the 

National Institute of Standards and Technology Report, 

“Technical Note 1781: Modeling and Measuring the Effects of Portable Gasoline Powered 

Generator Exhaust on Indoor Carbon Monoxide Level”*

March 2013 

The report titled, “Technical Note 1781: Modeling and Measuring the Effects of Portable 

Gasoline Powered Generator Exhaust on Indoor Carbon Monoxide Level,” presents the findings 

of research conducted by the National Institute of Standards and Technology (NIST) under an 

interagency agreement with the U.S. Consumer Product Safety Commission (CPSC).† This 

research was performed in support of CPSC’s advance notice of proposed rulemaking (ANPR) to 

address the carbon monoxide (CO) poisoning hazard associated with portable generators.‡ 

Under this agreement with CPSC, NIST performed certain generator testing and 

modeling studies. The tested generators included both commercial unmodified and low COemission prototype configurations. One series of tests characterized indoor CO concentrations 

resulting from portable generators operating in the attached garage of a single level, single 

family house (SFH) under various use and environmental conditions. NIST used those empirical 

test data to validate the ability of NIST’s indoor air quality (IAQ) model CONTAM to predict 

CO levels in the garage and the house. Another series of tests determined the generators’ CO 

emission and oxygen consumption rates by testing them in a single-zone enclosed space. These 

rates were used as inputs to the CONTAM model validation and in simulation analyses to

examine the potential performance of the low CO-emission prototype under a wider range of 

operating conditions than those actually tested with the generator operating in the attached 

garage of the SFH. 

For two different commercially available unmodified generators (i.e., with carbureted 

engines lacking CO-emission controls) tested in the single-zone enclosed space, CO emissions 

ranged from a low of around 500 grams per hour (g/h), at near ambient oxygen levels, to a high 

of nearly 4000 g/h as oxygen decreased to 17 percent. Tests of two modified, prototype 

generators (commercially available units adapted with closed-loop electronic fuel injection and a 

small catalyst integrated into the muffler) showed CO emissions reductions of more than

90 percent with most CO emission rates well below 500 g/hr; and no trend toward higher 

emission rates was seen as the oxygen level dropped. CPSC staff will use this information to 

develop a potential staff recommendation to the Commission as part of the process initiated with 

the ANPR described above. 

 

* This statement was prepared by CPSC staff, and the attached report was produced by NIST for CPSC staff. The statement and 

report have not been reviewed or approved by, and do not necessarily represent the views of, the Commission.

† CPSC-I-06-0012. ‡

 Portable Generators; Advance Notice of Proposed Rulemaking; Request for Comments and Information, Federal 

Register, 71 FR 74472, December 12, 2006.
NIST Technical Note 1781

Modeling and Measuring the Effects of Portable

Gasoline Powered Generator Exhaust on Indoor 

Carbon Monoxide Level

Steven J Emmerich

Andrew K Persily

Liangzhu (Leon) Wang

http://dx.doi.org/10.6028/NIST.TN.1781
NIST Technical Note 1781

Modeling and Measuring the Effects of Portable

Gasoline Powered Generator Exhaust on Indoor 

Carbon Monoxide Level

Steven J Emmerich

Andrew K Persily

Energy and Environment Division

Engineering Laboratory

Liangzhu (Leon) Wang

Concordia University

http://dx.doi.org/10.6028/NIST.TN.1781

February 2013

U.S. Department of Commerce 

Rebecca Blank, Acting Secretary

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director 
Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Technical Note 1781

Natl. Inst. Stand. Technol. Tech. Note 1781, 141 pages (February 2013) 

http://dx.doi.org/10.6028/NIST.TN.1781

CODEN: NTNOEF
iii

Abstract

The U.S. Consumer Product Safety Commission (CPSC) is concerned about the hazard of acute 

residential carbon monoxide (CO) exposures from portable gasoline powered generators that can 

result in death or serious and/or lasting adverse health effects in exposed individuals. In an initial 

approach to characterizing these hazards, CPSC measured the CO emission rates from generators by 

testing them in a small, laboratory test chamber (Brown 2006). CPSC subsequently contracted with 

the University of Alabama (UA) to develop a low CO-emission prototype generator by adapting offthe-shelf emission control technologies on a commercially available portable generator and then to

construct multiple units for testing. Under an interagency agreement with CPSC, NIST conducted a 

series of tests to characterize the indoor CO concentrations resulting from portable generators 

operating in the attached garage of a house under various use and environmental conditions so CPSC

staff could analyze the safety implications of operating generators under these conditions. The tested 

generators include both unmodified and modified low CO emission prototype configurations. NIST 

used those test data to validate the ability of the CONTAM indoor air quality (IAQ) model to predict 

CO levels in the garage and the house and to develop an estimate of the uncertainty of these 

predictions relative to measured values. NIST also conducted tests with the generators operating in a 

one-zone shed to derive their CO emission and O2 consumption rates. These rates were used both as 

inputs to the model validation effort as well as in simulation analyses conducted to examine the 

potential performance of the low CO-emission prototype under a wider range of operating 

conditions. 

To determine generator CO emission rates under more realistic conditions than those attained in a 

small, laboratory test chamber, NIST conducted tests on the unmodified and modified generators 

(i.e. without and with CO emission controls) in a single-zone shed. For two different unmodified 

generators (i.e., without CO emission controls), it was found that CO emissions ranged from a low of 

around 500 g/h at near ambient O2 levels to a high of nearly 4000 g/h as O2 approached 17 %. The 

rates of CO generation and O2 consumption in these unmodified generators were affected by 

multiple parameters, with the O2 level in the space and the actual electrical output of the generator 

being two of the most important. Tests performed below 17 % O2 showed a drop off in CO 

emissions due to poor engine performance under these conditions. Tests of two modified, prototype 

generators (i.e., with CO emission controls) showed CO emissions reductions of over 90 % 

depending on the specific emission controls and operating conditions and no trend toward higher 

emission rates was seen as O2 levels dropped to 18 %. 

A series of tests were also conducted to measure the emission and transport of CO when operating 

portable gasoline-powered generators in an attached garage. This series of tests included both 

unmodified and prototype generators operated in the garage attached to NIST’s manufactured test 

house. Testing was conducted under seven different test house/garage configurations to evaluate 

their impacts on the buildup of CO in the garage and its transport into different rooms in the house. 

The configurations studied included two different garage bay door positions (fully closed or open 

0.6 m); two connecting door settings between the garage and the family room (fully closed or open 

5 cm); and two house central heating, ventilating, and air conditioning (HVAC) fan settings (on or 

off). CO concentrations varied widely with peak house CO concentrations ranging from under 

10 μL/L to over 10,000 μL/L (note that μL/L are equivalent to ppmv). As expected, the highest 

concentrations in the house resulted from operation of the unmodified generator in the garage with 

the bay door closed and the house access door open 5 cm. The lowest concentrations resulted from 
iv

operation of a modified low CO-emission prototype in the garage with the bay door open 0.6 m and 

the house access door closed. These garage tests documented reductions of 85 % to 98 % in CO 

concentrations in the house due to emissions from two modified, prototype low CO-emission 

portable generators compared to a “stock” generator. Note that these results apply to the specific 

units tested and that other units, modifications and test conditions may produce different results. 

An extensive model validation effort using the multizone airflow and IAQ model CONTAM was 

carried out using the data from the seven tests that were conducted with a generator operating in the 

attached garage of the test house to compare predicted CO concentrations with measured values. The 

agreement between the measurements and predictions of the O2 concentrations in the garage and of 

the average CO calculated for the house zones was excellent for the collective set of data; all of the 

calculated statistical values met the ASTM D5157 criteria for comparing IAQ model predictions and 

measurements. The agreement, however, was somewhat worse for the garage CO concentrations, 

with some parameters falling slightly outside the ASTM criteria limits. Overall, the average 

individual house zone and garage CO concentration predictions and measurements were within 

about 20 % and 30 % respectively when averaged over all cases. 

Forty-two simulations were then performed with the NIST CONTAM model to examine the 

potential performance of the prototype generator under a wider range of conditions than studied 

during the experiments at the test house. All of the simulations were based on the NIST 

manufactured test house and the tested low CO emission prototype generator with and without a 

catalyst integrated in the muffler (referred to as cat muffler and noncat muffler, respectively). Model 

parameters that were varied included ambient conditions, CO emission rates, source locations and

door positions. The highest house CO concentrations were found for the generator with the noncat 

muffler operated in the utility room of the house, with indoor CO concentrations reaching 2000 μL/L

for some cases. Operation of the generator with the cat muffler substantially reduced CO 

concentrations, however, they still reached levels of 280 μL/L to 600 μL/L for cases with the 

generator located in the utility room. The lowest indoor CO concentrations resulted from operation 

of the generator with the cat muffler in the garage, with CO concentrations in the house reaching 

10 μL/L to 160 μL/L. Simulations also showed that, as expected, closed bedroom doors resulted in 

less uniform indoor concentrations and higher peak indoor zone concentrations, though the impact 

varied greatly from about a 10 % to about a 100 % increase.

Keywords

Generator; carbon monoxide; CONTAM; exposure; indoor air quality; health; measurements; 

multizone airflow model; simulation
v 

Table of Contents

Abstract .......................................................................................................................................... iii

Keywords ................................................................................................................................. iv

Table of Contents .............................................................................................................................v

List of Figures ......................................................................................................................... vii

List of Tables .............................................................................................................................x

Nomenclature ........................................................................................................................... xi

Introduction ......................................................................................................................................1

Background ................................................................................................................................1

Objective ....................................................................................................................................2

Contents of report ......................................................................................................................2

Experimental ....................................................................................................................................3

Methods and Equipment ............................................................................................................3

Instrumentation ..............................................................................................................3

Test House .....................................................................................................................3

Shed................................................................................................................................4

Generators and Loading .................................................................................................5

Shed Test Results .............................................................................................................................8

Analysis Method ........................................................................................................................8

Generator B ................................................................................................................................9

Generator X ..............................................................................................................................13

Generator SO1 .........................................................................................................................16

Discussion of Shed Tests .........................................................................................................19

Garage Test Results .......................................................................................................................20

Testing Configurations.............................................................................................................20

Results ......................................................................................................................................22

Summary of Garage Tests ........................................................................................................54

Simulation ......................................................................................................................................56

Modeling Method.....................................................................................................................56

Garage Pressurization Tests .........................................................................................58

Model Validation .....................................................................................................................61

Statistical Evaluation of Model Predictions .................................................................61

Testing Configurations.................................................................................................62

RESULTS OF MODEL VALIDATION .................................................................................64

Case #1 .........................................................................................................................64

Case # 2 ........................................................................................................................66
vi

Case # 3 ........................................................................................................................67

Case # 4 ........................................................................................................................68

Case # 5 ........................................................................................................................70

Case # 6 ........................................................................................................................72

Case # 7 ........................................................................................................................74

Summary ......................................................................................................................75

Prototype Generator Performance Simulations .......................................................................78

Garage source cases .....................................................................................................79

Utility source ................................................................................................................82

Closed bedroom door cases .........................................................................................84

Discussion of Simulation Results ............................................................................................85

Conclusion .....................................................................................................................................87

Shed Tests ....................................................................................................................87

Garage Tests.................................................................................................................87

Simulations ..................................................................................................................88

Acknowledgements ........................................................................................................................89

References ......................................................................................................................................89

Appendix A Uncertainty Analysis of Shed Measurements ...........................................................92

Appendix B Summary of Instrument Calibrations ........................................................................98

Appendix C Additional Garage Tests ............................................................................................99
vii

List of Figures

1 Aerial view of NIST manufactured test house 4 

2 Floorplan of NIST manufactured test house 4 

3 Front view of the test shed 5 

4 Inside view of the test shed 5 

5 Schematic of experimental setup in shed 6 

6 Schematic of a generator operating in a single-zone space 9 

7 Measured CO and O2 concentrations of Tests 1 and 13 11 

8 Five-minute averaged (a) CO emission rates and (b) O2 consumption rates at different 

O2 levels for Gen B 13 

9 Five-minute averaged (a) CO emission rates and (b) O2 consumption rates at different 

O2 levels for unmodified Generator X 14 

10 Five-minute averaged (a) CO emission rates and (b) O2 consumption rates at different 

O2 levels for modified Generator X 16 

11 Five-minute averaged (a) CO emission rates and (b) O2 consumption rates at different 

O2 levels for Generator SO1 with CAT muffler 17 

12 Five-minute averaged (a) CO emission rates and (b) O2 consumption rates at 

different O2 levels for Generator SO1 with nonCAT muffler 18 

13a CO and O2 concentrations in the garage and measured load for Test B (unmod Gen X, 

Configuration 1) 22 

13b CO (ppm range) concentrations in the house for Test B 

(unmod Gen X, Configuration 1) 23

13c CO (high range) concentrations in the house for Test B 

(unmod Gen X, Configuration 1) 23 

14a CO and O2 concentrations in the garage and measured load for Test O (mod Gen X, 

Configuration 1) 24 

14b CO concentrations in the house for Test O (mod Gen X, Configuration 1) 25

14c Temperatures and AFR measured in Test O (mod Gen X, Configuration 1) 25

15a CO and O2 concentrations in the garage and measured load for Test N (Gen SO1, 

Configuration 1) 26 

15b CO concentrations in the house for Test N (Gen SO1, Configuration 1) 27 

15c Temperatures and AFR measured on Gen SO1 in Test N (Gen SO1, Configuration 1) 28 

16a CO and O2 concentrations in the garage and measured load for Test F (unmod Gen X, 

Configuration 2) 29 

16b CO concentrations in the house for Test F (unmod Gen X, Configuration 2) 30 

17a CO and O2 concentrations in the garage and measured load for Test R (mod Gen X, 

Configuration 2) 31 

17b CO concentrations in the house for Test R (mod Gen X, Configuration 2) 32 

17c Temperatures and AFR measured in Test R (mod Gen X, Configuration 2) 32 

18a CO and O2 concentrations in the garage and measured load for Test T (Gen SO1, 

Configuration 2) 33 
viii

18b CO concentrations in the house for Test T (Gen SO1, Configuration 2) 34 

18c Temperatures and AFR measured in Test T (Gen SO1, Configuration 2) 34 

19a CO and O2 concentrations in the garage and measured load for Test I (unmod Gen X, 

Configuration 3) 35 

19b CO (ppm range) concentrations in the house for Test I 36 

19c CO (high range) concentrations in the house for Test I 36 

20a CO and O2 concentrations in the garage and measured load for Test Z (Gen SO1 

noncat, Configuration 3) 37 

20b CO concentrations in the house for Test Z (Gen SO1 noncat, Configuration 3) 38 

20c Temperatures and AFR measured on Gen SO1 in Test Z 39 

21a CO (high range) and O2 concentrations in the garage and measured load for Test J 

(unmod Gen X, Configuration 4) 40 

21b CO concentrations in the house for Test J (unmod Gen X, Configuration 4) 40 

22a CO and O2 concentrations in the garage and measured load for Test W (Gen SO1, 

Configuration 4) 41 

22b CO concentrations in the house for Test W (Gen SO1, Configuration 4) 42 

22c Temperatures and AFR measured in Test W (Gen SO1, Configuration 4) 42 

23a CO (high range) and O2 concentrations in the garage and measured load for Test D 

(unmod Gen X, Configuration 5) 43 

23b CO concentrations in the house for Test D (unmod Gen X, Configuration 5) 44 

24a CO and O2 concentrations in the garage and measured load for Test AH (Gen SO1 

noncat, Configuration 5) 45 

24b CO concentrations in the house for Test AH (Gen SO1 noncat, Configuration 5) 46 

24c Temperatures and AFR measured in Test AH (Gen SO1 noncat, Configuration 5) 46 

25a CO and O2 concentrations in the garage and measured load for Test G (unmod Gen X, 

Configuration 6) 47 

25b CO concentrations in the house for Test G (unmod Gen X, Configuration 6) 48 

26a CO and O2 concentrations in the garage and measured load for Test U (Gen SO1, 

Configuration 6) 49 

26b CO concentrations in the house for Test U (Gen SO1, Configuration 6) 50 

26c Temperatures and AFR measured in Test U (Gen SO1, Configuration 6) 50 

27a CO and O2 concentrations in the garage and measured load for Test K (unmod Gen X 

Configuration 7) 51 

27b CO concentrations in the house for Test K (unmod Gen X Configuration 7) 51 

28a CO and O2 concentrations in the garage and measured load for Test V (noncat Gen 

SO1, Configuration 7) 52 

28b CO concentrations in the house for Test V (noncat Gen SO1, Configuration 7) 53 

28c Temperatures and AFR measured in Test V (noncat Gen SO1, Configuration 7) 54 

29 CONTAM Sketchpad Representation of Test House 56 

30 Test house and garage fan pressurization configurations 59 

31 Predicted and observed CO and O2 concentrations for Case #1 65 

32 Predicted and observed CO and O2 concentrations for Case # 2 66 

33 Predicted and observed CO and O2 concentrations for Case # 3 68 
ix

34 Predicted and observed CO and O2 concentrations for Case # 4 70 

35 Predicted and observed CO and O2 concentrations for Case # 5 71 

36 Predicted and observed CO and O2 concentrations for Case # 6 73 

37 Predicted and observed CO and O2 concentrations for Case # 7 75 

38 Comparison of predicted and observed individual house zone average CO 

concentrations for all cases 77 

39 Garage source, open bay door, closed house door, cat muffler cases 80 

40 Utility source, closed utility door, cat muffler cases 83 

41 CO concentrations for garage source, closed bay door, open house door, mild weather, 

cat muffler with closed bedroom doors 85 
x 

List of Tables

1 Hourly cyclic load profile 7

2 Summary of Gen B test results 10

3 Tests Conducted in Attached Garage 21

4 Summary of garage test results 55

5 Air Leakage Values 57 

6 HVAC System Supply Flows 58 

7 Fan Pressurization Test Results 60 

8 Model Validation Tests Conducted 63 

9 Statistical parameters for Case #1 65 

10 Statistical parameters for Case # 2 67 

11 O2 dependent CO emission rate for Gen B 67 

12 Statistical parameters for Case # 3 68 

13 Hourly cyclic load profile for Gen SO1 cat 69 

14 Statistical parameters for Case # 4 70 

15 Hourly cyclic load profile for Gen SO1 cat 71 

16 Statistical parameters for Case # 5 72 

17 Hourly cyclic load profile for Gen SO1 cat 72 

18 Statistical parameters for Case # 6 73 

19 Hourly cyclic load profile for Gen SO1 noncat 74 

20 Statistical parameters for Case # 7 75 

21 Summary of statistical parameters from comparison of average observed and predicted 

zone CO concentrations for all cases 76 

22 Simulation results for garage source cases 81 

23 Simulation results for utility source cases 84 

24 Simulation results for closed bedroom door cases 85 
xi

Nomenclature

Ain hourly air change rate of a test space evaluated at the ambient temperature, h-1

Aout hourly air change rate of a test space evaluated at the space temperature, h-1

CCO CO concentration, mg/m3

12 C t,OH water vapor volumetric concentration at time t1, m3

/m3

22 C t,OH water vapor volumetric concentration at time t2, m3

/m3

*

O2 C non-dimensional O2 concentration

Cout gas volumetric concentration outside the space, m3/m3

C 6 ,SF in SF6 concentration in the intake of the generator, m3

/m3

,SF ex 6 C SF6 concentration in the exhaust of the generator, m3

/m3

Dgen displacement of the generator engine, m3

ELA Effective leakage area, cm2 at 4 Pa

K ,m ρ ρρ ,mout,m in , ratio of density outside and inside the test space of a gas component “m”

K 6 d,SF SF6 decomposition ratio in the generator

Lo output load of a generator *

Lo non-dimensional generator load

P partial pressure of water vapor, Pa

Psat saturated water vapor pressure, Pa

Qgen,in volumetric flow rate through the generator at the intake gas temperature, m3/h

Qin volumetric air inflow of a test space, m3/h

Qout volumetric air outflow of a test space, m3/h

Qgen,ex volumetric flow rate through the generator at the exhaust gas temperature, m3/h

RPM revolutions per minute of the generator engine, min-1

SC source emission rate of a gas “C” at the air temperature of the test space, kg/h * SCO non-dimensional CO emission rate

SCO,min

minimum CO emission rate, g/h

SCO max, maximum CO emission rate, g/h

S 2OH water emission rate from the generator, kg/h

Sgen mass added from the generator to the test space, kg/h *

O2 S non-dimensional O2 consumption rate

T gas temperature, K

Tgen,in temperature of the generator intake gas, K

Tgen,ex temperature of the generator exhaust gas, K

Vs volume of the test space, m3

φ water vapor relative humidity

η gen volumetric efficiency of the generator engine

ρ ,C in density of a gas component inside the test space, kg/m3

ρ out,C density of a gas component outside the space, kg/m3

ρ in gas mixture density in the test space, kg/m3

ρ out,m density of gas mixture (= ρ air ) outside the test space, kg/m3
xii
1 

Introduction

Background

The U.S. Consumer Product Safety Commission (CPSC) is concerned about the hazard of acute 

residential carbon monoxide (CO) exposures from portable gasoline powered generators that can 

result in death or serious and/or lasting adverse health effects in exposed individuals. As of April 

2012, CPSC databases contain records of at least 755 (695 from generator use alone, 60 from 

generator use in conjunction with another CO-producing consumer product) deaths from CO 

poisoning associated with consumer use of generators in the period of 1999 through 2011 

(Hnatov 2012). In addition, the percentage of estimated non-fire, consumer product-related CO 

poisoning deaths specifically associated with generators for CPSC’s four most recent years of 

data are 51 % (2005), 49 % (2006), 38 % (2007), and 49 % (2008) (Hnatov 2011). Typically, 

these deaths occur when consumers use a generator in an enclosed or partially enclosed space or 

outdoors near an open door, window or vent, and they often occur after severe weather events 

such as hurricanes and ice or snow storms. The initial health impact of CO is caused by anoxia: 

deprivation of oxygen supply. When inhaled, CO preferentially binds with the oxygen carrier in 

the red blood cells, hemoglobin (Hb), to form carboxyhemoglobin (COHb), which causes the 

anoxia (Stewart 1975). 

Since the possession of household generators in the U.S. has climbed continuously in recent 

years, from an estimated 9.2 million units in 2002 to 10.6 million units in 2005, CPSC is 

working to avoid future generator-related CO poisoning incidents, especially those associated 

with operating a generator indoors (CPSC 2006). Measures have been taken to educate people 

not to operate generators indoors and to require manufacturers of portable generators to warn 

consumers of CO hazards with a warning label (CFR 2007). In order to understand the CO 

exposures associated with such incidents and their potential reduction, the emission 

characteristics of these generators need to be better characterized. 

In an initial approach to characterizing these hazards, CPSC measured the CO emission rates 

from generators by testing them in a small, laboratory test chamber (Brown 2006). Using those 

data, CPSC performed preliminary indoor air quality (IAQ) modeling and estimated that a 92 % 

reduction in the CO emission rate based on these measurements would likely result in a 

significant delay and reduced severity of the CO exposure in areas of a home remote from the 

generator location (Inkster 2006). CPSC subsequently worked with the University of Alabama 

(UA) to develop a low CO-emission prototype generator by adapting off-the-shelf emission 

control technologies on a commercially-available portable generator. UA then constructed

multiple prototype generators by adapting the same emission control strategy onto other units 

powered by the same model engine. In conjunction with these efforts, CPSC established an 

interagency agreement with the National Institute of Standards and Technology (NIST) to model 

indoor time course profiles of CO concentrations throughout a single-family residence resulting 

from CO emissions from a portable generator operating in the attached garage of a home under 

various use and environmental conditions to enable CSPC staff to understand the safety 

implications of operating a portable generator in these conditions. The agreement was later 

expanded to include a series of tests to provide empirical data to further characterize the hazard 

by measuring the emission and transport of CO when generators are operated in an actual 

building, to test them in a one-zone shed to derive their CO emission and O2 consumption rates, 

and to provide model validation data. 
2 

An interim report by Emmerich (2011) presents data from a series of tests of both unmodified 

and UA-modified low CO emission prototype generators operated in the garage attached to 

NIST’s manufactured house, a test facility designed for conducting residential indoor air quality 

(IAQ) studies. This double-wide manufactured house is similar in size to homes commonly 

involved in fatal consumer incidents (Hnatov 2012). The garage tests described in the interim 

report documented reductions of 85 % to 98 % in CO concentrations due to emissions from two 

modified, prototype low CO-emission portable generators compared to an unmodified generator

in this house for the scenarios tested. 

Objective

The primary objective of this project was to measure and model CO emission and exposure, and 

O2 depletion, resulting from portable generators operating in a home’s attached garage under 

various use and environmental conditions in order to enable CPSC staff to analyze the safety 

implications of operating a portable generator under these conditions. 

Contents of report

This report contains four main sections. The Experimental section describes the experimental 

methods and equipment, test house and shed, and generators used in the experimental portion of 

this project. The Shed Test Results section presents the results of testing three different 

generators operating in the test shed. The Garage Test Results presents the results of testing three 

different generators operating in the attached garage of the NIST test house. The Simulation 

section describes the CONTAM model of the test house, the model validation effort and the 

simulations used to extend the measurement results to other scenarios. The report also contains 

three appendices including one describing the uncertainty analysis of the measurements, one on 

instrument calibrations, and one with additional garage test results beyond those presented in the 

main body of the report. 
3 

Experimental

Methods and Equipment

Instrumentation 

Gas concentrations were measured with two multi-gas engine exhaust analyzers (NOVA 

Analytics Model 7464), which are combination non-dispersive infrared (NDIR) and 

electrochemical sensor technologies, referred to as N1 and N2 in the rest of this report. These 

analyzers measured CO on two channels covering different ranges of 0 % to 1 % and 0 % to 10 

%, CO2 from 0 % to 20 %, hydrocarbons (as hexane) from 0 % to 2 % and O2 from 0 % to 25 % 

with a reported accuracy of 1 % of full scale for all five channels. An electrochemical sensor CO 

analyzer (NOVA Analytics Model 7461), referred to as N3 in this report, measured CO over a 

range of 0 ppmv to 2000 ppmv and with a reported accuracy of 1 % of full scale. Two additional 

NDIR CO analyzers were used, a Thermoelectron Model 48 (referred to as TE) and a Rosemount 

Model 880A (referred to as RM), both with ranges of 0 ppmv to 1000 ppmv and reported 

accuracy of 1 % of full scale. Finally, a portable O2 analyzer (Sybron Servomex O2 Analyzer OA 

580) was also used. Not all instruments were used during every test. Repeated calibrations during 

the test periods found that typical measurement uncertainties were consistent with the 

manufacturers’ reported accuracies. See Appendix B for more detail on calibrations. To protect 

the analyzers from condensed water and/or soot particles, desiccant and high efficiency 

particulate air (HEPA) filters were used in the sampling system. 

Air change rates were measured using tracer gas decay method (ASTM 2011). A pulse of sulfur 

hexafluoride (SF6) was injected and allowed to mix before being measured with a gas 

chromatograph with an electron capture detector. A local weather station was used to measure 

ambient conditions. All gas concentration, air temperature, and humidity data were recorded by 

an automated data acquisition system. Nabinger and Persily (2008) provide more details on the 

SF6, temperature, humidity, and ambient weather condition measurements including 

uncertainties.

Test House 

The test house used in this study was a manufactured house located on the NIST campus, which 

was erected in 2002 (Nabinger and Persily 2008). An aerial view and floorplan of the house are 

shown in Figures 1 and 2. The house includes three bedrooms (MBR, BR2, and BR3), a living 

room (LR), a family room (FAM), a kitchen (KIT), and an attached garage. The house has a 

floor area of 140 m2

 and a volume of 340 m3. The attached garage has a floor area of 36.5 m2, a 

volume of 90 m3 and was built as an addition to the house in 2007. The interior of the garage, 

including the ceiling, is finished with painted gypsum board. As part of the garage construction, 

the underlayment and siding of the exterior west wall of the house were removed and replaced 

with 3⁄4 inch gypsum board on studs with fibrous glass batt insulation in the wall cavity. 
4 

Figure 1 Aerial view of NIST manufactured test house 

 

Figure 2 Floorplan of NIST manufactured test house 

Measurements of gas concentrations were made at various points throughout the house using 

sample lines suspended 1.5 m above the floor in the center of each of the three bedrooms, the 

living room, the kitchen, and the family room, as well as five sample lines located near the four 

corners and center of the garage. The six individual living space locations were measured for one 

minute each in a repeating six minute cycle. The garage sample locations were measured 

separately, as well as a single mixed sample, the latter of which is reported here. Indoor air 

temperature and humidity were measured with sensors in each room of the house and on two 

opposite walls of the garage. The outdoor temperature and wind speed were measured at a 

weather station located about 6 m behind the house for tests conducted in 2008. For tests in 

2010, wind speed data were collected from a weather station located on the roof of Building 226 

on the NIST campus (about a mile from the test house), however, wind direction data were still 

collected at the local weather station. 

Shed 

Experiments were also conducted in a shed, with dimensions of 4.88 m (L) × 3.05 m (W) × 

2.90 m (H), as shown in Figures 3 and 4, for the purpose of measuring the CO emission rate and 

O2 consumption rate of the generators. The shed was used because it allowed better control of 

conditions than the garage and enabled a simplified single-zone analysis of the measurement 

results (see Shed Test Results section for description of analysis method). An explosion-proof 

Garage Family 

Room 

(FR)

Kitchen 

(KIT)

Living 

Room 

(LV)

Master Bedroom

Bed 

Room3

Bed 

Room2

N
5 

exhaust fan was installed in the wall opposite the door for shed ventilation at the end of a test and 

for quick exhaust during an emergency. This fan was also used in selected tests to create a high 

air change rate to obtain CO emission rates at near ambient conditions. The shed also had two 

operable windows at both sidewalls, which were adjusted to vary the air change rate. 

Figure 3 Front view of the test shed 

Figure 4 Inside view of the test shed

Separate sample lines were placed mid-height in the center of the shed (midway between the 

walls) for CO, O2, and SF6. Non-dispersive infrared and electrochemical sensor CO analyzers

(N1, N2 and N3 described in the Instrumentation section) and a portable O2 analyzer were used 

to measure CO and O2 respectively. A gas divider/diluter was also used to dilute the sampled CO 

for the CO analyzer for some tests. The air temperature and humidity in the shed were measured 

at two locations near two sidewalls. 

Generators and Loading 

Generators were selected with electrical power output ratings in the size range most commonly 

involved in fatal consumer incidents, which is 5.0 kilowatts (kW) to 6.5 kW (Hnatov 2012). 

Tests were conducted with three different generators that were configured in multiple ways. Two

unmodified ‘stock’ (i.e., in their as-purchased condition) generators were tested. The first

generator (referred to here as Gen B) has a full-load power rating of 5.5 kW with a 7.5 kW (10 

horsepower, hp) carbureted, single-cylinder gasoline engine and no specific CO emission control 
6 

technology. This same generator was also tested by CPSC in a small chamber as reported on by 

Brown (2006). 

Figure 5 Schematic of experimental setup in shed

The second generator is powered by a carbureted 8.2 kW (11 hp) single-cylinder gasoline engine 

made by a different manufacturer than Gen B and has an advertised full-load electric power 

rating of 5.0 kW. This generator was tested in both unmodified condition (referred to as unmod 

Gen X) and as a modified low-CO emission prototype (referred to as mod Gen X). The 

unmodified generator operates at air-fuel ratios (AFR, ratio of mass of air to mass of fuel) in the 

range of 10 to 13 AFR depending on the load, which is common for small, air-cooled carbureted 

engines. The modifications were made by the University of Alabama (UA) and included adding 

an engine management system (EMS) with associated sensors and actuators for electronic fuel 

injection (replacing the carburetor) and a muffler with a small catalyst integrated in it. The 

function of the EMS is to control ignition timing and fuel delivery through an engine control unit 

(ECU) microcomputer that receives input from a variety of system sensors. UA calibrated the 

ECU on the modified prototype to operate around a 14.6 AFR over the full range of loads. This 

AFR fuel control strategy is the primary means by which the prototype aims to achieve its 

reduction in CO emissions. The catalyst primarily targets reduction of oxides of nitrogen (NOx) 

and has relatively low catalytic activity because the EMS significantly reduces the available 

oxidation constituents in the exhaust stream.

For the third generator (referred to as Gen SO1), a model similar to Gen X was obtained which 

had the same model engine but with an alternator with an output rating of 7 kW. It was tested 

after UA modified it using the same fuel control strategy and largely the same emission control 

hardware that was used in mod Gen X. One difference is that Gen S01 had a different model 

ECU than that used on mod Gen X. Another difference noted during the testing is that its 

manufacturer included programming to maintain rich AFR operation until the oil temperature 

rose above approximately 60 °

C, resulting in an initial “spike” of CO when the engine was 

started cold. This ECU also includes an algorithm developed by UA that can be switched on or 

off by the test operator for testing purposes. The algorithm was intended to sense when the 
7 

generator was operating in an enclosed space, based on engine operation parameters and when 

enabled, was intended to shut off the engine before a life-threatening CO hazard develops. All 

the tests with Gen SO1 that are reported in the main body of this report were performed with the 

algorithm disabled. Gen SO1 was also tested in a configuration with a muffler that did not 

contain a catalytic converter (referred to as the noncat muffler). The purpose of testing with the 

two different muffler versions was to measure the CO emissions produced in the engine due to 

the fuel control strategy alone (from tests with the noncat muffler) as well as get an indication of 

the catalyst’s performance in further lowering those emissions (from the tests with the cat 

muffler). A full description of the prototype configuration of both mod Gen X and Gen SO1 is

provided in greater detail in UA’s report to CPSC (CPSC 2012). 

 

To monitor prototype engine operation, generators mod GenX and Gen SO1 were outfitted with 

thermocouples and a Lambda sensor to measure AFR (ECM Model Lambda 5220, with an AFR 

range of 6 to 364 and reported accuracy of 0.2 for 12 < AFR < 18). The Lambda sensor and a 

thermocouple for measuring engine-out exhaust temperature were mounted through ports that 

UA provided on the exhaust manifold pipe between the engine and muffler. Cylinder head 

temperature was measured with a ring thermocouple mounted under the spark plug. Engine oil 

temperature was measured with a thermocouple inserted into the sump. For some of the tests, 

muffler and shroud temperatures were also measured, using thermocouples mounted directly on 

their surfaces at the hottest locations previously identified by UA with infrared cameras during 

their prototype tests.

The generators were operated using reformulated gasoline with 10 % ethanol obtained from the 

NIST motor pool, which is purchased to the same specification year-round. The generators were 

placed on a spill-catching platform in the middle of the garage (or shed) with the exhaust pipe 

pointing towards the garage wall adjoining the house. 

A portable alternating current (AC) resistive load bank connected to the generator’s 240-volt 

receptacle was used to draw electrical power and thereby act as a surrogate for consumer 

appliance loads. The load bank has manual switches in 250 W increments with a maximum 

setting of 10 kW. Some tests involved a constant load while others followed a cyclic load profile 

(see Table 1). This profile is similar to the load profile used by UA during the durability and 

emission testing of their low CO emission prototype generator. It was derived from a test cycle 

developed by industry to replicate typical in-use operation of small utility engines when used in 

all types of engine-driven products (CPSC 2012). Because the actual delivered power did not 

always match the load bank settings, particularly when oxygen depletion was occurring, the 

delivered power was measured and recorded during all tests. 

Table 1 Hourly cyclic load profile

Load bank 

setting (W)

Duration 

(min) 

no load 3 

500 4 

1500 18 

3000 17.5 

4500 12 

5500 5.5
8 

Shed Test Results 

Few studies have been conducted directly on CO emission and O2 consumption rates associated 

with gasoline-powered generators running indoors. Brown (2006) studied the CO emission rates 

from four different commercially-available generators in an enclosed experimental chamber, 

where air temperature and air change rate were controlled to provide different operating 

conditions. Also, the air change rates during these chamber tests were generally quite high 

compared with typical residences. Steady-state CO concentrations were found to be more than 

7400 μL/L and O2 levels as low as 18.5 %. CO emission and O2 consumption rates at steady 

state were also calculated, and were found to be affected by O2 level, generator loading, and/or 

chamber air temperature. 

While generator tests conducted in laboratory chambers offer a high degree of control, they do 

not allow consideration of the real-world impacts of varying outdoor weather or the lower and 

variable air change rates that occur in residential structures. Alternatively, operating a generator 

in an enclosed space such as a garage or a storage shed, as opposed to a laboratory chamber, will 

be subject to uncontrolled temperatures and to lower ventilation rates determined by ambient 

weather conditions. To study generator CO emission rates under more realistic conditions 

including emission controls, tests were conducted in a single-zone space. This section reports on 

the measurement of CO emission rates from generators, operating in the unmodified carbureted 

configuration as well as in the low CO emission prototype configuration, (i.e. without and with 

CO emission controls) in a test shed located outdoors. The CO emission rates and O2

consumption rates are used for the prediction of CO emission, migration and exposure in 

simulations. 

Analysis Method

To model a generator running inside an enclosed single-zone space, a theoretical model was 

constructed based on Figure 6. This model was used to calculate CO emission and O2

consumption rates from the concentrations measured during the tests. Assuming a gas 

component, C, is either generated (SC > 0) or consumed (SC < 0) in the zone, a differential mass 

balance equation for C during a period of 12 ∆ −= ttt can be expressed as

ρC,inVs

dC

dt = SC − ρC,inCQout + ρC,outCoutQin (1)

by assuming the following:

the gas component, C, is an ideal gas, 

the concentration of C is uniform in the zone, 

ρ C , SC, and Q are constant during ∆t , and 

the mass of fuel added from the generator to the zone air is neglected. 

Note: See the Nomenclature list for all terms in the equations.
9 

 Figure 6 Schematic of a generator operating in a single-zone space 

The density of C in Eq. (1) can be obtained by the ideal gas law. SC is positive when referred to 

as SCO, the emission rate of CO, and negative for SO2, the consumption rate of O2. In the 

experiments presented in this report, the concentration of a gas component, C, was measured by 

a gas analyzer, as described above. The air change rate, Q, was determined by using the tracer 

gas decay method using SF6. Given that SF6 may decompose at temperatures above 200  ̊C (Air 

Products 2006), the potential consumption of SF6 in the generator engine, SSF6 (<0), was 

examined but was found to be insignificant (Wang and Emmerich 2010). 

After determining the air change rate of the space from the decay rate of the SF6 (see Wang and 

Emmerich 2010 for details), SCO and SO2 can be solved from Eq. (1) for the time period of t1 to t2

tA

tA

CO t CO t

CO CO in sout out

out

e

eCC

S VA ∆−

∆−

−

− = 1

2, 1, ρ ,

 (2) 

outo

m

soutoutO

tA

tA

tOtO

O O in sout C

K

VA

e

eCC

S VA out

out

,

,

1,2, ,

, 2

2 2 2

2 2 1 ρ

ρ

ρ − −

− = ∆−

∆−

 (3) 

In addition, several gas concentration uniformity tests were conducted by collecting samples at 

five different locations in the shed. It was found that the thermal plume, which was driven by the 

heat from the running generator, mixed the shed very well throughout testing. The variations 

among the five sample locations were less than 5 % for SF6, which met the uniformity 

requirement of 10 % in the ASTM standard (ASTM-E741-00 2006). 

Generator B

Using the same generator (B), Brown (2006) found that CO emission rate was closely related to 

generator load and O2 level in small chamber tests. In the current study, 13 tests of Gen B were 

conducted for load settings of 2.5 kW (half of the maximum load of the generator) and 5.0 kW 

(full load) for different air change rates (which result in different O2 levels) and air temperature 

as shown in Table 2. Different air change rates were achieved by adjusting the shed windows to 

be opened with a 0.05 m × 0.2 m crack (OW) or completely closed (CW). Each test was also 

conducted at least twice to confirm the validity of the measurement data. Each test in Table 2 is 

named by its load setting, window adjustment and repetition, for example, 2.5kW-OW-1 

indicates the first test running under a 2.5 kW load setting and with open windows. Note that 

Tests 6, 11, and 12 were conducted after the generator engine was serviced (S = serviced), which 

entailed changing the oil and cleaning the air filter. For Test 13, the shed was tightened to 

achieve a low air change rate (LA). 

Qin Qout

SC 

Generator
10

Table 2 Summary of Gen B Shed Test Results

Load

setting 

(kW) 

Test Name

Run 

Time 

min

Max CO 

(μL/L) 

Min 

O2

(%) 

CO 

Emission

rates 

(g/h)

O2

Consumption 

rates 

(g/h)

Air 

change 

rate

(h-1)

2.5 

1 2.5kW-OW-1 80 3190 19.3 340 to 900 3300 to 4800 6.5 

2 2.5kw-OW-2 63 5290 18.9 40 to 900 3300 to 4000 3.7 

3 2.5kw-CW-1 86 23950 16.9 1100 to 

3100 4600 to 5900 2.6 

4 2.5kw-CW-2 101 23270 16.8 1000 to 

3000 4200 to 5700 3.1 

5 2.5kw-CW-3 80 20090 17.1 880 to 

2600 4100 to 5100 2.9 

6 2.5kw-CW-S1 106 8160 18.2 500 to 

1400 4200 to 5100 3.6 

5.0 

7 5.0kw-OW-1 90 18210 17.9 120 to 

3700 5700 to 7100 4.7 

8 5.0kw-OW-2 81 8420 19.1 1500 to 

2600 4100 to 6500 7.6 

9 5.0kw-CW-1 62 21320 17.4 1100 to 

3600 5600 to 6700 3.7 

10 5.0kw-CW-2 74 21200 17.1 1700 to 

3000 4700 to 6400 2.9 

11 5.0kw-CW-S1 65 22470 16.9 1600 to 

3500 5800 to 7600 3.0 

12 5.0kw-CW-S2 66 22280 17.0 70 to 3800 1400 to 7100 3.6 

13 5.0kw-CW-LA 86 25500 16.2 2000 to 

3400 3900 to 6200 0.7 

Table 2 shows large variations in CO and O2 levels between tests with Gen B under the different 

constant load settings and window positions. The CO level reached only 3190 μL/L (note that 

μL/L are equivalent to the commonly used unit ppmv) within 80 min in Test 1 when the 

generator was at half-load with the shed windows opened, but reached a maximum of 25500 

μL/L for an 86-min run-time under full load in Test 13 when the shed was tightened. The shed 

O2 concentration dropped significantly below ambient levels when operating the generator in this 

enclosed space. The largest decline occurred for Test 13, when the O2 level was 16.2 % with the 

low air change rate and full loading. Table 2 also provides the measured air change rates and 

time-averaged weather conditions. When the shed was tightened in Test 13, the air change rate 

reached its lowest value, 0.7 h-1, which explains it reaching the highest CO level and lowest O2

level. For the rest of the tests, the shed window positions played an important role. The air 

change rates of open-window cases were all as high as or higher than those with closed windows. 

As a result, the opened window tests always had higher O2 levels than those with closed 

windows for the same load. The CO levels in opened-window tests were generally lower than 

closed-window cases. Note also that the weather conditions during these tests varied 

significantly. The ambient temperature ranged from 4.0 °C to 17.2 °C and the wind speed from 

1.1 m/s to 6.8 m/s. The variations of weather provided realistic generator operating conditions 

for these experiments as compared to controlled chamber experiments. Note that the CO and O2
11

levels in Table 2 are only provided to indicate the range of test conditions, as these tests were 

primarily performed to determine CO emission and O2 consumption rates for use in the 

simulation effort described later in this report.

As an illustration of individual shed tests, Figure 7 shows the CO and O2 measured for Tests 1 

and 13 from Table 2. The patterns of CO concentration in both tests are almost an inverse to that 

of the O2 level for this unmodified generator. The CO level is low at the beginning of generator 

startup and increases steadily as the O2 level drops. As the O2 drops further and causes a very 

rich fuel mixture in the engine, CO reaches a maximum level. Test 13 in Figure 7 shows an 

extreme case in which the generator eventually produces a zero electrical load when the O2 drops 

to around 16.4 %, although it was set at a full load and the engine crankshaft was still rotating. 

Figure 7 Measured CO and O2 concentrations of Tests 1 and 13. 

Figure 7 also shows that steady state was never reached for either test. A relatively stable period 

occurred at about 40 min for Test 1 and 45 min for Test 13, but they only held for a few minutes. 

These results differed from chamber experiments, where CO concentrations becomes constant 

after a period of time as complete steady state can be achieved under the controlled environment 

and higher air change rate. While these chambers tests are useful, the results from the shed 

confirm the importance of studying CO emission as a transient process under real weather 

conditions and more realistic air change rates to better understand generator performance in the 

field.

In order to generalize these test results to other conditions beyond this particular test facility, it is 

important to convert the results into CO emission and O2 consumption rates. As seen in these 

tests, many factors can affect these rates directly or indirectly: space ventilation conditions, 

combustion conditions in the engine, O2 level in the space, load setting, and the time over which 

the generator has been running. 

16

16.5

17

17.5

18

18.5

19

19.5

20

20.5

21

0

5000

10000

15000

20000

25000

30000

35000

0 10 20 30 40 50 60 70 80 90 100

O2 (%)

CO (μL/L)

Time (min)

CO(Test 1) CO(Test 13) O2(Test 1) O2(Test 13)

Generator off(Test 1)

Generator on(Test 13)

Generator on(Test 1)

Zero load output(Test 13)

Generator off(Test 13)
12

Figures 8(a) and 8(b) illustrate how 5-min average CO emission rates and O2 consumption rates 

(∆t = 5 min in Eqs. (7) and (8)) change with O2 levels in the thirteen shed tests. Figure 8(a) 

shows that for both full and half load settings CO emission rates increase with decreasing O2, 

reach maximum values when O2 drop to about 17 % to 18 %, and then decline at lower O2 levels. 

Under the extreme case of Test 13 (5.0kw-CW-LA), the CO rate decreases dramatically as the 

O2 level reaches around 16.4 % with an electrical output of zero. Figure 8(b) shows an opposite 

trend of O2 consumption rate compared to CO. The O2 consumption rate decreases notably near 

16.4 %, corresponding to the zero electrical output caused by O2 deficiency. Given that the 

engine crankshaft was still rotating, the O2 consumption rate was not quite zero, which explains 

the continual drop of O2 in Test 13 of Figure 7, although the electrical output is zero. 

The solid points in Figure 8 are data points for a half-load setting (2.5 kW) and the hollow ones 

for a full load setting (5.0 kW). As seen in previous small chamber test results in Brown (2006), 

a higher load setting generally results in more CO generated and O2 consumed until the O2 level 

reaches about 17 %, where data for full and half loads come together. This overlap corresponds 

to the drop in electrical output with the decrease of O2. Note that Figure 8 also shows the 

calculated uncertainty for each data point of CO emission and O2 consumption rates, which was 

mostly less than 20 % with a confidence level of 95 %. Appendix A discusses the uncertainty 

analysis in detail.

Figure 8(a)

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

16 16.5 17 17.5 18 18.5 19 19.5 20 20.5 21

CO Emission Rate (g/h)

O2 (%)

2.5kw-OW-1 2.5kw-OW-2 2.5kw-CW-1 2.5kw-CW-2 2.5kw-CW-3

2.5kw-CW-S1 5.0kw-CW-1 5.0kw-CW-2 5.0kw-OW-1 5.0kw-OW-2

5.0kw-CW-S1 5.0kw-CW-S2 5.0kw-CW-LA
13

Figure 8(b)

Figure 8 Five-minute averaged (a) CO emission rates and (b) O2 consumption rates at different 

O2 levels for Gen B

Generator X

Generator X was tested in both unmodified and modified (low CO emission) prototype 

configurations. The primary difference between the tests with Gen X and Gen B was the 

generator loading. Gen X was tested at load points selected to approximately match the points of 

the load profile used by UA (See Table 1) during the durability and emission testing of their low 

CO emission prototype generator. 

Figures 9(a) and 9(b) present the CO emission rates and O2 consumption rates as a function of O2

levels for unmodified Generator X. Although the tests of Gen X and Gen B were not identical, 

Figure 9(a) show similar results in that the CO emission rates range from a low of around 500 

g/h at near ambient conditions to a high of nearly 4000 g/h as O2 approaches 17 %. Unlike Gen 

B, however, the emission rate is only clearly load-dependent when the O2 drops below about 

19 %. Fewer tests were performed on Generator X below 17 % O2 but the results indicate a 

similar drop off in CO emissions due to poor engine performance under these conditions. 

-7,500

-7,000

-6,500

-6,000

-5,500

-5,000

-4,500

-4,000

-3,500

-3,000

-2,500

-2,000

-1,500

16 17 18 19 20 21

O2 Consumption Rate (g/h)

O2 (%)

2.5kw-OW-1 2.5kw-OW-2 2.5kw-CW-1 2.5kw-CW-2 2.5kw-CW-3

2.5kw-CW-S1 5.0kw-CW-1 5.0kw-CW-2 5.0kw-OW-1 5.0kw-OW-2

5.0kw-CW-S1 5.0kw-CW-S2 5.0kw-CW-LA
14

Figure 9(a) 

Figure 9(b)

Figure 9 Five-minute averaged (a) CO emission rates and (b) O2 consumption rates at different 

O2 levels for unmodified Generator X 

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

16 17 18 19 20 21

CO Emission Rate (g/h)

O2 Level (%)

Idle(CO) 0.5kW(CO) 1.5kW(CO) 2.5kW(CO)

4.0kW(CO) 5.5kW(CO) 5.5kw-2(CO) 4.5kw(CO)

3.0kW(CO) 5.5kW-2-NOVA2(CO) 1.5kW-2(CO) 0.5kW-2(CO)

Idle-2(CO) 4.5kW-2(CO) 3.0kW-2(CO) 5.5kW-3(CO)

0.5kW-3(CO) 3.0kW-3(CO) 5.5kW-4(CO) Idle-4(CO)

0.5kW-4(CO) 1.5kW-4(CO) 3.0kW-4(CO) 3.0kW-4(CO)

4.5kW-5(CO) 5.5kW-4b(CO) 5.5kW-5(CO) 5.5kW-6(CO)

-10,000

-9,000

-8,000

-7,000

-6,000

-5,000

-4,000

-3,000

-2,000

-1,000

0

16 17 18 19 20 21

O2 Consumption Rate (g/h)

O2 Level (%)

Chart Title Idle(O2) 0.5kW(O2) 1.5kW(O2) 2.5kW(O2)

4.0kW(O2) 5.5kW(O2) 5.5kW-2(O2) 4.5kw(O2)

3.0kW(O2) 5.5kW-2-NOVA2(O2) 4.5kw-2(O2) 3.0kW-2(O2)

1.5kW-2(O2) 0.5kW-2(O2) Idle-2(O2) 4.5kW-2(O2)

3.0kW-2(O2) 5.5kW-3(O2) 0.5kW-3(O2) 3.0kW-3(O2)

5.5kW-4(O2) Idle-4(O2) 0.5kW-4(O2) 1.5kW-4(O2)

3.0kW-4(O2) 4.5kW-4(O2) 4.5kW-5(O2) 5.5kW-4(O2)

5.5kW-5(O2) 5.5kW-6(O2)
15

Figures 10(a) and 10(b) present the CO emission rates and O2 consumption rates as a function of 

O2 levels for modified Generator X. Although modified Gen X was not tested as many times as 

unmodified Gen X, comparing Figure 10(a) to Figure 9(a) shows the dramatic reduction in CO 

emission rates due to the low CO emission modifications included on the prototype. Most of the 

modified Gen X CO emission rates were well below 500 g/h. Although not enough low O2 tests 

were performed to be conclusive, the CO emission rates at the highest loads did tend to increase 

as O2 dropped. 

Figure 10(a)

0

500

1000

1500

2000

2500

16 17 18 19 20 21

CO Emission Rate (g/h)

O2 Level (%)

Idle - GenXmod(CO) 0.5kW - GenXmod(CO) 1.5kW-GenXmod(CO)

3.0kW - GenXmod(CO) 4.5kW - GenXmod(CO) 5.5kW - GenXmod(CO)
16

Figure 10(b)

Figure 10 Five-minute averaged (a) CO emission rates and (b) O2 consumption rates at different 

O2 levels for modified Generator X

Generator SO1

Generator SO1was tested at the same load points as Generator X but was not tested in an 

unmodified configuration. Instead, it was tested in two modified configurations – with and 

without a catalyst integrated in the muffler (referred to as cat muffler and noncat muffler, 

respectively). Figure 11(a) and 11(b) present the CO emission rates and O2 consumption rates as 

a function of O2 levels for Generator SO1 with the cat muffler (referred to as Gen SO1 cat). 

Comparing Figure 11(a) to Figure 10(a) shows that Gen SO1 cat performed somewhat better 

than modified Gen X. All measured CO emission rates for Gen SO1 cat were well below 500 

g/h, and no trend toward higher emission rates was seen as O2 levels dropped to 18 %. However, 

as with Gen X, no tests were performed at levels as low as 17 %.

-10,000

-9,000

-8,000

-7,000

-6,000

-5,000

-4,000

-3,000

-2,000

-1,000

0

16 17 18 19 20 21

O2 Consumption Rate (g/h)

O2 Level (%)

Idle-Xmod(O2) 0.5kW-Xmod(O2) 1.5kW - GenXmod(O2)

3.0kW - GenXmod(O2) 4.5kW - GenXmod(O2) 5.5kW-GenXmod(O2)
17

Figure 11(a)

 Figure 11(b)

Figure 11 Five-minute averaged (a) CO emission rates and (b) O2 consumption rates at different 

O2 levels for Generator SO1 with cat muffler

0

50

100

150

200

250

300

350

400

450

500

16 17 18 19 20 21

CO Emission Rate (g/h)

O2 Level (%)

1.5kWSO1-2(CO) 3.0kWSO1cat-1(CO) 4.5kWSO1cat-1(CO)

Idle-1 SO1cat(CO) 0.5-1 SO1cat(CO) 5.5 kW-1 SO1cat(CO)

1.5kW-1 SO1cat(CO) Idle-2 SO1cat(CO) 0.5 kW-SO1cat2(CO)

5.5kw SO1cat-2(CO)

-10000

-9000

-8000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

16 17 18 19 20 21

O2 Consumption Rate (g/h)

O2 Level (%)

1.5kW-SO1cat-2(O2) 3.0kW-SO1cat-1(O2) 4.5kW-SO1cat-1(O2)

Idle-1 SO1cat(O2) 0.5 kW-1 SO1cat(O2) 5.5 kW-1 SO1cat(O2)

1.5kW-1 SO1cat(O2) Idle-2 SO1cat(O2) 0.5 kW SO1cat-2(O2)

5.5 kW SO1cat-2(O2)
18

Figure 12(a)

Figure 12(b)

Figure 12 Five-minute averaged (a) CO emission rates and (b) O2 consumption rates at different 

O2 levels for Generator SO1 with noncat muffler 

0

100

200

300

400

500

600

700

800

900

1000

16 17 18 19 20 21

CO Emission Rate (g/h)

O2 Level (%)

Idle-1-SO1nonCat(CO) 0.5kw-1-SO1noncat(CO) 1.5kW-1-GenSO1noncat(CO)

3.0kW-1-SO1noncat(CO) 4.5kw-1-SO1noncat(CO) 5.5kw-1-SO1noncat(CO)

4.5kW-2-SO1noncat(CO) 3.0kW-3-SO1noncat(CO) 1.5kW-4-SO1noncat(CO)

3.0kW-4-SO1noncat(CO) 4.5kW-4-SO1noncat(CO) 5.5kW-4-SO1noncat(CO)

idle-2-GenSO1noncat(CO) 5.5kW-5-SO1noncat(CO) 0.5kW-2-GenSO1noncat(CO)

1.5kW-2-GenSO1noncat(CO) 4.5kW-5-SO1noncat(CO) 3.0kW-2-GenSO1noncat(CO)

0.5kW-5-SO1noncat(CO) idle-4-SO1noncat(CO) 0.5kW-4-SO1noncat(CO)

idle-5-SO1noncat(CO) idle-3-SO1noncat(CO) 0.5kW-3-SO1noncat(CO)

-10000

-9000

-8000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

16 17 18 19 20 21

O2 Consumption Rate (g/h)

O2 Level (%)

Idle-1-GenSO1noncat(O2) 0.5kw-1-GenSO1noncat(O2) 1.5kW-1-GenSO1noncat(O2)

3.0kW-1-SO1noncat(O2) 4.5kw-1-SO1noncat(O2) 5.5kW-1-GenSO1noncat(O2)

4.5kW-2(SO1noncat(O2) 1.5kW-4-SO1noncat(O2) 3.0kW-4-SO1noncat(O2)

4.5kW-4-SO1noncat(O2) 5.5kW-4-SO1noncat(O2) idle-2-GenSO1noncat(O2)

0.5 kW-2-GenSO1noncat(O2) 1.5 kW-2-GenSO1noncat(O2) 3.0 kW-2-GenSO1noncat(O2)

idle-3-SO1noncat(O2) 0.5kW-3-SO1noncat(O2) 3.0kW-3-SO1noncat(O2)

5.5kW-5-SO1noncat(O2) 4.5kW-5-SO1noncat(O2)
19

Figures 12(a) and 12(b) present the CO emission rates and O2 consumption rates as a function of 

O2 levels for Generator SO1 with the noncat muffler (referred to as Gen SO1 noncat). 

Comparing Figure 12(a) to Figure 11(a) shows that Gen SO1 noncat had higher CO emission 

rates than Gen SO1 cat. However, the measured CO emission rates for Gen SO1 noncat were still 

substantially lower than the emission rates of the unmodified generators, and no trend toward 

higher emission rates was seen as O2 levels dropped close to 17 %. 

Discussion of Shed Tests

A series of experiments were conducted to measure CO emission and O2 consumption rates of 

portable, spark-ignited, gasoline-powered electric generators (in both unmodified carbureted 

configuration and prototype low CO emission configuration) operated in a single-zone shed. 

These tests were used to derive the CO emission and O2 consumption rates to be used as inputs 

to a model validation effort, as well as for the simulation analyses conducted to examine the 

potential performance of the low CO-emission prototype under a wider range of operating 

conditions. For two different unmodified generators (i.e., without CO emission controls), it was 

found that CO emission ranged from a low of around 500 g/h at near ambient O2 levels, to a high 

of nearly 4000 g/h as O2 approached 17 %. The rates of CO generation and O2 consumption were 

affected by multiple parameters, with the O2 level and the actual electrical output of the 

generator being two of the most important. Tests performed below 17 % O2 showed a drop off in 

CO emissions due to poor engine performance under these conditions. Tests of two modified

prototype low CO emission generators (i.e., with CO emission controls) showed reductions of 

CO emissions of over 90 % depending on the specific emission controls and operating 

conditions. 
20

Garage Test Results 

NIST also conducted a series of tests to provide empirical data to characterize the emission and 

transport of CO due to operating portable gasoline-powered generators in an attached garage. 

This section of this report presents data from these tests of both unmodified and UA-modified 

prototype generators operated in the garage attached to NIST’s manufactured test house. Note 

that these results apply to the specific units tested and that other units, modifications, houses and 

test conditions may produce different results. 

Testing Configurations

Testing was conducted under seven different test house configurations to evaluate their impacts 

on the buildup of CO in the garage and its transport into the different rooms in the house. These 

configurations included two different garage bay door positions (fully closed or open nominally 

0.6 m), two connecting door settings between the garage and the family room (fully closed or 

open nominally 5 cm), and two house central heating, ventilating, and air conditioning (HVAC) 

fan settings (on or off). All internal house doors were kept open throughout all tests. 

There were multiple purposes in conducting tests under these different configurations. The 

garage-house door positions directly affect the rate of engine exhaust transfer from the garage 

into the house. The status of the HVAC fan, which circulates the interior air throughout the 

rooms of the house, affects the CO distribution within the house. The fan operation also affects 

the house air change rate due to air distribution duct leakage within the crawl space (Nabinger 

and Persily 2008). It is also relevant to consider the fan status, even when there is a power 

outage, because the consumer may use the generator to provide power to the home’s central 

heating system, which includes providing power to the HVAC fan. Another reason for testing 

under different garage door and garage-house door positions is that, with the generator operating 

in the garage, it is possible that the engine will consume the oxygen in the garage at a faster rate 

than the rate at which natural air change replenishes oxygen. The degree of either door’s opening 

will impact whether or not the garage’s oxygen level can be maintained at ambient level and, if 

not, how low it will drop. Testing with different door positions enabled observations of the 

effects of different oxygen levels on generator engine performance. Variations in house 

configuration can be found in CPSC’s investigation reports of fatal CO poisonings involving 

generators (Hnatov 2010). These reports include cases in which consumers were aware of the 

CO poisoning hazard but attempted to provide what they considered “proper ventilation” by 

operating the generator in a partially-open garage. A bay door opening of 0.6 m was selected in 

part based on it being within the range of openings that can be modeled using the multizone 

airflow and IAQ model CONTAM. The house door opening of 5 cm was selected because it is a 

reasonable opening to allow the passage of an extension cord from the generator into the house. 

Table 3 includes a summary of the tests conducted including information on the generator tested, 

the test house configuration (defined by door positions and fan status), a test identification code, 

the date the test was conducted, the average ambient temperature and wind speed, and the CO 

analyzers used. As described in the Instrumentation Section, several different analyzers were 

used during the tests to span the full range of CO concentrations, but data is presented only from 

the analyzer considered most appropriate for the CO concentration range in each test. All tests 

listed in Table 3 were “cold start” tests. Additional garage tests are reported in Appendix C. 
21

Table 3 Tests Conducted in Attached Garage

Generator House 

Configuration 

Garge 

bay 

door 

Garage 

to house 

door 

HVAC 

fan 

Test 

ID

Date Outdoor 

Temp

(°C)

Wind 

speed 

(m/s)

CO 

analyzers 

in garage 

CO 

analyzers 

in house

unmod GenX 1 Closed Open OFF B 04/22/08 20.1 6.5 N1 N2, N3

modGenX 1 Closed Open OFF O 04/02/10 22.0 6.5 N2, N3 N1, R1

SO1 1 Closed Open OFF N 04/01/10 19.9 6.3 N2, N3 N1, R1

unmod GenX 2 Open Closed OFF F 05/06/08 22.8 7.7 N1 N2, N3

modGenX 2 Open Closed OFF R 04/12/10 19.9 6.7 N2, N3 N1, R1

SO1 2 Open Closed OFF T 04/14/10 13.4 6.9 N2, N3 N1, R1

unmod GenX 3 Closed Open ON I 05/15/08 22.8 7.4 N1 N2, N3

SO1 with 

noncat muffler

3 Closed Open ON Z 05/05/10 28.3 6.7 N2, N3 N1, R1

unmod GenX 4 Closed Closed ON J 05/21/08 18.2 9.6 N1 N2, N3

SO1 4 Closed Closed ON W 04/29/10 17.8 9.5 N2, N3 N1, R1

unmod GenX 5 Closed Closed OFF D 04/30/08 12.2 8.2 N1 N2, N3

SO1 with 

noncat muffler

5 Closed Closed OFF AH 05/13/10 15.6 6.5 N2, N3 N1, R1

unmod GenX 6 Open Open ON G 05/07/08 25.1 7.0 N1 N2, N3

SO1 6 Open Open ON U 04/22/10 20.4 7.8 N2, N3 N1, R1

unmod GenX 7 Open Open OFF K 05/23/08 13.84 7.0 N1, T1 N2, N3

SO1 with 

noncat muffler

7 Open Open OFF V 04/23/10 15.8 6.5 N2, N3 N1, R1
22

Results

Figures 13 through 28 show the measurement results for all 16 tests listed in Table 3, including 

CO concentration in the house and garage, O2 concentration in the garage, and the measured 

electric load supplied by the generator. In all tests, the generator was started at time 0 and was 

manually shut off by the test operator using a wireless switch that interrupted the engine’s 

ignition. The data in the figures are plotted up until the time mechanical venting was initiated, 

which typically immediately followed generator shut-off. In some tests, where time and 

circumstances permitted, natural decay was allowed to occur after the generator was stopped, 

before mechanical venting was initiated. In those tests, the natural decay is plotted. 

Figures 13a, 13b, and 13c show the results for Test B, which was a three hour test of unmod Gen 

X in Configuration 1 (garage bay door closed, garage access door to house open nominally 5 cm, 

and the house central HVAC fan off). Since it was a three hour test, the hourly cyclic load profile 

in Table 1 was repeated three times. At the end of the third cycle, the generator was stopped, and 

the garage was mechanically vented. 

Figure 13a CO and O2 concentrations in the garage and measured load for Test B (unmod Gen 

X, Configuration 1)

Figure 13a shows the concentration of CO in the garage reached a peak of over 19,500 μL/L and 

the volume fraction of O2 in the garage dropped by 3.8 % to nearly 17 % when the generator was 

stopped. It also shows that in the first load cycle, the delivered electrical output was less than the 

load bank settings for the two highest loads in the load cycle, 4500 W and 5500 W, which were 

applied when the oxygen was already below 19 %. As the oxygen continued to drop in the 

subsequent load cycles, the delivered power for these load points decreased further.

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 30 60 90 120 150 180 210

CO (

% Oxygen

μL/L) Load (W)

Time (min)

N1 HI-CO N1 LO-CO Delivered Load O2
23

Figure 13b CO (ppm range) concentrations in the house for Test B 

(unmod Gen X, Configuration 1)

Figure 13c CO (high range) concentrations in the house for Test B 

(unmod Gen X, Configuration 1) 

Figures 13b and 13c show the CO concentration in six rooms of the test house (see Figure 2 for 

room locations) as measured with the N3 ‘ppm range’ (where the CO concentration plot plateaus 

at the instrument’s 2000 μL/L limit) and N2 ‘high range’ CO instruments, respectively. The CO 

0

1000

2000

3000

4000

5000

6000

7000

8000

0 30 60 90 120 150 180 210

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM

0

1000

2000

3000

4000

5000

6000

7000

8000

0 30 60 90 120 150 180 210

CO (

μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
24

reached a peak concentration of over 6500 μL/L in the family room, with peak concentrations in 

the other rooms ranging from about 3500 μL/L to 6000 μL/L. As described in the 

Instrumentation section, samples were taken for one minute at each location on a rotating six 

minute cycle.

Figures 14a, 14b, and 14c show the results for Test O, which was a four and a half hour test of 

mod Gen X with the same test house configuration (Configuraiton 1) as used in Test B of unmod 

Gen X (shown in Figure 13). After the generator was stopped, the garage and house were 

mechanically ventilated.

Figure 14a CO and O2 concentrations in the garage and measured load for Test O (mod Gen X, 

Configuration 1) 

As shown in Figure 14a, the garage CO concentration reached a peak of nearly 3000 μL/L while 

the garage O2 concentration dropped by 1.7 % to 19.5 % after completing the fourth cycle of the 

load profile. Note that the ppm instrument (N3) briefly topped out at this time, about 230 min 

into the test. Also, the initial O2 concentration is shown as slightly above 20.9 % for this and 

some other tests due to the limitations in instrument accuracy. The generator was intentionally 

stopped midway through the fifth load cycle. 

At three hours into this test, the garage CO concentration was approximately 1400 μL/L. Under 

fairly similar ambient conditions between this test and Test B, this CO concentration is a 93 % 

reduction compared to that measured with unmod Gen X in Test B. In that case, the garage CO 

was over 19,500 μL/L at the same time during the test. 

In the first load cycle, as the oxygen dropped, the delivered electrical output was less than the 

load bank settings for the three highest loads in the load cycle, 3000 W, 4500 W, and 5500 W. 

While the electrical output stayed near constant for the four cycles, the CO levels increased 

progressively and the oxygen decreased slightly with each additional cycle. 

17.0

18.0

19.0

20.0

21.0

22.0

0

1000

2000

3000

4000

5000

0 30 60 90 120 150 180 210 240 270

CO(

% Oxygen

μL/L), Load (W)

Time (min)

N2 HI-CO N3 CO Delivered Load O2
25

Figure 14b CO concentrations in the house for Test O (mod Gen X, Configuration 1)

As seen in Figure 14b, the peak CO concentration throughout the house was about 800 μL/L, 

with a relatively uniform distribution in all the rooms despite the HVAC fan being off. By 

comparison, unmod Gen X in Test B produced a peak concentration of over 6500 μL/L in the 

family room. 

Figure 14c Temperatures and AFR measured in Test O (mod Gen X, Configuration 1)

0

100

200

300

400

500

600

700

800

900

1000

0 30 60 90 120 150 180 210 240 270

CO (

μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM

10

11

12

13

14

15

16

17

18

0

100

200

300

400

500

600

700

800

0 30 60 90 120 150 180 210 240 270

AFR

Temperature ( C)

Time (min)

Engine-out exhaust temp Cyl head temp Oil sump temp

Muffler surface temp Air-fuel ratio
26

The AFR (provided as a general indicator of the prototype engine’s performance for this and 

other tests) and temperatures measured on modGen X during Test O are shown in Figure 14c. 

During this test, the engine performed off design with AFR largely ranging from around 14 to 

around 15.4 during each load cycle and reducing to rich operation when transitioning between 

the load cycles as well as during the high loads. 

Figures 15a, 15b, and 15c show the results for Test N, which was a two hour test of Gen SO1

with the same test house configuration used in Test B of unmod Gen X and Test O of mod GenX 

(Configuration 1). This test was terminated earlier than planned after a fuse blew on the load 

bank after 114 min of operation, dropping half the load. The generator was turned off 138 min 

after it was started. A natural decay period of 45 min was included after the generator was 

stopped, followed by mechanical venting. 

Figure 15a CO and O2 concentrations in the garage and measured load for Test N (Gen SO1, 

Configuration 1) 

As shown in Figure 15a, there was an initial increase of CO to almost 220 μL/L in the first 

12 min after the generator was started. This rise is due to the rich operation upon cold engine 

start until the oil warms and the ECU transitions to the calibrated AFR fuel control. This initial 

increase is observed at the start of each of the tests with Gen SO1. The garage CO concentration 

reached a peak of around 300 μL/L, and the garage O2 concentration dropped by 1.6 % to 19.4 % 

before the generator was stopped. The garage CO concentration after two hours is about 98 % 

lower than the concentration at two hours with unmod Gen X in Test B, which was about 

13,000 μL/L. In the first load cycle, as the oxygen dropped, the delivered electrical output was 

less than the load bank setting for the highest load in the load cycle, 5500 W. This difference 

increased in the subsequent load cycle as the oxygen level decreased. Comparing the 

performance of mod Gen X (Figure 14a) and Gen SO1 (Figure 15a) shows that, under similar 

17.0

17.5

18.0

18.5

19.0

19.5

20.0

20.5

21.0

21.5

22.0

0

1000

2000

3000

4000

5000

0 30 60 90 120 150 180

% Oxygen

CO (

μL/L), Load (W)

Time (min)

N3 CO Delivered Load N2 O2
27

conditions (Configuration 1), Gen SO1 resulted in significantly lower CO concentrations at the 

2 h mark. 

Figure 15b CO concentrations in the house for Test N (Gen SO1, Configuration 1)

As shown in Figure 15b, the concentration throughout the house was about 130 μL/L when the 

generator was stopped after 114 min. There is a relatively even CO distribution among the rooms 

in spite of the HVAC fan being off. For the following 45 min in which the exhaust was allowed 

to naturally decay, CO continued to infiltrate from the garage into to the house, slightly 

increasing the house concentration to about 140 μL/L before the concentration began dropping. 

By comparison, unmod Gen X in Test B produced a peak concentration of over 3500 μL/L in the 

family room after 2 h.

0

50

100

150

200

0 30 60 90 120 150 180

CO (

μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
28

Figure 15c Temperatures and AFR measured on Gen SO1 in Test N (Gen SO1, Configuration 1)

The AFR and temperatures measured on Gen SO1 during Test N are shown in Figure 15c. With 

the exception of two periods of AFR excursion after the engine warmed up (i.e., after 

approximately 10 min), the engine operated at the calibrated AFR of 14.5 as the oxygen level 

dropped. The spike in AFR at the end of the test occurred when the engine was turned off. 

Figures 16a and 16b show the results for Test F, which was a four hour test of unmod Gen X 

with Configuration 2 (garage bay door open, garage access door to house closed, and house 

central HVAC fan off). After the generator was stopped, the garage concentration was allowed to 

naturally decay for one hour before the garage and house were mechanically vented.

10

11

12

13

14

15

16

17

18

0

100

200

300

400

500

600

700

800

0 30 60 90 120 150 180

AFR

Temperature (oC)

Time (min)

Engine-out exhaust temp Muffler surface temp Cyl head temp

Shroud surface temp Oil sump temp Air-fuel ratio
29

Figure 16a CO and O2 concentrations in the garage and measured load for Test F (unmod Gen 

X, Configuration 2) 

The garage CO concentration peaked during each load cycle during the 1500 W load bank 

setting. The peak concentration rose slightly in each load cycle, reaching a maximum 

concentration somewhat under 1500 μL/L in the fourth load cycle. For this test, the garage was 

not instrumented with a low concentration CO analyzer, and the instrument uncertainty is large 

relative to measured concentrations below 500 μL/L. 

During this test, with the garage bay door open, the garage oxygen level decreased only slightly, 

down by 0.5 % to 20.5 %, and the delivered electrical output was consistent during each cycle, 

largely meeting the load bank setting with the exception of the 5500 W setting. 

16.0

17.0

18.0

19.0

20.0

21.0

22.0

0

1000

2000

3000

4000

5000

6000

0 30 60 90 120 150 180 210 240 270 300

% Oxygen

CO (

μL/L), Load (W)

Time (min)

N1 LO-CO Delivered Load O2
30

Figure 16b CO concentrations in the house for Test F (unmod Gen X, Configuration 2) 

As shown in Figure 16b, the maximum house CO concentration was measured in the family 

room at just over 200 μL/L about 15 min after the generator was stopped after a 4 h runtime. 

The master bedroom had the lowest peak concentration among all the rooms, reaching just over 

150 μL/L about 30 min after the generator was stopped. 

Figures 17a, 17b, and 17c show the results for Test R, which was a four hour test of mod Gen X 

with the same test house configuration as used in Test F of unmod Gen X (Configuration 2). 

Mechanical venting was initiated right after the generator was stopped. 

0

50

100

150

200

250

0 30 60 90 120 150 180 210 240 270 300

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
31

Figure 17a CO and O2 concentrations in the garage and measured load for Test R (mod Gen X, 

Configuration 2) 

As seen in Figure 17a, the garage CO concentration was nominally steady at 30 μL/L (though the 

uncertainty of the instrument is large relative to this level), and the oxygen stayed nominally at 

ambient throughout the test. This is about a 98 % reduction in CO compared to the nearly 

1500 μL/L measured with unmod Gen X in Test F. 

The delivered electrical output was less than the load bank settings for the three highest loads in 

the load cycle, which occurred with no significant oxygen depletion. After this test, the unit was 

thoroughly inspected, including the wiring between the generator head and the 240-volt 

receptacle, but no anomalies were found. During UA’s development of this prototype, they 

observed on several occasions that these wires and associated connector melted.

17.0

17.5

18.0

18.5

19.0

19.5

20.0

20.5

21.0

21.5

22.0

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 30 60 90 120 150 180 210 240

% Oxygen

CO (

μL/L), Load (W)

Time (min)

N3 CO Delivered Load O2
32

Figure 17b CO concentrations in the house for Test R (mod Gen X, Configuration 2) 

The CO concentration throughout the house was nominally steady at 5 μL/L in all rooms 

throughout the test (though the instrument uncertainty is large relative to this concentration). By 

comparison, unmod Gen X in Test F produced a maximum CO concentration in the family room 

at just over 200 μL/L, corresponding to a reduction of around 98 % in Test R. 

Figure 17c Temperatures and AFR measured in Test R (mod Gen X, Configuration 2)

0

10

20

30

40

50

60

70

80

90

100

0 30 60 90 120 150 180 210 240

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM

10

11

12

13

14

15

16

17

18

0

100

200

300

400

500

600

700

800

0 30 60 90 120 150 180 210 240

AFR

Temperature (oC)

Time (min)

Engine-out exhaust temp Cyl head temp Oil sump temp

Muffler surface temp Air-fuel ratio
33

The AFR and temperatures measured on modGen X during Test R are shown in Figure 17c. 

During each load cycle, the engine primarily ran lean, with the AFR ranging from about 14.5 to 

15.6. The spike in AFR at the end of the test corresponds to when the engine was turned off. 

Figures 18a, 18b, and 18c show the results for Test T, which was a three hour test of Gen SO1

with the same test house configuration as used in Test F and Test R of unmod Gen X and 

modGenX respectively (Configuration 2). The generator was stopped when a circuit breaker on 

the 240-volt receptacle tripped. Mechanical venting was initiated right after the generator was 

stopped. 

Figure 18a CO and O2 concentrations in the garage and measured load for Test T (Gen SO1, 

Configuration 2) 

As shown in Figure 18a, there was an initial spike of CO in the garage of over 300 μL/L when 

the engine was started and as the oil warmed before operation transitioned to the calibrated AFR. 

The CO concentration then dropped and maintained a level of about 20 μL/L throughout the test 

(though the uncertainty of the instrument is large relative to this level). With the garage bay door 

open, the garage oxygen level stayed nominally at ambient. With the exception of the early peak, 

this CO concentration is over a 98 % reduction compared to the peak garage CO measured with 

unmod Gen X in Test F. Throughout the test, the delivered electrical output was consistent 

during each cycle, largely meeting the load bank setting with the exception of the 5500 W 

setting. Comparing the performance of mod Gen X (Figure 17a) and Gen SO1 (Figure 18a) 

shows that, for Configuration 2, both generators resulted in similar low CO concentrations after 

the initial spike in Test T.

16.0

16.5

17.0

17.5

18.0

18.5

19.0

19.5

20.0

20.5

21.0

21.5

22.0

0

1000

2000

3000

4000

5000

6000

0 30 60 90 120 150 180

% Oxygen

CO (μL/L), Load (W)

Time (min)

N3 CO Delivered Load N2 O2
34

Figure 18b CO concentrations in the house for Test T (Gen SO1, Configuration 2) 

As shown in Figure 18b, an initial spike of CO exceeding 50 μL/L was measured in the family 

room about 25 min after the generator was started, but 5 min after that it dropped below 10 μL/L 

and continued to drop for the remainder of the test. By comparison, unmod Gen X in Test F 

produced a maximum CO concentration in the family room at just over 200 μL/L. 

Figure 18c Temperatures and AFR measured in Test T (Gen SO1, Configuration 2)

0

10

20

30

40

50

60

70

80

90

100

0 30 60 90 120 150 180

CO (

μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM

10

11

12

13

14

15

16

17

18

0

100

200

300

400

500

600

700

800

0 30 60 90 120 150 180

AFR

Temperature (oC)

Time (min)

Engine-out exhaust temp Cyl head temp Oil sump temp Air-fuel ratio
35

The AFR and temperatures measured on Gen SO1 during Test T are shown in Figure 18c. The 

engine operated at the calibrated AFR after the engine oil temperature warmed to about 70 °C. 

After this series of tests was conducted, due to limitations in the test program resources that 

would not support continued testing of both prototypes, a decision was made to continue the 

testing only the newer prototype Gen SO1 for drawing comparisons between performance of the 

prototype and stock generator.

Figures 19a, 19b, and 19c show the results for Test I, which was a four hour test of unmod Gen 

X in Configuration 3 (garage bay door closed, garage access door to house open 5 cm, and the 

house central HVAC fan on). These conditions are similar to the three hour Test B with unmod 

Gen X under Configuration 1 except for the HVAC fan status. Since the operation of HVAC fan 

primarily affects the airflow between rooms in the house and is not expected to significantly 

impact the airflow between the house and garage, this allows a comparison between the garage 

CO and oxygen levels in Tests I and B. After the generator was stopped, the building naturally 

decayed for one hour before the garage and house were mechanically vented. 

Figure 19a CO and O2 concentrations in the garage and measured load for Test I (unmod Gen X, 

Configuration 3)

Figure 19a shows that the concentration of CO in the garage reached a peak of about 

18,600 μL/L and the concentration of O2 in the garage had dropped by 3.7 % to 17.5 % when the 

generator was stopped. It also shows that in the first load cycle the delivered electrical output 

was less than the load bank settings for the two highest loads in the load cycle, 4500 W and 

5500 W, which were applied as the oxygen was approaching 19 %. As the oxygen continued to 

drop in the subsequent load cycles, the delivered power for these load points decreased further. 

These results are fairly similar to those in Test B. 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 30 60 90 120 150 180 210 240 270 300

% Oxygen

CO (μL/L), Load (W)

Time (min)

N1 HI-CO N1 LO-CO Delivered Load N1 O2
36

Figure 19b CO (ppm range) concentrations in the house for Test I 

(unmod Gen X, Configuration 3)

Figure 19c CO (high range) concentrations in the house for Test I 

(unmod Gen X, Configuration 3)

0

2000

4000

6000

8000

10000

12000

0 30 60 90 120 150 180 210 240 270 300

CO (

μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM

0

2000

4000

6000

8000

10000

12000

0 30 60 90 120 150 180 210 240 270 300

CO (

μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
37

Figures 19b and 19c show the CO concentration in the rooms of the test house, as measured in

the ‘ppm range’ (where the CO concentration plot plateaus at the instrument’s 2000 μL/L limit)

and with the ‘high range’ CO instruments, respectively. The CO reached a peak concentration of 

around 10,500 μL/L in the family room, with peak concentrations in the other rooms ranging 

from about 8,000 μL/L to 10,000 μL/L. With the HVAC fan on in this test, there is a relatively 

more uniform distribution of CO compared to Test B in which the HVAC fan was off. 

Figures 20a, 20b, and 20c show the results for Test Z, which was a 4.75 h test of Gen SO1 with 

the noncat muffler (Configuration 3). The test ended when the generator ran out of fuel. (Note: 

this run time does not indicate a limit on potential run-time as the tank was not full at the 

beginning of the test.) The test house configuration conditions are the same as that in the 4 h Test 

I with unmod Gen X. They are also the same as that used in the 2 h Test N with Gen SO1 except 

that the HVAC fan was off in Test N and Gen SO1 had the catalyst-installed muffler (referred to 

as cat muffler) in Test N. Since the operation of the HVAC fan is not expected to significantly 

impact the airflow between the house and garage, the effect of the catalytic and non-catalytic 

muffler on the resulting garage CO and oxygen levels between Tests Z and N (Configuration 3 

and 1, respectively) up to the 2 h point can be seen. After the generator was stopped, the garage 

and house were mechanically vented.

Figure 20a CO and O2 concentrations in the garage and measured load for Test Z (Gen SO1 

noncat, Configuration 3)

As shown in Figure 20a, the CO concentration in the garage initially rose to about 470 μL/L 

upon start, then lowered after the engine warmed up. It further increased and decreased cyclically 

with each successive load cycle. By the end of the fourth load cycle, it had reached a nominal 

peak of 630 μL/L and the oxygen dropped 1.6 % to 19.5%. This peak CO concentration is a 

97 % reduction compared to that measured with unmod Gen X in Test I in which the garage CO 

reached about 18,600 μL/L at the end of the fourth load cycle. 

17.0

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18.5

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19.5

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0

1000

2000

3000

4000

5000

0 30 60 90 120 150 180 210 240 270

% Oxygen

CO (μL/L), Load (W)

Time (min)

N3 CO Delivered Load O2
38

 

Figure 20a also shows that the delivered electrical output was progressively less than the load 

bank settings for the two highest loads in the load cycle as the oxygen dropped throughout the 

test. 

Figure 20b CO concentrations in the house for Test Z (Gen SO1 noncat, Configuration 3)

As shown in Figure 20b, the CO concentration reached a peak of nominally 360 μL/L at 4 h in 

the family room. There is a relatively even distribution, with all the rooms reaching at least 

300 μL/L, as would be expected with the HVAC fan on. By comparison, unmod Gen X in Test I 

produced a peak CO concentration of around 10,600 μL/L in the family room, with peak 

concentrations in the other rooms ranging from about 8,000 μL/L to 10,000 μL/L. 

0

100

200

300

400

500

0 30 60 90 120 150 180 210 240 270

CO (

μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
39

Figure 20c Temperatures and AFR measured on Gen SO1 in Test Z (Gen SO1 noncat, 

Configuration 3)

The AFR and temperatures measured on Gen SO1 during Test Z are shown in Figure 20c. With 

the exception of a few short periods of AFR excursion after the engine warmed up, the engine 

operated at the calibrated AFR of about 14.5. The spike in AFR at the end of the test corresponds 

to when the engine was turned off. 

Engine operation was, by and large, comparable between Test Z and Test N, which were with 

Gen SO1 and the cat muffler respectively. Therefore, the garage CO concentrations at the same 

time in each test can be compared to get an indication of the catalyst’s performance in further 

lowering the CO emissions. At 2 h into Test Z, the garage CO concentration was 500 μL/L and 

the oxygen was 19.7 %. By comparison, at the end of the 2 h Test N, the garage CO 

concentration reached a peak of around 300 μL/L and the garage O2 concentration dropped to 

19.4 %. Therefore, the CO concentrations were approximately 40 % lower for Test Z with the 

catalyst than for Test N with the EMS alone. An unknown portion of the difference may be due 

to differences in ambient or other test conditions. 

Figures 21a and 21b show the results for the two and a quarter hour test of unmod Gen X, Test J,

in Configuration 4 (garage bay door closed, garage access door to house closed, and the house 

central HVAC fan on). After the generator was stopped, the garage was mechanically vented. 

For this test, the load cycle was applied in reverse order to that shown in Table 1. 

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11

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18

0

100

200

300

400

500

600

700

0 30 60 90 120 150 180 210 240 270

AFR

Temperature (oC)

Time (min)

Engine-out exhaust temp Cyl head temp Oil sump temp

Muffler surface temp Air-fuel ratio
40

Figure 21a CO (high range) and O2 concentrations in the garage and measured load for Test J

(unmod Gen X, Configuration 4)

As shown in Figure 21a, at the time the generator was stopped, the garage CO concentration 

reached a peak of over 21,300 μL/L and the oxygen dropped by 4.7 % to about 16 %. It also 

shows that in the first load cycle, the delivered electrical output matched the load bank settings 

with the exception of the 5500 W setting. However, during the third load cycle, as the oxygen 

level dropped significantly, the generator’s ability to meet the load was severely compromised 

and the test was ended due to poor generator operation. 

Figure 21b CO concentrations in the house for Test J (unmod Gen X, Configuration 4)

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25000

0 30 60 90 120

% Oxygen

CO (μL/L), Load (W)

Time (min)

N1 HI-CO N1 LO-CO Delivered Load O2

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600

800

1000

1200

1400

1600

1800

2000

0 30 60 90 120 150 180

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
41

As shown in Figure 21b, the CO reached a peak concentration of about 1,800 μL/L in the family 

room, with peak concentrations in the other rooms ranging from about 1,200 μL/L to 

1,650 μL/L. 

Figures 22a, 22b, and 22c show the results for a six hour test of Gen SO1, Test W, with the same 

test house configuration as used in Test J of unmod Gen X (Configuration 4). The load cycle 

was applied with the same profile as that in Table 1, with the load going from low to high. After 

the generator was stopped, the garage was mechanically vented. 

Figure 22a CO and O2 concentrations in the garage and measured load for Test W (Gen SO1, 

Configuration 4) 

As shown in Figure 22a, the CO concentration in the garage initially rose to 680 μL/L, and then 

decreased after the engine warmed up. In the fourth load cycle, it reached a peak of about 

960 μL/L and the oxygen lowered by 2.8 % to 18.2 %. 

At two and one quarter hours into this test, the garage CO concentration was nominally 

640 μL/L. Although the tests are not entirely comparable due to the opposite loading pattern, this 

CO concentration is a 97 % reduction compared to that measured with unmod Gen X in Test J in 

which the garage CO was over 21,300 μL/L at the same time during the test. 

In the first load cycle, the delivered electrical output exceeded the load bank settings except for 

the two highest loads. In the subsequent load cycles, as the oxygen level dropped, the delivered 

load was less than the load bank settings for the three highest loads in the cycle.

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1000

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2000

2500

3000

3500

4000

4500

5000

0 30 60 90 120 150 180 210 240 270 300 330 360

% Oxygen

CO (μL/L), Load (W)

Time (min)

N3 CO Delivered Load O2
42

Figure 22b CO concentrations in the house for Test W (Gen SO1, Configuration 4) 

As shown in Figure 22b, the CO reached a peak concentration of about 145 μL/L in the family 

room, with peak concentrations in the other rooms relatively evenly distributed just below that, 

down to 100 μL/L. By comparison, unmod Gen X in Test J produced a peak CO concentration of 

over 1,800 μL/L in the family room after 2 h of operation.

Figure 22c Temperatures and AFR measured in Test W (Gen SO1, Configuration 4)

0

100

200

300

400

0 30 60 90 120 150 180 210 240 270 300 330 360

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM

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200

300

400

500

600

700

800

0 30 60 90 120 150 180 210 240 270 300 330 360

AFR

Temperature (oC)

Time (min)

Engine-out exhaust temp Cyl head temp Oil sump temp Air-fuel ratio
43

The AFR and temperatures measured on Gen SO1 during Test W are shown in Figure 22c. After 

the engine warmed up, the engine operated at the calibrated AFR for the next 30 min. There were 

then occasional periods of lean as well as rich operation, with most of them occurring during the 

transition between the load cycles when the load bank was switched from 5500 W to no load. 

The spike in AFR at the end of the test corresponds to when the engine was turned off. 

Figures 23a and 23b show the results for the two hour test of unmod Gen X, Test D, in 

Configuration 5 (garage bay door closed, garage access door to house closed, and the house 

central HVAC fan off). These conditions are the same as the two and a quarter hour Test J with 

unmod Gen X except in that test the HVAC fan was on. Since the operation of the HVAC fan is 

not expected to significantly impact the airflow between the house and garage, especially with 

the house door closed, this allows some degree of comparison to be made for the resulting garage 

CO and oxygen levels between Tests D and J. After the generator was stopped, the garage was 

mechanically vented. 

Figure 23a CO (high range) and O2 concentrations in the garage and measured load for Test D

(unmod Gen X, Configuration 5)

Figure 23a shows the concentration of CO in the garage reached a peak of almost 23,000 μL/L, 

and the concentration of O2 in the garage dropped by 5.0 % to below 16 % when the generator 

was stopped. It also shows that in the first load cycle the delivered electrical output was less than 

the load bank settings for the two highest loads in the load cycle, 4500 W and 5500 W, which 

were applied as the oxygen was approaching 18 %. As the oxygen continued to drop in the 

subsequent load cycle, the delivered power for these load points decreased further. The results 

are similar to those in Test J despite the reversal of the load cycled pattern in Test J. 

12

13

14

15

16

17

18

19

20

21

22

0

5000

10000

15000

20000

25000

0 30 60 90 120

% Oxygen

CO (

μL/L), Load (W)

Time (min)

N1 HI-CO N1 LO-CO Delivered Load O2
44

Figure 23b CO concentrations in the house for Test D (unmod Gen X, Configuration 5) 

Figure 23b shows the CO reached a peak concentration of almost 1660 μL/L in the family room 

with peak concentrations in the other rooms ranging from about 600 μL/L to 1400 μL/L. This is 

a comparable peak CO concentration to the 1670 μL/L measured in the family room at the 2 h 

point in Test J. When comparing the other room time course profiles with those at the 2 h point 

in Test J, it can be observed that the mixing due to the operation of the HVAC fan made the most 

difference in the master bedroom. This effect is not consistent during all tests as other factors 

affecting mixing (such as temperature) differ from test to test.

Figures 24a, 24b, and 24c show the results for Test AH, which was a five hour test of Gen SO1 

with the noncat muffler and the same conditions of the test house as used in the 2 h Test D with 

unmod Gen X (Configuration 5). These conditions are also the same as that used in the 6 h Test 

W with Gen SO1 except that in Test W Gen SO1 had the cat muffler and the HVAC fan was on. 

Since the operation of the HVAC fan is not expected to significantly impact the airflow between 

the house and garage, this allows some degree of comparison to be made for the resulting garage 

CO and oxygen levels between Tests AH and W. After the generator was stopped, the exhaust 

decayed naturally for 45 min and then the garage and house were mechanically vented.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 30 60 90 120 150 180

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
45

Figure 24a CO and O2 concentrations in the garage and measured load for Test AH (Gen SO1 

noncat, Configuration 5)

As shown in Figure 24a, the CO concentration in the garage initially rose to nominally 670 μL/L 

upon start, then continued to climb until it reached a nominal peak of 2300 μL/L and oxygen 

lowered 3.5 % to 17.8 % in the garage during the second load cycle. This CO concentration is a 

90 % reduction compared to that measured with unmod Gen X in Test D in which the CO in the 

garage at the end of the second load cycle was almost 23,000 μL/L.

Figure 24a also shows that in the first load cycle the delivered electrical output was less than the 

load bank settings for the two highest loads in the load cycle. During the subsequent load cycles 

the delivered power degraded even further as the garage oxygen approached and then dropped 

below 18 %. 

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1000

2000

3000

4000

5000

0 30 60 90 120 150 180 210 240 270 300 330

% Oxygen

CO (μL/L), Load (W)

Time (min)

N2 HI-CO N3 CO Delivered Load O2
46

Figure 24b CO concentrations in the house for Test AH (Gen SO1 noncat, Configuration 5) 

As shown in Figure 24b, the CO reached a peak concentration of about 470 μL/L throughout the 

house, with even distribution among the rooms even though the HVAC fan was off. At 2 h into 

this test, the CO in the house was about 180 μL/L. By comparison, the 2 h operation of unmod 

Gen X in Test D produced a peak CO concentration of almost 1660 μL/L in the family room, 

with peak concentrations in the other rooms ranging from about 600 μL/L to 1400 μL/L. 

Figure 24c Temperatures and AFR measured in Test AH (Gen SO1 noncat, Configuration 5)

0

100

200

300

400

500

600

700

800

900

1000

0 30 60 90 120 150 180 210 240 270 300 330

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM

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18

0

100

200

300

400

500

600

700

800

0 30 60 90 120 150 180 210 240 270 300 330

AFR

Temperature (oC)

Time (min)

Engine-out exhaust temp Cyl head temp Oil sump temp

Muffler surface temp Air-fuel ratio
47

The AFR and temperatures measured on noncat Gen SO1 during Test AH are shown in Figure 

24c. After the engine warmed up, it operated at the calibrated AFR for the next 30 min, but then 

had periods of off-design operation throughout the remainder of the test. The spike in AFR at the 

end of the test corresponds to when the engine was turned off. 

Since engine performance during the first 40 min in Test AH was similar to that in Test W with 

Gen SO1 and the cat muffler, a comparison of each test’s garage CO concentration at that point 

in time suggests the prototype’s catalyst is providing about a 50 % reduction in the CO emissions 

compared with that provided by the EMS alone. At 40 min, the garage CO concentrations were 

about 410 μL/L and 820 μL/L in Tests W and AH, respectively. This reduction is somewhat 

larger than the 40 % reduction observed when comparing the garage CO concentrations in Tests 

Z and N. 

Figures 25a and 25b show the results for Test G, a 2 h test of unmod Gen X in Configuration 6 

(garage bay door open, garage access door to house open 5 cm, and the house central HVAC fan 

on). After the generator was stopped, the garage was mechanically vented. 

Figure 25a CO and O2 concentrations in the garage and measured load for Test G (unmod Gen 

X, Configuration 6)

As shown in Figure 25a, the CO in the garage peaked at around 1100 μL/L during the second 

load cycle (though the instrument uncertainty is large relative to the concentrations). With the 

garage bay door open, the oxygen level decreased 0.5 % to about 20.5 %. Throughout the test, 

the delivered electrical output met or exceeded the load bank settings.

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5000

6000

0 30 60 90 120

% Oxygen

CO (

μL/L), Load (W)

Time (min)

N1 HI-CO N1 LO-CO Delivered Load O2
48

Figure 25b CO (ppm range) concentrations in the house for Test G (unmod Gen X, 

Configuration 6) 

Figure 25b shows the CO reached a peak concentration of about 220 μL/L in the family room, 

with slightly lower peak concentrations in the other rooms of around 190 μL/L to 200 μL/L.

 

Figure 26a CO and O2 concentrations in the garage and measured load for Test U (Gen SO1, 

Configuration 6)

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50

100

150

200

250

300

350

400

450

500

0 30 60 90 120

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM

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4500

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5500

6000

0 30 60 90 120 150

% Oxygen

CO (

μL/L), Load (W)

Time (min)

N3 CO Delivered Load O2
49

Figures 26a, 26b, and 26c show the results for Test U, which was a 2 h test of Gen SO1 with the 

same conditions of the test house as used in the 2 h Test G with unmod Gen X (Configuration 6). 

After the generator was stopped, the exhaust decayed naturally for 30 min and then the garage 

and house were mechanically vented.

As shown in Figure 26a, after an initial spike to nominally 260 μL/L of CO in the garage shortly 

after the generator was started, it dropped and maintained a level below 30 μL/L throughout the 

test. After the initial spike, this CO concentration reflects about a 97 % reduction compared to 

that measured with unmod Gen X in Test G in which the CO in the garage ranged from around 

300 μL/L to 1100 μL/L for portions of the second load cycle. With the garage bay door open, the 

oxygen level stayed nominally at ambient. 

Throughout the test, the delivered electrical output met or exceeded the load bank settings with 

the exception of the highest load setting. 

Figure 26b CO concentrations in the house for Test U (Gen SO1, Configuration 6) 

As shown in Figure 26b, the CO concentration in the family room initially spiked to about 

90 μL/L and then dropped to an even distribution in all rooms of the house around 30 μL/L with 

a continual decline to below 20 μL/L before mechanical venting was initiated. By comparison, 

unmod Gen X in Test G produced a nominal peak CO concentration of 220 μL/L in the family 

room with a minimum peak concentration in the other rooms just below 190 μL/L. 

0

50

100

150

200

0 30 60 90 120 150

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
50

Figure 26c Temperatures and AFR measured in Test U (Gen SO1, Configuration 6)

The AFR and temperatures measured on Gen SO1 during Test U are shown in Figure 26c. The 

engine operated at the calibrated AFR after the engine oil temperature warmed to nominally 

70 °C. The spike in AFR at the end of the test corresponds to when the engine was turned off. 

Figures 27a and 27b show the results for Test K, which was a 2 h 10 min test of unmod Gen X in 

Configuration 7 (garage bay door and garage access door to house open, and house central 

HVAC fan off). For this test, the load cycle was applied in reverse order to that shown in Table 

1. The test house conditions for this test are similar to the 2 h Test G with unmod Gen X except 

in that test the HVAC fan was on. Since the operation of the HVAC fan is not expected to have 

a significant impact on the airflow between the house and garage, this allows some degree of 

comparison to be made for the resulting garage CO and oxygen levels between Tests K and G. 

After the generator was manually stopped, the garage and house were mechanically vented. 

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400

500

600

700

800

0 30 60 90 120 150

AFR

Temperature (oC)

Time (min)

Engine-out exhaust temp Cyl head temp Oil sump temp Air-fuel ratio
51

Figure 27a CO and O2 concentrations in the garage and measured load for Test K (unmod Gen 

X Configuration 7)

As shown in Figure 27a, the CO in the garage peaked at about 680 μL/L. This compares to the 

1100 μL/L reported in Test G with unmod Gen X that was measured with a high range CO 

analyzer. With the garage bay door open, the garage oxygen level decreased to about 20.4 %.

Throughout the test, the delivered electrical output exceeded the load bank settings with the 

exception of the highest load setting. 

Figure 27b CO concentrations in the house for Test K (unmod Gen X Configuration 7)

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0 30 60 90 120

% Oxygen

CO (μL/L), Load (W)

Time (min)

T1 CO Delivered Load O2

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450

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0 30 60 90 120

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
52

Figure 27b shows the CO reached a peak concentration of 320 μL/L in the family room with 

peak concentrations in the other rooms just below that value, nominally 260 μL/L when 

mechanical venting was initiated.

Figures 28a, 28b, and 28c show the results for Test V, which was a 2 h 15 min test of Gen SO1 

with the noncat muffler and the same test house configuration as used in the 2 h Test K with 

unmod Gen X (Configuration 7). To match the reverse order load profile used Test K, the load 

cycle for this test was also applied in reverse order to that shown in Table 1. The test house 

conditions for this test are also the same as that used in the 2 h Test U with Gen SO1 except that 

in Test U Gen SO1 had the cat muffler and the house central HVAC fan was on. After the 

generator was stopped, the garage and house were mechanically vented. Due to a software error, 

about 15 min of data were not recorded approximately 1 h into the test. 

Figure 28a CO and O2 concentrations in the garage and measured load for Test V (noncat Gen 

SO1, Configuration 7)

As shown in Figure 28a, after an initial spike to nominally 430 μL/L of CO in the garage shortly 

after the generator was started, it dropped to a level near 50 μL/L before rising to about 80 μL/L 

during the brief third load cycle. Note that the missing data included the high load portion of the 

second load cycle and a peak during this time cannot be ruled out. Excluding the initial peak of 

Test V, this is a reduction of 85 % to 88 % compared to that measured with unmod Gen X in 

Test K, in which the CO in the garage ranged from 350 μL/L to 650 μL/L. 

With the garage bay door open, the garage oxygen level stayed nominally at ambient. 

Throughout the test, the delivered electrical output met or exceeded the load bank settings, with 

the exception of a slight drop at the highest setting during the third load cycle.

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3000

4000

5000

6000

0 30 60 90 120

% Oxygen

CO (μL/L), Load (W)

Time (min)

N3 CO Delivered Load O2
53

Figure 28b CO concentrations in the house for Test V (noncat Gen SO1, Configuration 7)

As shown in Figure 28b, the CO concentration in the family room initially spiked to 135 μL/L

and then dropped to a uniform concentration throughout the house of around 75 μL/L, with a 

continual decline to 50 μL/L, at which point the mechanical venting was initiated. With the 

exception of the first 25 min of the test, the distribution was very uniform despite the HVAC fan 

being off. By comparison, unmod Gen X in Test K produced a less uniform house distribution, 

with a peak CO concentration of nominally 320 μL/L in the family room and concentrations in 

the other rooms just below that, down to 260 μL/L. 

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50

75

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125

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CO (

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Time (min)

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54

Figure 28c Temperatures and AFR measured in Test V (noncat Gen SO1, Configuration 7)

The AFR and temperatures measured on Gen SO1 during Test V are shown in Figure 28c. The 

engine operated at the calibrated AFR after the engine oil temperature warmed to about 70 °

C. 

Engine performance in this test was similar to that in Test U with Gen SO1 and the cat muffler, 

with the caveats that the loads were applied in opposite order and some data were missed in Test 

V. A comparison of the 50 μL/L garage CO concentration in this test with the 20 μL/L in Test U 

indicates the prototype’s catalyst is providing about up to a 60 % reduction in CO emissions 

from that provided by the EMS alone. This somewhat larger difference than found when 

comparing Tests Z to N and Tests AH to W could be due to changes in infiltration rates or other 

factors. 

Summary of Garage Tests

A series of tests were conducted in which portable gasoline-powered electric generators were 

operated in the attached garage of the NIST manufactured test house. The data include CO 

emission and O2 depletion in the garage, CO migration into the test house and engine operation 

parameters. A summary of the test results is provided in Table 4. These tests document 

reductions of 85 % to 98 % in CO concentrations due to emissions from two modified, prototype 

low CO-emission portable generators compared to an unmodified generator. The second 

prototype (Gen SO1) resulted in lower CO concentrations during similar tests with the garage 

bay door closed while both prototypes resulted in low CO concentrations during tests with the 

garage bay door open. Note that these results apply to the specific units tested and that other 

units, modifications, and test conditions may produce different results.

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11

12

13

14

15

16

17

18

0

100

200

300

400

500

600

700

800

0 30 60 90 120

AFR

Temperature (oC)

Time (min)

Engine-out exhaust temp Cyl head temp Oil sump temp Air-fuel ratio
55

Table 4 Summary of Garage Test Results

Generator Test 

ID

Garage bay 

door, house 

door, 

HVAC

Test 

Duration 

(h)

Peak Garage 

CO 

Concentration 

(μL/L)

% Reduction

in peak 

garage CO 

relative to 

unmod GenX

Peak CO 

concentration 

in house

(μL/L)

unmod 

GenX

B Closed, 

open, off

3 19,500

(12,800 at 2 h)

NA 6500

modGenX O Closed, 

open, off

4.5 3000

(1,400 at 3 h)

93 800

SO1 N Closed, 

open, off

2 300 98 140

unmod 

GenX

F Open, 

closed, off

4 1,500 NA 200

modGenX R Open, 

closed, off

4 30 98 5

SO1 T Open, 

closed, off 

3 300

(20 after initial 

spike)

98 50

unmod 

GenX

I Closed, 

open, on

4 18,600 NA 10,600

SO1 with 

noncat 

muffler

Z Closed, 

open, on 

4.75 630 97 360

unmod 

GenX

J Closed, 

closed, on

2.25 21,300 NA 1,800

SO1 W Closed, 

closed, on

6 960

(640 at 2.25 h)

97 145

unmod 

GenX

D Closed, 

closed, off

2 23,000 NA 1660

SO1 with 

noncat 

muffler

AH Closed, 

closed, off 

5 2,300 90 470

unmod 

GenX

G Open, open,

on

2 1,100 NA 220

SO1 U Open, open,

on 

2 260

(< 30 after 

initial spike)

97 90

unmod 

GenX

K Open, open,

off

>2 680 NA 320

SO1 with 

noncat 

muffler

V Open, open,

off 

>2 430

(50 to 80 after 

initial spike)

85 to 88 135

Notes: 

Unmod Gen X is an unmodified (stock) generator with a carbureted engine. 

Mod Gen X is a modified (prototype) generator with electronic fuel injection, an engine control 

unit and a catalytic converter.

Gen SO1 is a modified (prototype) generator with electronic fuel injection, an engine control unit 

(different than mod Gen X), and a catalytic converter (not used in ‘noncat’ configuration). 

% reduction in peak garage CO concentration excludes initial spike.
56

Simulation

This section of the report describes the CONTAM model of the test house used to extend the 

measurement results to other scenarios, (e.g. weather conditions and house configuration. The 

primary objective of these modeling efforts was to examine the potential performance of the 

prototype generators under a wider range of conditions than studied during the experiments at the 

test house. This section starts with a detailed description of the CONTAM model of the test 

house, then describes a number of model validation cases, and finally presents simulation results 

for a broad range of cases. The purpose of the model validation effort was to establish the ability 

of the CONTAM model to predict CO levels in the garage and the house and to develop an 

estimate of the uncertainty of these predictions relative to the measured values. 

Modeling Method

The simulations in this study used CONTAM (Walton and Dols 2010), a multizone IAQ and 

ventilation model developed in the Engineering Laboratory (EL) at NIST. The multizone 

approach is implemented by constructing a building model as a network of elements describing 

the flow paths (e.g. HVAC ducts, doors, windows, cracks, etc.) between the zones (primarily 

rooms) of a building. The network nodes represent the zones, which are modeled with a 

hydrostatically varying pressure, and uniform temperature and pollutant concentration within 

each zone. After calculating the airflow between zones and the outdoors, zone pollutant 

concentrations are calculated by applying mass balance equations to the zones. CONTAM has 

frequently been used to study a variety of residential IAQ issues in past simulation studies 

(Emmerich and Persily 1996, Emmerich et al. 2005). For more detail on the CONTAM model, 

see the user’s manual which is available online along with the latest version of the CONTAM 

program (www.bfrl.nist.gov/IAQanalysis).

The graphical representation of the main floor of the test house as it appears in CONTAM is

shown in Figure 29. The test house crawlspace and attic were included in the model but are not 

shown. The layout of the test house within CONTAM and the division of the zones were defined 

to represent the actual floorplan of the test house as seen in Figure 2, therefore each zone 

typically represents one room although the kitchen, living, dining and family room spaces are 

combined into a single CONTAM zone, referred to as zone LFK, due to the lack of any 

partitions separating them. 

Figure 29 CONTAM Sketchpad Representation of Test House 

Garage

LFK

MBR

BR2

BR3
57

Nabinger and Persily (2008) and Nabinger et al. (2010) describe a study of ventilation rates and 

energy consumption in the test house before and after a retrofit effort to improve the tightness of 

both the building envelope and air distribution systems. That study included fan pressurization 

tests of both the envelope and air distribution systems, measurements of HVAC system air flows 

and tracer gas decay tests of air change rates. A CONTAM airflow model of the building was 

created and the predicted infiltration rates agreed well with the measured values over a range of 

weather and system operation conditions. Tables 5 and 6 list the air leakage values (in terms of 

the effective leakage area (ELA) at a reference pressure difference of 4 Pa) and HVAC system 

airflows from Nabinger et al. (2010). The CONTAM model used a detailed duct model for the 

HVAC system and, as such, the modeled flow values varied somewhat from the measured values 

in Table 6. Also, a duct leak of about 30 L/s was included in the belly. The belly, which is 

located between the floor of the living space and the crawlspace, contains the supply duct work 

within an insulated membrane. 

Table 5 Air Leakage Values

ELA at 4 Pa

Airflow path Post-retrofit

Living space envelope Exterior wall 0.13 cm2

/m2

Ceiling wall interface 0.73 cm2/m

Floor wall interface 1.12 cm2/m

Window #1 5.00 cm2

Window #2 1.94 cm2

Corner interface 0.73 cm2/m

Exterior doors 18.7 cm2

Living space to belly 1.43 cm2/m2

Interior airflow paths Interior walls 2 cm2

/m2

Bedroom doorframe 410 cm2

Open interior doors 2 m x 0.9 m

Bathroom doorframe 330 cm2

Interior doorframe 250 cm2

Closet doorframe 4.6 cm2

Attic Attic floor 2 cm2

/m2

Roof vents 0.135 m2/each

Eave vents 296 cm2/m 

Crawl space and belly Exterior walls of crawl space 25 cm2

/m2

Rear crawl space vents 323 cm2

Front crawl space vents 465 cm2

Crawl space access door 206 cm2

Crawl space to “belly” 181 cm2

Duct leak into belly 58 cm2
58

Table 6 HVAC System Supply Flows

Airflow rate L/s 

Room Vent No. Post-retrofit

Family 1 54

Family 2 44

Kitchen 3 39

Dining 4 41

Bath2 5 40

Bed3 6 62

Bed2 7 21

Living 8 27

Living 9 28

Bed1 10 29

Bed1 11 25

Bath1 12 17

Utility 13 24

Total 450

The CONTAM airflow model described by Nabinger et al. 2010 was modified to add the 

attached garage, including leakage paths from the garage to ambient and to the house. The 

modeled garage leakage was based on fan pressurizations tests conducted in April, July and 

August 2008 with analysis following the methods described by Emmerich et al. 2003. 

Garage Pressurization Tests 

Additional pressurization tests were conducted on the house to measure the leakage of the house, 

garage, and the house-garage (HG) interface. The pressurization tests were generally conducted 

according to ASTM Standard E 779-10 (ASTM 2010). Three configurations were used on the 

test house as shown in Figure 30: 

1. Blower door (or pressurization fan) in the living space with the garage door open. 

2. Blower door in the garage access door with the living space doors open. 

3. Blower door in the living space with the HG interface door open. 

In the figure and the analysis, each building is represented as two zones; a house zone (H) and a 

garage (G) zone separated by the house-garage interface. Arrows indicate the location of the 

blower door for each test and the direction of the airflow from the blower door fan. The pressure 

difference, ∆PHG, is the pressure difference across the house-garage interface. The pressure 

differences across the living space exterior envelope for each test configuration are designated as 

∆PH, while ∆PG designates the pressure differences across the garage exterior envelope.
59

Figure 30 Test house and garage fan pressurization configurations 

For each blower door test, measured airflows were recorded at multiple pressures ranging from 

10 Pa to 70 Pa (not all spaces were pressurized to 70 Pa). The airflows and pressures were then 

fit to a power law equation: Q = C (∆P)n. For configuration 1, depressurizing the living space 

(H), while the garage door is open, effectively changes the garage space to an ambient zone. The 

house zone is bounded by the exterior envelope surfaces of the living space, H, and the HG 

interface surface. The reverse is performed for the garage in test 2, in which the garage zone is 

bounded by the exterior envelope surfaces of the garage, G, and the HG interface surface. 

Finally, in configuration 3, the combined house and garage spaces are pressurized.

As described in Emmerich et al. 2003, calculations for the test house are based on continuity, 

effective leakage area, and power law orifice equations. Applying the law of conservation of 

mass and the power law orifice equation yields a three by three matrix of airflow rates: 

H QQ HG

n PCQ +=∆⋅= 1

11 (4) 

G QQ HG

n PCQ +=∆⋅= 2

22 (5) 

QQ HG

n PCQ +=∆⋅= 3

33 (6) 

Symbol Legend for Figures and Equations in this section

H – exterior envelope of house (living space) 

H’ – house exterior envelope and HG interface combined (H+HG) 

G – exterior envelope of garage

G’ – garage exterior envelope and HG interface combined (G+HG)

HG – HG interface

Q# or L – Airflow rate from blower door in configuration # or for surface designation (L) 

∆P# or L – Pressure difference across a surface

C# or L – flow coefficient for Q# or L

 
60

Here, C1 and n1 are the flow parameters obtained from the curve fit to the data for configuration 

1 (tested twice in depressurization mode only), C2 and n2 are obtained from the data for 

configuration 2 (tested 3 times in depressurization mode and once in pressurization mode), and 

C3 and n3 are obtained from the data for configuration 3 (tested twice in depressurization mode 

only). With three equations and three unknowns, this matrix can be solved for QH, QG, and QHG, 

all at a specific reference pressure across surfaces H, G, and HG. Using these three airflow rates, 

the zone envelope and HG interface ELA’s are calculated using the effective leakage area 

equation (ASHRAE 2009): 





 



 ∆⋅

=

ρ

P

Q ELA

2

 

(7)

Table 7 Fan Pressurization Test Results

C1 (m3/h·Pan) 290

C2 (m3/h·Pan) 157

C3 (m3/h·Pan) 296

n1 0.61 

n2 0.56 

n3 0.65

ACH50,H (h-1) 11.8

ACH50,G (h-1) 15.5 

ELA4,H (cm2) 574 

ELA4,G (cm2) 210 

ELA4,HG (cm2) 157

The total leakage from the house to the garage (ELA4,HG) of 157 cm2 was equal to the average 

total leakage from the house to the garage for the five houses tested by Emmerich et al. (2003). 

The effective leakage areas for G and HG from Table 7 were distributed vertically along the 

walls of the garage and on the ceiling in the CONTAM model. 

Temperatures were measured in most of the individual rooms, garage, crawlspace and attic of the 

test house during the tests and used as input to the model. Temperatures in the LFK zone were 

based on an average of the living room, family room, and kitchen measurements. Both BR2 and 

BR3 temperatures were based on measured BR3 temperatures as there was no measurement 

available for BR2. Temperatures in the Utility zone were based on the adjacent family room 

temperature except when the door to the garage was open in which case they were based on an 

average of the temperature in the LFK zone and the garage. Temperatures in other zones without 

measurements such as bathrooms were based on an average of all room (i.e., not including 

garage, crawlspace or attic) measurements.
61

Model Validation

This section describes the model validation efforts that were carried out to establish the ability of 

the CONTAM model to predict CO levels in the garage and the house and to develop an estimate 

of its uncertainty of these predictions relative to the measured values. A number of past studies 

(e.g., Emmerich and Nabinger (2001) and Emmerich et al. (2004)) have also examined the issue 

of multizone IAQ model validation as reviewed in Emmerich (2001). 

Statistical Evaluation of Model Predictions

As part of this validation effort, predicted CO concentrations were compared with measured 

values from a series of experiments in the test house. The predictions and measurements were 

compared using ASTM D5157 Standard Guide for Statistical Evaluation of Indoor Air Quality 

Models, which provides quantitative and qualitative tools for evaluation of IAQ models (ASTM 

2008). It provides guidance in choosing data sets for model evaluation and focuses on evaluating 

the accuracy of indoor concentrations predicted by a model. As part of the comparison of 

CONTAM predictions to measurements of CO and O2 concentrations, the ASTM D5157 

suggested criteria were applied. The data sets collected during this study meet the ASTM D5157 

criteria for model evaluation, as they are entirely independent of the data used to develop the 

model and to estimate model inputs. Also, the data are of sufficient detail to evaluate the 

CONTAM predictions of individual zonal CO concentrations.

ASTM D5157 provides three statistical tools for evaluating the accuracy of IAQ model 

predictions and two additional statistical tools for assessing bias. Values for these statistical 

criteria are provided to indicate whether the model performance is adequate. The tools for 

assessing agreement between predictions and measurements, and the criteria for model 

evaluation, include: 

1) The correlation coefficient of predictions and measurements should be 0.9 or greater. 

2) The line of regression between the predictions and measurements should have a slope 

between 0.75 and 1.25 and an intercept less than 25 % of the average measured 

concentration. 

3) The normalized mean square error (NMSE) should be less than 0.25. The NMSE is 

calculated as: 

∑=

−=

N

i

NMSE oipi CCCC po

1

2 2/)( (8) 

where Cp is the predicted concentration and Co is the observed concentration, and the over-bar 

represents an average over the N data points during the test period for each test case. 

ASTM D5157 also provides two statistical measures of bias with values for judging adequate 

model performance. These measures of bias include:

1) Normalized fractional bias (FB) of the mean concentrations. Fractional bias should be 0.25 or 

lower and is calculated as:

( ) FB +−= CCCC opop /)(2 (9) 

2) Fractional bias based on the variance (FS) which should be 0.5 or lower. FS is calculated as: 
62

( ) 2 2 2 2 FS =− + 2( ) / σ σ p p σ σ o o (10) 

where σp is the standard deviation of the predicted concentrations and σo is the standard 

deviation of the observed concentrations. 

Testing Configurations

As previously discussed in the Garage Test Results section and as part of the model validation 

effort, testing was conducted under seven different test house configurations to evaluate their 

impacts on the ability to predict the build-up of CO in the garage and its transport into the 

different rooms in the house. These configurations included two different garage bay door 

positions (fully closed or open nominally 0.6 m), two connecting door positions between the 

garage and the house (fully closed or open nominally 5 cm), and two house central heating, 

ventilating, and air conditioning (HVAC) fan settings (on or off). When the garage bay door was 

open, there was also a vertical opening of approximately 0.15 m at the top of the bay door. All 

internal house doors were kept open throughout all validation tests and model analyses. 

Table 8 includes a summary of the validation tests conducted including information on the 

generator tested, the test house configuration (door positions and fan status), a test identification 

code, the date the test was conducted, the average ambient temperature and wind speed, and the 

CO analyzers used. The validation tests in Table 8 with Gen SO1 were included in the garage 

test results section, but those with Gen B were used exclusively for model validation. 
63

Table 8 Model Validation Tests Conducted

Generator Garage bay 

door 

Garage to house 

entry door 

HVAC 

fan 

Date Outdoor 

Temp

(°C)

Wind 

speed 

(m/s)

Load CO analyzers in 

garage 

CO analyzers 

in house

B Closed Open OFF 10/8/08 18.4 4.5 2.5 

kW

N1, T1 N2, N3

B Open Open OFF 10/23/08

a.m.

8.8 2.2 5 kW N1, T1 N2, N3

B Closed Closed ON 10/23/08

p.m.

13.1 3.8 5 kW N1, T1 N2, N3

SO1 cat Closed Open OFF 04/01/10 19.9 6.3 Cyclic N2, N3 N1, R1

SO1 cat Open Open ON 04/22/10 20.4 7.8 Cyclic N2, N3 N1, R1

SO1 cat Closed Closed ON 04/29/10 17.8 9.5 Cyclic N2, N3 N1, R1

SO1 with noncat 

muffler

Closed Closed OFF 05/13/10 15.6 6.5 Cyclic N2, N3 N1, R1
64

RESULTS OF MODEL VALIDATION

As shown in Table 8, seven validation cases were simulated under a variety of conditions 

between October 2008 and May 2010. The tests consisted of operating one of the 

generators in the attached garage and measuring the concentrations in the house and 

garage for two to six hours. 

Case #1

In the first validation case, Gen B was operated with a nominal load of 2.5 kW for 2 h in 

the garage with the garage bay door closed, house entry door open and the HVAC system 

off. Based on the shed test results and the fact that the O2 in the garage did not drop 

below about 19 % during this test, a constant CO emission rate of 760 g/h and a constant 

O2 consumption rate of 4140 g/h were applied in the simulations of this case. These 

values are based on the average shed test results plotted on Figures 8a and 8b for O2 at 19 

% or above. 

 

The observed and predicted CO and O2 concentrations for the garage zone and the LFK

zone in the house are shown in Figure 31. Table 9 shows average observed and predicted 

zone CO concentrations and percent differences, along with the ASTM D5157 statistical 

parameters (described previously) calculated for the zone average concentrations. The 

suggested ASTM D5157 statistical criteria were evaluated for overall zone average 

concentrations for the entire testing period for all cases. The columns of Table 9 (and 

subsequent tables) include the zone average observed concentration (Co), average 

predicted concentration (Cp), standard deviation of observed concentrations (σo), standard 

deviation of predicted concentrations (σp), correlation coefficient (R), regression slope 

(m), regression intercept divided by the average observed concentration (b/Co), 

normalized mean square error (NMSE), fractional bias of the mean concentrations (FB), 

and fractional bias based on the variance (FS). The average concentration in the bottom 

row of the tables is a linear average of the five zone concentrations. 
65

Figure 31 Predicted and observed CO and O2 concentrations for Case #1 (Notes: Plotted 

house concentration is from the LFK zone for Figures 31 through 37. The x-axis shows 

clock time for Figures 31 through 37.)

Table 9 Statistical parameters for Case #1 (concentrations in units of μL/L)

Zone Co Cp Co -Cp

% 

diff

GAR 2802 3066 -265 9

LFK 435 414 21 -5

MBR 307 337 -30 10

Bed2 308 383 -75 24

Bed3 289 369 -81 28

Average 

|% difference| 

15

Co Cp σo σp R m 

b/Co

(%) NMSE FB FS

Room Average

Concentrations 828 914 1100 1200 1.00 1.09 1.5 0.02 0.10 0.27

The bold values in Table 9 are those that meet the ASTM suggested criteria. Based on the 

statistical parameters, this case exhibits excellent agreement between measurements and 

predictions. Specifically, the values for R, m, b/Co, NMSE, FS and FB calculated for the 

average zone concentrations all meet the ASTM D5157 suggested limits. Additionally, 

18.5

19

19.5

20

20.5

21

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

14:02 14:31 15:00 15:28 15:57 16:26

O2 (%)

CO (

μL/L)

Garage CO (observed)

Garage CO (predicted)

House CO (observed)

House CO (predicted)

Garage O2 (observed)

Garage O2 (predicted)
66

the average of the absolute percent differences between the zone values of Co and Cp was 

15 %. 

Case # 2 

Case # 2 involved operation of Gen B in the garage with the bay door open, the house 

entry door open and the HVAC system fan off. A constant nominal load of 5 kW was 

applied for this case. Based on the shed test results and the fact that the O2 in the garage 

did not drop below 20.8 % during this test, a constant CO emission rate of 1090 g/h 

(corresponding to the highest O2 point during the shed testing [see Fig 6a]) and a constant 

O2 consumption rate of 5640 g/h (see Fig 6b) were applied to the simulations.

The observed and predicted concentrations for garage zone and the LFK zone in the 

house are shown in Figure 32, and the calculated D5157 statistical parameters are shown 

in Table 10. Based on the statistical parameters, the agreement between measurements 

and predictions was not as good as for Case #1. Specifically, only the values for R and 

FB calculated for the average zone concentrations meet the ASTM D5157 suggested 

limits while m, b/Co, NMSE, and FS fall outside the limits. Also, the average of the 

absolute percent differences between the zone values of Co and Cp, i.e., 24 %, was worse

than Case 1. This larger average value was driven primarily by a large difference in the 

garage zone, though smaller differences are seen in the house zones. One possible reason 

for poorer agreement is the lack of shed test data for an O2 level above 20.5 % (see 

Figure 8a).

Figure 32 Predicted and observed CO and O2 concentrations for Case # 2 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

100

200

300

400

500

600

700

800

900

1000

10:19 11:24 12:28

O2 (%)

CO Concentration (μL/L)

Garage CO (predicted)

House CO (Observed)

House CO (Predicted)

Garage CO (Observed)

Garage O2 (Observed)

Garage O2 (Predicted)
67

Table 10 Statistical parameters for Case # 2 (concentrations in units of μL/L)

Zone Co Cp Co-Cp % diff

GAR 329 470 -142 43

LFK 96 61 34 -36

MBR 55 49 6 -11

Bed2 73 52 21 -28

Bed3 50 51 -1 2

Average 

|% difference| 

24

Co Cp σo σp R m b/Co (%) NMSE FB FS

Room Average

Concentrations 120 137 118 186 0.99 1.57 -43 0.26 0.19 1.0

Case # 3 

Case # 3 involved operation of Gen B in the garage with the bay door closed, the house 

entry door closed and the HVAC system fan on. A constant nominal load of 5 kW was 

applied for this case. Based on average values from the shed test results in Figure 8, a 

constant O2 consumption rate of 5940 g/h was applied and the CO emission rate was

dependent on the predicted O2 level per the relationship in Table 11. 

Table 11 O2 dependent CO emission rate for Gen B

O2 level (%) CO emission 

(g/h)

O2>20 1370

19<O2<20 2070

18<O2<19 3000

17<O2<18 3150

O2<17 2140

The observed and predicted transient concentrations for the garage zone and the LFK

zone in the house are shown in Figure 33, and the D5157 statistical parameters calculated 

for the zone average concentrations are shown in Table 12. Based on the statistical 

parameters, this case resulted in agreement between measurements and predictions

similar to that seen in Case 2. Specifically, the values for R and b/Co calculated for the 

comparison of average zone concentrations meet the ASTM D5157 suggested limits 

while those for m, NMSE, FB, and FS fall outside the limits. However, the average of the 

absolute percent differences between Co and Cp, i.e., 16 %, was better than Case 2. Again, 

there are large differences in the garage zone concentrations, but differences in the house 

zones are smaller. 
68

Figure 33 Predicted and observed CO and O2 concentrations for Case # 3 

Table 12 Statistical parameters for Case # 3 (concentrations in units of μL/L)

Zone Co Cp Co-Cp % 

diff

GAR 9542 12978 -3436 36

LFK 434 322 111 -26

MBR 275 254 20 -7

Bed2 289 289 0 0

Bed3 262 289 -27 10

Average 

|% difference|

16

Co Cp σo σp R m b/Co

(%)

NMS

E

FB FS

Room 

Average

Concentration

2160 2827 4130 5580 1.00 1.37 -6.6 0.39 0.27 0.85 

Case # 4 

Case # 4 involved operation of Gen SO1 with the CAT muffler in the garage with the bay 

door closed, the house entry door open, the HVAC system fan off and a cyclic electrical

16

17

18

19

20

21

22

0

5000

10000

15000

20000

25000

14:15 14:44 15:12 15:41 16:10

O2 (%)

CO Concentration (μL/L)

Garage CO (Predicted)

House CO (Observed)

House CO (Predicted)

Garage CO (Observed)

Garage O2 (Observed)

Garage O2 (Predicted)
69

load. During the test, a load bank fuse failed and the load dropped in half for a few 

minutes before the fuse was replaced and the test continued. Also, this test includes a 

natural decay period of about 40 min (included in plots and analysis below) following 

generator shut-off. 

Based on average values from the shed test results shown in Figure 11, load-dependent 

O2 consumption and CO emission rates were used for this case (see Table 13), with both 

rates dependent on the electrical load. In addition, past measurement of Gen SO1 

operation in both the shed and garage showed that an initial spike of CO is consistently 

observed immediately following a cold start (note the ECU algorithm is programmed to 

operate rich upon start-up). Based on analysis of cold start-up data following the method 

described previously for shed testing, a 430 g/h source of CO was included in the model 

at start-up in place of the Table 13 value until the measured AFR exceeded 13.5 (the first 

five minutes of operation for this case). 

Table 13 Hourly cyclic load profile for Gen SO1 cat

Load bank setting

(W)

Duration 

(min) 

CO emission

(g/h)

O2 consumption

(g/h)

no load 3 20 2620

500 4 15 3880

1500 18 17 3080

3000 17.5 47 4820

4500 12 96 4370

5500 5.5 102 3710

The observed and predicted transient concentrations for the garage zone and LFK zone in 

the house are shown in Figure 34, and the D5157 statistical parameters calculated for the 

zone average concentrations are shown in Table 14. Based on the statistical parameters, 

this case resulted in agreement between average concentration measurements and 

predictions similar to Case #2. Specifically, only the value for R and FB calculated for 

the comparison of average zone concentrations met the ASTM D5157 suggested limits

while m, b/Co, NMSE, and FS fall outside the limits. Additionally, the average of the 

absolute percent differences between zone values of Co and Cp was 26 %, which was 

driven by a large difference in the garage zone but much smaller differences in the LFK 

zone. 
70

Figure 34 Predicted and observed CO and O2 concentrations for Case # 4 

Table 14 Statistical parameters for Case # 4 (concentrations in units of μL/L)

Zone Co Cp Co-Cp % diff

GAR 175 326 -151 86

LFK 85 73 12 -14

MBR 83 69 14 -17

Bed2 77 69 7 -10

Bed3 69 70 0 1

Average 

|% difference|

26

Co Cp σo σp R m b/Co (%) NMSE FB FS

Room Average

Concentrations 98 121 43 114 0.99 2.6 -135 0.39 0.21 1.6

Case # 5 

Case # 5 involved operation of Gen SO1 with CAT muffler in the garage with the bay 

door open, the house entry door open and the HVAC system fan on. A cyclic load was 

applied for this case. Also, this test includes a natural decay period of about 25 min 

following generator shut-off. Based on average values from the shed test results shown in 

Figure 11, load-dependent O2 consumption and CO emission rates were used for this case 

17.5

18

18.5

19

19.5

20

20.5

21

21.5

0

100

200

300

400

500

600

700

800

900

1000

10:12 11:12 12:12 13:13

O2 (%)

CO Concentration (μL/L)

Garage CO (Predicted)

House CO (Observed)

House CO (Predicted)

Garage CO (Observed)

Garage O2 (Observed)

Garage O2 (Predicted)
71

(see Table 15) along with a 430 g/h source of CO at start-up in place of the Table 15 

emission until the measured AFR exceeded 13.5 (ten minutes for this case). 

Table 15 Hourly cyclic load profile for Gen SO1 cat

Load bank setting (W) Duration (min) CO emission

(g/h)

O2 consumption

(g/h)

no load 3 20 2620

500 4 15 3880

1500 18 17 3080

3000 17.5 47 4820

4500 12 96 4370

5500 5.5 102 3710

The observed and predicted transient concentrations for the garage zone and the LFK

zone in the house are shown in Figure 35, and the D5157 statistical parameters calculated 

for the zone average concentrations are shown in Table 16. Based on the statistical 

parameters, this case resulted in good agreement between measurements and predictions. 

The average absolute value of the percent differences between the zone measurements 

and predictions was 20 %, and the absolute difference was less than 10 μL/L for all 

zones. Additionally, the values for R, NMSE, and FB calculated for the comparison of 

average zone concentrations meet the ASTM D5157 suggested limits, while m, b/Co, and 

FS fall somewhat outside the limits.

Figure 35 Predicted and observed CO and O2 concentrations for Case # 5

19

19.5

20

20.5

21

21.5

0

100

200

300

400

500

12:14 12:43 13:12 13:40 14:09 14:38

O2 (%)

CO Concentration (μL/L)

Garage CO (Predicted)

House CO (Observed)

House CO (Predicted)

Garage CO (Observed)

Garage O2 (Observed)

Garage O2 (Predicted)
72

Table 16 Statistical parameters for Case # 5 (concentrations in units of μL/L)

Zone Co Cp Co-Cp % diff

GAR 41 41.9 -0.5 1

LFK 26 29.2 -2.8 11

MBR 23 28.4 -5.8 25

Bed2 21 28.5 -7.5 36

Bed3 23 28.7 -5.9 26

Average 

|% difference|

20

Co Cp σo σp R m b/Co (%) NMSE FB FS

Room Average

Concentrations 27 31 8.4 5.9 0.98 0.69 48 0.03 0.15 -0.54

Case # 6

Case # 6 involved operation of Gen SO1 with CAT muffler in the garage with the bay 

door closed, the house entry door closed and the HVAC system fan on. A cyclic load was 

applied for this case. Based on the average values from the shed test results shown in 

Figure 11, load-dependent O2 consumption and CO emission rates were used for this case 

(see Table 17) along with a 430 g/h source of CO at start-up in place of the Table 17 

emission until the measured AFR reached 13.5 (ten minutes for this case). There was a 

problem with the Family room temperature measurement for this case so the utility room 

temperature was set equal to the kitchen temperature.

Table 17 Hourly cyclic load profile for Gen SO1 cat

Load bank setting (W) Duration (min) CO emission

(g/h)

O2 consumption

(g/h)

no load 3 20 2620

500 4 15 3880

1500 18 17 3080

3000 17.5 47 4820

4500 12 96 4370

5500 5.5 102 3710

The observed and predicted transient concentrations for the garage zone and the LFK

zone in the house are shown in Figure 36, and the D5157 statistical parameters calculated 

for the zone average concentrations are shown in Table 18. The average absolute value of 

the percent differences between the zone measurements and predictions was 16 %. Based 

on the statistical parameters, this case resulted in excellent agreement between 

measurements and predictions. Specifically, the values for R, m, b/Co, NMSE, FB and 
73

FS calculated for the comparison of average zone concentrations all meet the ASTM 

D5157 suggested limits. 

Figure 36 Predicted and observed CO and O2 concentrations for Case # 6

Table 18 Statistical parameters for Case # 6 (concentrations in units of μL/L)

Zone Co Cp Co-Cp % diff

GAR 631 686 -55 9

LFK 87 88 0 0

MBR 72 84 -12 17

Bed2 63 84 -22 34

Bed3 72 86 -13 19

Average 

|% difference|

16

Co Cp σo σp R m b/Co (%) NMSE FB FS

Room Average

Concentrations 185 206 250 269 1.00 1.07 3.5 0.02 0.10 0.25

16

17

18

19

20

21

22

0

100

200

300

400

500

600

700

800

900

1000

10:40 11:40 12:40 13:40 14:40 15:40 16:40

O2 (%)

CO Concentration (μL/L)

Garage CO (Predicted)

House CO (Observed)

House CO (Predicted)

Garage CO (Observed)

Garage O2 (Observed)

Garage O2 (Predicted)
74

Case # 7 

Case # 7 involved operation of Gen SO1 with nonCAT muffler in the garage with the bay 

door closed, the house entry door closed and the HVAC system fan off. A cyclic load was 

applied for this case. This case included a natural decay for about 45 min after generator 

shut-off (included in plots and analysis below). Based on average values of the shed test 

results shown in Figure 12, load-dependent O2 consumption and CO emission rates were 

used for this case (see Table 19) along with an 840 g/h source of CO at start-up in place 

of the Table 19 emission until the AFR reached 13.5 (the first ten minutes of operation 

for this case). 

Table 19 Hourly cyclic load profile for Gen SO1 noncat

Load bank setting (W) Duration (min) CO emission

(g/h)

O2 consumption

(g/h)

no load 3 150 2580

500 4 88 2780

1500 18 97 3450

3000 17.5 236 4110

4500 12 271 4190

5500 5.5 236 4440

The observed and predicted transient concentrations for the garage zone and the LFK

zone in the house are shown in Figure 37, and the D5157 statistical parameters calculated 

for the zone average concentrations are shown in Table 20. The average absolute value of 

the percent differences between the zone measurements and predictions was 17 %. Based 

on the statistical parameters, this case resulted in excellent agreement between 

measurements and predictions. Specifically, the values for R, m, b/Co, NMSE, FB and 

FS calculated for the comparison of average zone concentrations all meet the ASTM 

D5157 suggested limits. 
75

Figure 37 Predicted and observed CO and O2 concentrations for Case # 7

Table 20 Statistical parameters for Case # 7 (concentrations in units of μL/L)

Zone Co Cp Co-Cp % diff

GAR 1712 1944 -232 14

LFK 266 223 42 -16

MBR 264 212 52 -20

Bed2 263 216 47 -18

Bed3 261 216 46 -18

Average 

|% difference|

17

Co Cp σo σp R m b/Co (%) NMSE FB FS

Room Average

Concentrations 553 562 648 772 1.00 1.19 -18 0.04 0.02 0.36

Summary 

Table 21 summarizes the statistical parameters calculated for all the cases based on the 

average zone concentrations. The bold values in Table 21 are those that meet the ASTM 

suggested criteria. As seen in the table, statistical agreement varies for the individual 

cases. The agreement was generally better for the cases with both the bay and house entry 

16

17

18

19

20

21

22

0

500

1000

1500

2000

2500

3000

10:55 11:55 12:55 13:55 14:55 15:55

O2 (%)

CO Concentration (μL/L)

Garage CO (Predicted)

House CO (Observed)

House CO (Predicted)

Garage CO (Observed)

Garage O2 (Observed)

Garage O2 (Predicted)
76

doors closed (3, 6, and 7). This improved agreement is not a surprise, as the cases with 

better agreement rely primarily on CONTAM’s ability to model airflow through smaller

leaks. While CONTAM includes elements to model flow through larger openings (used 

for the open bay and entry doors of Cases 1, 2, 4 and 5), those flow predictions are 

thought to be less reliable given the physical theory on which CONTAM is based. Table 

21 shows that the statistical agreement was generally somewhat worse for those cases. 

Table 21 Summary of statistical parameters from comparison of average observed

and predicted zone CO concentrations for all cases

Case

Co Cp Avg 

|%diff|

σo σp R m b/Co 

(%) 

NMSE FB FS

1 828 914 15 1100 1200 1.00 1.09 1.5 0.02 0.10 0.27

2 120 137 24 118 186 0.99 1.57 -43 0.26 0.19 1.0

3 2160 2827 16 4130 5580 1.00 1.37 -6.6 0.39 0.27 0.85 

4 98 121 26 43 114 0.99 2.6 -135 0.39 0.21 1.6

5 27 31 20 8.4 5.9 0.98 0.69 48 0.03 0.15 -

0 54

6 185 206 16 250 269 1.00 1.07 3.5 0.02 0.10 0.25

7 553 562 17 648 772 1.00 1.19 -18 0.04 0.02 0.36

Overall summary statistics for all tests 

Garage CO 2180 2790 28 3400 4620 1.00 1.36 -7.6 0.28 0.25 0.81 

Garage O2 19.5 19.1 2.5 1.1 1.3 0.94 1.18 -20 0.001 -0.02 0.41 

House zones 

CO 165 160 17 130 128 0.96 0.94 3.0 0.05 -0.03 -0.08

While the statistical evaluation of the individual cases is useful to understand strengths 

and weaknesses of the model, a more important statistical evaluation is the comparison of 

the entire set of cases. This overall evaluation indicates the model’s ability to predict the 

relative outcome when individual parameters are changed (e.g., door open vs. closed, or 

one type of generator vs. another). The last three rows of Table 21 include summary

statistics for the average CO and O2 concentrations in the garage and for the average CO 

calculated for the house zones. The agreement between the measurements and predictions 

for this entire set of cases was excellent for both the garage O2 concentrations and the 

house CO concentrations. All of these statistical values met the D5157 criteria. The 

agreement, however, was somewhat worse for the garage CO concentrations, with some 

parameters falling slightly outside the ASTM criteria limits. The average concentrations 

for individual house zones (i.e., LFK, MBR, BR2 and BR3) from all cases are plotted in 

Figure 38 to show the comparison of predictions and measurements. To summarize, 

average individual house zone and garage CO concentration predictions and 
77

measurements were within about 20 % and 30 % respectively when averaged over all 

cases.

Figure 38 Comparison of predicted and observed individual house zone average CO 

concentrations for all cases

As discussed previously (Emmerich 2001), absolute validation of a complex model, such 

as CONTAM, is impossible as there are infinite possible building models that can be 

created by a user. However, the primary objective of this experimental validation is to 

evaluate the accuracy of the CONTAM model predictions for this specific application, to 

identify large sources of error, and to determine the level of confidence that we may have 

in the predictions for the other cases simulated. For the cases modeled in this effort, no 

significant errors in the CONTAM model were identified. Additionally, some of the 

discrepancies between model predictions and experimental measurements may be due to 

measurement limitations instead of model deficiencies. For example, this effort involved 

a fairly rich data set in terms of number of variables monitored and spatial and temporal 

detail. Still, there are additional measurements that would have been helpful for the 

simulations. Specifically, the utility room provided the primary pathway for contaminant 

transport from the garage into the house; however, temperatures and concentrations were 

not measured in this zone. Also, uncertainties in experimental measurements include 

more than simply the instrument accuracy. Many of the reported measurements are a 

single point that was used to represent an average room concentration. The ability of this 

single point measurement to represent the room may be questionable at times. 

y = 0.94x + 4.92

R2 = 0.92

0

100

200

300

400

500

0 100 200 300 400 500

Predicted (

μL/L)

Observed (μL/L)
78

Prototype Generator Performance Simulations 

As previously mentioned, a primary objective of these simulations was to examine the 

potential performance of the prototype generator under a wider range of conditions than 

studied during the experiments at the test house, such as a longer period of generator 

operation and operation in a house location other than the garage. These additional 

simulations were performed to allow analyses of the safety implications of operating a 

portable generator in these conditions. All of these simulations, a total of 42, were based 

on the NIST manufactured test house model and the tested generators, which were 

extensively validated as described above. 

Model parameters that were varied included ambient conditions, CO emission rates, 

source locations (location of the operating generator) and door positions. All simulations 

used steady state weather with three ambient temperatures (mild, 15 °C; cold, 0 °C; and 

hot 30 °C) and one wind condition (2 m/s blowing towards the garage bay door). 

Since the garage measurements reported above clearly establish the danger of operating 

the unmodified generators in an attached garage, it was decided to focus the simulations 

on the potential performance of the reduced CO emission prototype. Therefore, the CO 

emission rates in the simulation were based on operation of Gen SO1 (both with and 

without cat muffler) under the cyclic load profile for 18 hours followed by 6 hours with 

the generator off. The 18 hour period of operation is a conservative estimate based on the 

fuel consumption rate at a low load and a 5 gallon gas tank capacity; the actual run time 

would likely be shorter under real operation but using 18 hours provides a more 

conservative estimate of the CO levels and exposure. Based on the shed test results, the 

previously-validated load-dependent CO emission rates of Table 13 were used for Gen 

SO1 with cat muffler along with a constant 430 g/h of CO at start-up in place of the Table 

13 emission for the first 10 minutes of operation (typical time to reach an AFR of 13.5 for 

the model validation cases). Similarly, the previously-validated, load-dependent CO 

emission rates of Table 19 were used for Gen SO1 with noncat muffler along with a 

constant 840 g/h of CO at start-up for the first 10 minutes of operation. 

The simulations included the generator located in either the attached garage or in the 

utility room of a modified test house model with the garage removed. For garage source 

cases, 4 door position combinations were considered: bay door (closed or 0.6 m open) 

and garage/utility access door positions (closed or 5 cm open). For the utility source cases, 

two positions of the utility/family room connecting door were modeled (closed and 5 cm 

open). The temperature, source strength, location and door factors resulted in a matrix of 

36 cases. See Tables 22 and 23 for lists of the garage and utility source cases, 

respectively.

The HVAC system fan was modeled as off for all cases. Typically all other interior house 

doors were modeled as open but, for a set of 6 additional cases, the bedroom doors were 

modeled as closed (See Table 24 for closed bedroom door cases). Interior zone 

temperatures were set at 23 °C except for source zones and zones adjacent to source 

zones. Source zones were initially 23 °C, then experienced a linear increase over 2 h to 

40 °C, which was held constant until the generator stopped operating, followed by a 
79

linear decrease over the final 6 h back to 23 °C. Zones adjacent to the source zone 

experienced a linear increase over 2 h to 30 °C until the generator stops operating and a 

linear decrease over the final 6 h to 23 °C. 

Garage source cases

Figure 39 presents the simulation results for the cold, mild and hot ambient temperature 

cases with Gen SO1 with cat muffler in the garage with the bay door open and the house 

door closed. For these cases, the CO concentration in the garage spikes to around 

200 μL/L to 300 μL/L, but then drops and remains well below 100 μL/L and does not 

exceed 30 μL/L in the house. It can also be seen in Figure 39 that the temperature 

difference between the rooms of the house with the open doors resulted in very uniform 

concentrations regardless of ambient temperature. The other garage source cases have 

mostly similar patterns with the primary difference among all the cases with different 

door positions being the peak garage and house concentrations reached. 

Table 22 presents the peak CO concentrations in the house (not including utility room) 

and garage for all of the garage source cases. In general, for a specific set of door 

positions, increasing the ambient temperature from cold to mild to hot resulted in higher 

garage and house CO concentrations with the impact of a change from cold to hot being 

as large as a factor of 3. The results for cases 4 through 6 show that adding the open 

house door increased the peak house CO concentration, but the concentrations were still 

at or below 50 μL/L in the house. 

As expected, the results for cases 7 through 9 with the garage source, closed bay door, 

closed house door and cat muffler show that with the garage bay door closed the CO 

concentration reaches much higher peak concentrations in both the garage (850 μL/L to 

1300 μL/L) and house (30 μL/L to 100 μL/L) compared to the garage bay door open 

cases. As shown for cases 10 through 12, opening the house door results in slightly lower 

garage CO (710 μL/L to 810 μL/L) but higher house CO (70 μL/L to 160 μL/L) 

compared to cases 7 through 9 as more CO migrates to the house from the garage through 

the open door. 

The larger CO source for the prototype with noncat muffler (cases 13 through 24 in Table 

22) resulted in peak garage CO concentrations approximately 2 to 3 times higher than for 

the equivalent case with cat muffler. The increase in peak house CO concentrations due 

to the noncat muffler source ranged from a factor of 3 to 4. The noncat muffler source 

results also display the same impacts due to door opening/closing and ambient 

temperature as the cat muffler source cases. The highest peak garage CO concentrations 

were for the noncat source with closed bay door and closed house door at 4300 μL/L 

while the highest peak house CO concentration was for the noncat source with closed bay 

door and open house door at 530 μL/L.
80

 

Figure 39 Garage source, open bay door, closed house door, cat muffler cases (a) cold 

(0 °C), (b) mild (15 °C), (c) hot (30 °C) ambient temperature

0

5

10

15

20

25

30

35

40

0

50

100

150

200

250

300

350

400

0:002:004:006:008:00

10:0012:0014:0016:0018:0020:0022:00

0:00

House CO Concentration (μL/L)

Garage CO Concentration (μL/L)

Garage LFK MBR

BR2 BR3

0

5

10

15

20

25

30

35

40

0

50

100

150

200

250

300

350

400

0:002:004:006:008:00

10:0012:0014:00

16:0018:0020:0022:00

0:00

House CO Concentration (μL/L)

Garage CO Concentration (μL/L)

Garage LFK

MBR BR2

BR3

0

5

10

15

20

25

30

35

40

0

50

100

150

200

250

300

350

400

0:002:004:006:008:00

10:0012:0014:0016:0018:0020:0022:00

0:00

House CO Concentration (μL/L)

Garage CO Concentration (μL/L)

Garage LFK

MBR BR2

BR3
81

Table 22 Simulation results for garage source cases

Sim

ID

Source 

location 

Generator Bay 

door 

House 

Door 

Weath

er

Garage Peak 

CO 

Concentration 

(μL/L)

House Peak CO

Concentration, 

(Excluding 

Utility Room) 

(μL/L)

1 garage SO1 cat open closed cold 170 10

2 garage SO1 cat open closed mild 260 20

3 garage SO1 cat open closed hot 300 30

4 garage SO1 cat open open cold 170 20

5 garage SO1 cat open open mild 240 30

6 garage SO1 cat open open hot 300 50

7 garage SO1 cat closed closed cold 850 30

8 garage SO1 cat closed closed mild 1000 60

9 garage SO1 cat closed closed hot 1300 100

10 garage SO1 cat closed open cold 710 70

11 garage SO1 cat closed open mild 770 110

12 garage SO1 cat closed open hot 810 160

13 garage SO1 noncat open closed cold 330 40

14 garage SO1 noncat open closed mild 500 60

15 garage SO1 noncat open closed hot 600 100

16 garage SO1 noncat open open cold 310 70

17 garage SO1 noncat open open mild 470 90

18 garage SO1 noncat open open hot 600 150

19 garage SO1 noncat closed closed cold 2700 100

20 garage SO1 noncat closed closed mild 3300 190

21 garage SO1 noncat closed closed hot 4300 350

22 garage SO1 noncat closed open cold 2000 250

23 garage SO1 noncat closed open mild 2300 380

24 garage SO1 noncat closed open hot 2600 530
82

Utility source 

Simulations were also performed with the generator operating in the living space of the 

house as opposed to the garage. Figure 40 presents the simulation results for the cold, 

mild and hot ambient temperature cases with Gen SO1 operating in the utility room with 

the door closed. For these cases, the CO concentration in the house (not including the 

utility room in which the peak concentration exceeds 5000 μL/L) reaches a peak of 280 

μL/Lto 520 μL/L. The variation on the house peak CO concentration due to variations in

the ambient temperature was close to a factor of two, with higher concentration 

associated with higher temperature. The other utility room source cases (listed in Table 

23) showed a similar effect of ambient temperature. However, there was no notable effect 

on the CO distribution among the different rooms in the house. 

 

Table 23 presents the peak CO concentrations in the house (not including utility room) 

and in the utility room (not including the initial concentration spike) for all of the utility 

source cases. The results for cases 28 through 30 show that adding the open utility door 

increased the peak house CO concentrations by up to 20 % compared to the closed utility 

door cases. As seen in Table 23 for cases 31 through 36, the larger CO source for the 

prototype with noncat muffler results in the peak CO concentration in the house (not 

including the utility room) increasing by a factor of 3 to 4 times the equivalent cat 

muffler cases. The worst case is the prototype with noncat muffler in the utility room 

with the door open and hot weather which results in a peak house CO concentration of 

about 2000 μL/L.
83

Figure 40 Utility source, closed utility door, cat muffler cases (a) cold (0o

C), (b) mild 

(15o

C), (c) hot (30oC) ambient temperature

0

100

200

300

400

500

600

700

0:002:004:006:008:00

10:0012:0014:0016:0018:0020:0022:00

0:00

House CO Concentration (μL/L)

LFK

MBR

BR2

BR3

0

100

200

300

400

500

600

700

0:002:00

4:006:008:00

10:0012:0014:0016:0018:0020:0022:00

0:00

House CO Concentration (μL/L)

LFK

MBR

BR2

BR3

0

100

200

300

400

500

600

700

0:002:004:006:008:00

10:0012:0014:0016:0018:0020:0022:00

0:00

House CO Concentration (μL/L)

LFK

MBR

BR2

BR3
84

 

Table 23 Simulation results for utility source cases

Sim

ID

Source 

location 

Generator Utility 

door 

Weather Utility Room 

Peak CO

Concentration 

(excluding 

initial spike)

(μL/L)

House Peak CO

Concentration, 

(excluding 

utility room) 

(μL/L)

25 utility SO1 cat closed cold 2200 280

26 utility SO1 cat closed mild 2500 460

27 utility SO1 cat closed hot 3500 520

28 utility SO1 cat open cold 1200 330

29 utility SO1 cat open mild 1400 490

30 utility SO1 cat open hot 1500 600

31 utility SO1 

noncat 

closed cold 6000 950

32 utility SO1 

noncat 

closed mild 6600 1500

33 utility SO1 

noncat 

closed hot 7500 1900

34 utility SO1 

noncat 

open cold 3500 1100

35 utility SO1 

noncat 

open mild 3700 1600

36 utility SO1 

noncat 

open hot 4100 2000

Closed bedroom door cases

The cases presented in Tables 22 and 23 all included open bedroom doors, which results 

in significant mixing between house zones. Selected cases were also simulated with 

closed bedroom doors to examine the impact of this factor, in part to understand the CO 

exposure for cases in which occupants sleep with their bedroom doors closed. 

Figure 41 presents the simulation results for mild ambient temperature case with Gen 

SO1 with cat muffler operating in the garage with closed bay door, open house door, and 

closed bedroom doors. As expected, this case results in far less uniform house zone 

concentrations than the open bedroom door cases, which range from a peak of 20 μL/L in 

the BR2 zone to 130 μL/L in the LFK zone. 

Table 24 presents the peak house CO concentration for all 6 closed bedroom door cases 

which show an increase in peak concentration of up to about 30 % in the LFK zone 
85

compared to the equivalent cases with open bedroom doors; however, the bedrooms had 

generally lower concentrations, with the impact ranging from about an 80 % decrease to 

about a 2 % increase.

Figure 41 CO concentrations for garage source, closed bay door, open house door, mild 

ambient temperature, cat muffler with closed bedroom doors 

Table 24 Simulation results for closed bedroom door cases

Sim

ID

Source 

location 

Generator Bay or 

Utility 

door 

House 

door 

Weather House Peak CO

Concentration 

(excluding 

utility room) 

(μL/L)

37 garage SO1 cat closed open mild 130

38 garage SO1 cat closed closed cold 40

39 garage SO1 

noncat 

open open hot 160

40 garage SO1 

noncat 

closed open mild 450

41 utility SO1 cat open NA mild 490

42 utility SO1 

noncat 

closed NA cold 1000

Discussion of Simulation Results

A set of 42 simulations were performed to examine the potential performance of the 

prototype generator with and without the catalyst in the muffler under a wider range of 

0

20

40

60

80

100

120

140

160

180

200

0:002:004:006:008:00

10:0012:0014:0016:0018:0020:0022:00

0:00

House CO Concentration (

μL/L)

LFK

MBR

BR2

BR3
86

conditions than studied during the experiments at the test house. All of the simulations 

were based on the NIST manufactured test house model and the reduced CO emission 

prototype generator, which was extensively validated as described above. Model 

parameters that were varied included ambient conditions, CO emission rates, source 

locations and door positions. 

As expected the highest house CO concentrations were found for the generator with 

noncat muffler operated in the utility room of the house, with indoor CO concentrations 

reaching 2000 μL/L for one case. Operation of the generator with cat muffler in the utility 

room substantially reduced CO concentrations, however, they still reached high levels of 

280 μL/L to 600 μL/L. The lowest indoor CO concentrations resulted from operation of 

the cat muffler generator in the garage with peak house CO concentrations as low as 

10 μL/L (after an initial spike on start-up). Simulations also showed that, as expected, 

closed bedroom doors resulted in less uniform indoor concentrations and higher peak 

concentrations in the LFK zone (up to a 30 % increase) and typically lower peak 

concentrations in the bedrooms (up to an 80 % decrease). 
87

Conclusion

Under an interagency agreement with CPSC, NIST conducted a series of tests to 

characterize the indoor time course profiles of CO concentrations resulting from portable 

generators. These tests include both unmodified as well as modified low CO emission 

prototype configurations, operating in the attached garage of a home under various use 

and environmental conditions, and were conducted so CPSC staff could analyze the 

safety implications of operating the generators under these conditions. NIST used those 

test data to validate the ability of the CONTAM model to predict CO levels in the garage 

and the house and to develop an estimate of the uncertainty of its predictions relative to 

the measured values. NIST also conducted tests with the generators operating in a onezone shed to derive their CO emission and O2 consumption rates, to be used as input to 

their model validation effort as well as for simulations conducted to examine the potential 

performance of the low CO-emission prototype under a wider range of operating 

conditions. 

Shed Tests 

Very limited study has been conducted directly on CO emission and O2 consumption 

rates associated with gasoline-powered generators running indoors. Brown (2006) studied 

the CO emission rates from four different commercially-available generators in an 

enclosed experimental chamber, where air temperature and air change rate were 

controlled to provide different operating conditions. However, the air change rates were 

generally quite high compared with typical residences. Operating a generator in an 

enclosed space such as a garage or a storage shed, as opposed to a laboratory chamber, 

will be subject to uncontrolled temperatures and to lower ventilation rates determined by 

ambient weather conditions. To determine generator CO emission rates, tests were 

conducted in a single-zone shed on generators operating in the unmodified carbureted 

configuration as well as in the low CO emission prototype configuration. A literature 

search did not reveal previous studies on CO emission from generators in real conditions, 

where O2 levels can become significantly lower than ambient, and thereby impact CO 

emission. 

For two different unmodified generators (i.e., without CO emission controls), it was 

found that CO emissions ranged from a low of around 500 g/h at near ambient O2 levels

to a high of nearly 4000 g/h as O2 approach 17 %. The rates of CO generation and O2

consumption in these unmodified generators were affected by multiple parameters, with 

the O2 level in the space and the actual electrical output of the generator being two of the 

most important. Tests performed below 17 % O2 showed a drop off in CO emissions due 

to poor engine performance under these conditions. Tests of two modified low CO 

emission prototype generators (i.e., with CO emission controls) showed reductions of CO 

emissions of over 90 % depending on the specific emission controls and operating 

conditions and no trend toward higher emission rates was seen as O2 levels dropped to 

18 %. 

Garage Tests

A series of tests were also conducted to measure the emission and transport of CO due to 

operating portable gasoline-powered generators in an attached garage. This series of tests 
88

included both unmodified and UA-modified prototype generators operated in the garage 

attached to NIST’s manufactured test house. Testing was conducted under seven different 

test house/garage configurations to evaluate their impacts on the buildup of CO in the 

garage and its transport into the different rooms in the house. These configurations 

included two different garage bay door positions (fully closed or open 0.6 m), two 

connecting door settings between the garage and the family room (fully closed or open 

5 cm), and two house central heating, ventilating, and air conditioning (HVAC) fan

settings (on or off). CO concentrations varied widely with peak house CO concentrations 

ranging from under 10 μL/L to over 10,000 μL/L. As expected, the highest 

concentrations resulted from operation of the unmodified generator in the garage with the 

bay door closed and the house access door open. The lowest concentrations resulted from 

operation of a reduced-emission prototype in the garage with the bay door open and the 

house access door closed. 

These garage tests documented reductions of 85 % to 98 % in CO concentrations due to 

emissions from two modified, prototype low CO-emission portable generators compared 

to a “stock” unmodified generator. Note that these results apply to the specific units 

tested and that other units, modifications and test conditions may produce different 

results.

Simulations 

A set of 42 simulations were performed with the NIST CONTAM model to examine the 

potential performance of the prototype generators under a wider range of conditions than 

studied during the experiments at the test house. All of the simulations were based on the 

NIST manufactured test house model and the tested reduced CO emission prototype 

generator. Model parameters that were varied included ambient conditions, CO emission

rates, source locations and door positions. 

An extensive model validation effort was first carried out using results from seven garage 

tests to establish the ability of the CONTAM model to predict CO levels in the garage 

and the house and to develop an estimate of its uncertainty of these predictions relative to 

the measured values. An absolute validation of a complex model, such as CONTAM, is 

impossible as there are infinite possible building models that can be created by a user. For 

the cases modeled in this effort, no significant errors in the CONTAM model were 

identified. The agreement between the measurements and predictions of the O2

concentrations in the garage and of the average CO calculated for the house zones for the 

collective set of data was excellent. All of the calculated statistical values met the ASTM 

D5157 criteria. The agreement, however, was somewhat worse for the garage CO 

concentrations, with some parameters falling slightly outside the ASTM criteria limits. 

Overall, the average individual house zone and garage CO concentration predictions and 

measurements were within about 20 % and 30 % respectively when averaged over all 

cases.

As expected, the highest house CO concentrations were found for the generator with 

noncat muffler operated in the utility room of the house, with indoor CO concentrations 

exceeding 2000 μL/L for some cases. Operation of the generator with cat muffler 
89

substantially reduced CO concentrations; however, they still reached levels of 280 μL/L

to 600 μL/L. The lowest indoor CO concentrations resulted from operation of the 

generator with cat muffler in the garage with CO concentrations in the house reaching 

10 μL/L to 160 μL/L. Simulations also showed that, as expected, closed bedroom doors 

resulted in less uniform indoor concentrations and higher peak concentrations in the LFK 

zone (up to a 30 % increase) and typically lower peak concentrations in the bedrooms (up 

to an 80 % decrease).

Acknowledgements

This research was supported by the U.S. Consumer Product Safety Commission (CPSC) 

under interagency agreement CPSC-I-06-0012. The authors would like to acknowledge 

the contributions of Donald W. Switzer, Janet Buyer, Christopher J. Brown, Sandy 

Inkster, Han Lim and Susan Bathalon from CPSC, and Gregory T. Linteris and Steven J. 

Nabinger from NIST.

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Brown, C. J. 2006. Engine-drive Tools, Phase 1 Test Report for Portable Electric 

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notice of proposed rulemaking (ANPR); U.S. Consumer Product Safety Commission: 

Bethesda, MD, 2006; p 295. 
90

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Commission: Bethesda, MD, November, 2007; p 140. 

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Review” (2001) ASHRAE Transactions Vol. 107.2. 49. 

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Attached Garages to Residential Living Spaces-Literature Review and Field Test (2003) 

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Emmerich, S. J., & Persily, A. K. (1996). Multizone modeling of three residential indoor 

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Predictions for Tracer Gas in a Townhouse (2004) Building Services Engineering 

Research and Technology, Vol. 25 (4). 

Emmerich, S. and Wang, L. 2011. Measured CO Concentrations at NIST IAQ Test 

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Tools, 1999-2011; U.S. Consumer Product Safety Commission: Bethesda, MD. 
91

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Consumer Products, 2008 Annual Estimates, U.S. Consumer Product Safety 

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Generation, Dispersion and Exposure from Indoor Operation of Gasoline-powered 

Electric Generators under Actual Weather Conditions. Building and Environment 56: 

283-290. 
92

Appendix A Uncertainty Analysis of Shed Measurements

This Appendix presents the uncertainty analysis method for calculation of the CO 

emission and O2 consumption rates determined from testing the generators in the test 

shed. Sample calculations are presented in detail. 

The uncertainty of Aout can be expressed as:

2A1AK

V

Q

1t2t

(lnC ln )C A d,SF

s

1t,SF 2t,SF gen,in

out 6

6 6 + += −

− =

So the propagation of uncertainty for Aout

 u(A1, 2 (A2)u (A1)u )(Au A2) 2 2

out

2

c ++=

u(A1) is calculated from

2/1)2N(

r(tan(arccos (ln )))t,C |1A|)1A(u − =

r(lnC, t) is the correlation coefficient of lnC and t. 

From our pretests of checking SF6 decomposition, the covariance of A1 and A2

 u(A1, A2) −= 0.07341%

),(

2

)()()()()2( 2 ,,

,,

,

, 22

,

2 , 22

6

6

6

6

gen in dSF

s

dSF gen in

dSF

s

gen in

gen in

s

dSF KQu

V

QK Ku

V

Q Qu V

K

Au = + +

K 6 d,SF = -4 %, which were obtained from our pretests of the SF6 decomposition. 

To get the uncertainty of Qgen,in, we can look at the uncertainty propagation of Eq. (6) 

)(u) 2

RPM D()Q(u gen

22

gen,in = gen η

η gen is 85 % ± 5 % (Luce et al. 2006) so based on Eq. (A-7) of NIST uncertainty 

guidance(Taylor et al. 1994), we have 

%89.2

3

%5

u η gen )( ==

Then 99.0)(Qu gen,in = m3/h. If we assume 0),( , , 6 gen in KQu dSF = , then 
93

0301.0K

V

Q 2A d,SF

s

gen,in

6 = −=

)K(u) V

Q

()Q(u) V

K

()2A(u d,SF

22

s

gen,in

gen,in

22

s

2 d,SF

6

6 = +

= 0131.0)2A(u

Example of Aout Uncertainty Calculation

To calculate the uncertainty of air change rate in Test 1 of Table 2: 

 

)1A(u = 0.18

 u(A1, A2) −= % 0.07341 

= 0131.0)2A(u

 

So u(A1, 2 (A2)u (A1)u )(Au A2) 2 2

outc ++= = 0.2. For a confidence level of 95%, select 

k = 2, then )(Auk outc ⋅ = 0.4, Aout = 6.5 ± 0.4 hr-1

Uncertainty of CO Emission Rate

)C,C,A,(f e1

eCC

S VA )2t1t(A CO,in out CO 2t, CO 1t,

)2t1t(A

CO 2t, CO 1t,

CO CO,in sout out

out

ρ = ρ −

− = −

−

The uncertainty propagation for SCO was estimated as

)C(u) C

f ()C(u) C

f ()A(u) A

f ()(u) f ()S(u CO 1t,

22

CO 1t,

CO 2t,

22

CO 2t,

out

22

out

CO,in

22

CO,in

CO

2

c ∂

∂

+

∂

∂

+

∂

∂

+

∂

∂ = ρ

ρ

where

CO,in

CO

)2t1t(A

)2t1t(A

CO 2t, CO 1t,

sout

CO,in

S

e1

eCC

VA f

out

out

ρ ρ = −

− = ∂

∂ −

−

] )e1(

)eCC(e)2t1t(

e1

e)2t1t)(C( [VA A

S

A

f

)2t1t(A 2

)2t1t(A

CO 2t, CO 1t,

)2t1t(A

)2t1t(A

)2t1t(A

CO 1t,

CO,in sout

out

CO

out

out

out out

out

out

−

− −

−

−

−

− − +

−

− − +=

∂

∂

ρ
94

CO,in sout )2t1t(A

CO 2t, out e1

1 VA

C

f − − = ∂

∂

ρ

)2t1t(A

)2t1t(A

CO,in sout

CO 1t, out

out

e1

e VA

C

f −

−

− −= ∂

∂

ρ

To account for the uncertainty of using averaged density during ∆t

2/12

CO,in

N

1i

CO,in CO,in i, ( ]) )1N(N

1

ρ [)(u ∑ ρρ =

− − =

)C(u CO 2t, and CO 1t, )C(u were obtained from calibration curves of the CO analyzer as 

follows. 

Suppose the regression function from a calibration curve is 

⋅+= X2B1BY

where X is the expected value, for example, a known concentration of a gas cylinder, and 

Y is the measured value during calibration. 

During a formal test, if Y is the measured raw data

c −= 2B/)1BY(X

Where Xc is the corrected data by using the calibration curve, whose uncertainty can be 

determined from: 

)2B,1B(uCC2)2B(uC)1B(uC)Y(uC)X(u 2B1B

2 2

2B

2 2

1B

2 2

c = Y + + +

where 

2B

1

Y

X C c

Y = ∂

∂ =

2B

1

1B

X C c

1B −= ∂

∂ =

2

c

2B 2B

)1BY(

2B

X C − −= ∂∂ =
95

3

a )Y(u = , which can be either determined as a Type B uncertainty from a, the 

accuracy of the analyzer, or as a standard deviation for a Type A uncertainty if multiple 

observations of Y are available.

)2B(uX)2B,1B(u 2 −=

where X is the averaged X of the expected value during a calibration.

An Example of CO Emission Rate Uncertainty Calculation

The maximum CO emission rate of Test 1 in Table 2 is 900 g/h. 

CO,in

f

∂ρ

∂

= 0.81; Aout

f

∂

∂

= 

0.10; CCO 2t,

f

∂

∂

= 531.14; CCO 1t,

f

∂

∂

= -337.59; ρ CO,in )(u = 0.001

From a calibration curve of the CO analyzer for this test, CO 1t, )C(u = 91.03 mg/m3

; 

)C(u CO 2t, = 88.09 mg/m3

; So the uncertainty of SCO when SCO = 878 g/h is

)C(u) C

f ()C(u) Cf ()A(u) Af ()(u) f ()S(u CO 1t,

22

CO 1t,

CO 2t,

22

CO 2t,

out

22

out

CO,in

22

CO,in

c CO ∂

∂

+

∂

∂

+

∂

∂

+

∂

∂ = ρ

ρ

≈ 60 g/hr

For a confidence level of 95 % and k = 2, SCO = 900 ± 120 g/h 

Uncertainty of O2 Consumption Rate 

)K,C,C,A,(fC K

VA

e1

eCC

S VA out,o ,O in ,m1t,O2t,Oout

,m

soutout,O

)2t1t(A

)2t1t(A

1t,O2t,O

,OO in sout 2 2 2 2

2

out

out

2 2

2 2 ρ

ρ

ρ

ρ

ρ − = −

− = −

−

The uncertainty propagation for O2 S was

)S(u) S

f ()K(u) Kf (

)C(u) C

f ()C(u) Cf ()A(u) Af ()(u) f ()S(u

gen

22

gen

,m

22

,m

1t,O

22

1t,O

2t,O

22

2t,O

out

22

out

,O in

22

,O in

O

2

c 2

2

2

2

2

2

2

∂

∂

+

∂

∂

+

∂

∂

+

∂

∂

+

∂

∂

+

∂

∂ =

ρ

ρ

ρ

ρ

where

)2t1t(A

)2t1t(A

1t,O2t,O

sout

,O in out

out

2 2

2 e1

eCC

VA f −

−

−

− = ∂

∂

ρ
96

ρ

ρ

ρ ρ

,m

out,osout,O

)2t1t(A 2

)2t1t(A

1t,O2t,O

)2t1t(A

)2t1t(A

)2t1t(A

1t,O

,O in sout )2t1t(A

)2t1t(A

1t,O2t,O

,O in s

out

K

CV

] )e1(

)eCC(e)2t1t(

e1

e)2t1t)(C( [VA e1eCC

V

A

f

2 2

out

out

2 2

out

out

out

2

out 2

out

2 2

2

− −

− − +

−

− − + −− = ∂

∂

−

− −

−

−

−

−

,O in sout )2t1t(A

2t,O

2 out

2 e1

1 VA

C

f − − = ∂∂

ρ

)2t1t(A

)2t1t(A

,O in sout

1t,O out

out

2

2 e1

e VA

C

f −

−

− −= ∂

∂

ρ

out,o

,m2

soutout,O

,m

2

2 C

K

VA

K

f

ρ ρ

ρ

= ∂

∂

2/12

,O in

N

1i

,O in ,O in i, ( ]) )1N(N

1 [)(u 2 2 2 ρ ∑ ρρ =

− − =

2 2 1/2

out )u(A [u ++= A2)] u(A1, 2 (A2)u (A1)

2t,O )C(u 2

 and 1t,O )C(u 2 can be obtained using the same method as )C(u CO 2t, and 

CO 1t, )C(u . 

m,in

air out, K ,m ρ

ρ

ρ =

2/12

,m

N

1i

,m i,,m ])KK( )1N(N

1

ρ [)K(u ∑ ρ ρ

=

− − =

Example of O2 Consumption Rate Uncertainty Calculation

The maximum O2 consumption rate of Test 1 in Table 2 is 4800 g/hr. 

2 ,O in

f

∂ρ

∂

= 55.79; 

Aout

f

∂

∂

= -0.69; C 2 2t,O

f

∂

∂

= 743.30; C 2 1t,O

f

∂

∂

= -395.40; K ,m ρ

f

∂

∂

= 65.41; ,O in )(u 2 ρ = 

4.6×10-4; 2t,O )C(u 2

= 1t,O )C(u 2= 0.01% 

When neglecting the added fuel from the generator to the shed, gen = 0S
97

)K(u) K

f (

)C(u) C

f ()C(u) C

f ()A(u) A

f ()(u) f ()S(u

,m

22

,m

1t,O

22

1t,O

2t,O

22

2t,O

out

22

out

,O in

22

,O in

O

2

c 2

2

2

2

2

2

2

ρ

ρ

ρ

ρ

∂

∂

+

∂

∂

+

∂

∂

+

∂

∂

+

∂

∂ =

)S(u Oc 2

≈ 170 g/h. For a confidence level of 95% and k = 2, O2 S = 4800 ± 340 g/h
98

Appendix B Summary of Instrument Calibrations

This table summarizes the calibrations of the CO and O2 analyzers covering the testing 

periods included in this report. The table includes the date of the calibrations, the 

standard error for each instrument channel for each calibration, and the average standard 

error and the average standard error relative to the full scale for each device based on all 

of the calibrations. Not all analyzer channels were calibrated on each date due to 

instrument failure or other issues. Table 3 in the report describes which instrument was 

used for each test, and the instruments are described in the Instrumentation section of the 

main body. For comparison, the manufacturer’s stated accuracy for all of these analyzers 

is 1 % of full scale.

N2 N1 N2 N2 N1 N1 Nova3 TE RM

Date O2 O2 hi CO lo CO hi CO lo CO CO CO CO

std 

error

(%)

std 

error

(%)

std 

error

(%)

std 

error

(%)

std 

error

(%)

std 

error

(%)

std 

error

(ppm)

std 

error

(ppm)

std 

error

(ppm)

3/17/2008 0.0105 0.0191 0.0160 0.0036 0.0096 0.0056 NA NA

4/17/2008 0.0203 0.0243 NA NA NA 0.0094 26.3 NA

4/21/2008 0.482 0.0290 0.0107 0.0033 0.0033 0.0072 23.4 NA NA

4/29/2008 0.0317 0.0299 0.0090 0.0035 0.0026 0.0031 18.1 NA NA

5/5/2008 0.0210 0.0344 0.0052 0.0035 0.0028 0.0056 18.1 NA NA

5/13/2008 0.0255 0.0794 0.0397 0.0229 0.0074 0.0094 10.8 23.0 NA

5/21/2008 0.0192 0.0305 0.0026 0.0059 0.0062 0.0094 26.0 18.0 NA

6/2/2008 0.0551 0.0225 0.0108 0.0074 0.0065 0.0035 NA NA NA

6/10/2008 0.0140 0.0298 0.0086 0.0108 0.0081 0.0155 44.4 NA NA

3/17/2010 0.239 NA 0.0090 NA 0.0070 0.0045 14.4 NA NA

4/9/2010 0.0543 NA 0.0029 0.0065 0.0091 0.0067 13.8 NA 0.387

4/28/2010 0.0625 NA 0.0056 0.0004 0.0028 0.0003 11.0 NA NA

5/12/2010 0.0798 NA 0.0088 0.0253 0.0028 0.0057 6.87 NA 3.22

5/27/2010 0.0745 NA 0.0144 0.0215 0.0076 0.0225 11.6 NA 4.62

7/1/2010 0.0443 NA 0.0086 0.0123 0.0447 0.0056 15.9 NA 6.36

Average of 

all 

calibrations

0.0822 0.0332 0.0108 0.0098 0.0086 0.0076 18.5 20.5 3.65

Percent of 

full scale

0.33 0.13 0.36 1.08 0.29 0.85 1.03 2.27 0.41
99

Appendix C Additional Garage Tests

Table C1 provides a complete listing of all tests of the portable generators conducted in 

the test house garage. The results of these tests were not included in the interim report 

(Emmerich and Wang 2011), but are provided here to support additional analysis of the 

results. Plots of the test results that were not included in the main body of this report are 

included in Figures C1 through C27. The tests are grouped by test house configuration 

(see Table C2 for description of test house configurations). 
100

Table C1 Additional Tests Conducted in Attached Garage

Generator Test 

ID

Date Load 

Profile

Test 

house 

Config

Outdoor 

Temp 

(°C)

Wind 

speed 

(m/s)

Approx. 

Run 

Time (h)

Garage Peak 

CO 

Concentration 

(μL/L)

Lowest 

Garage O2

Concentration 

(%)

Automoatic 

shut-off

activated 

Notes

unmod 

GenX

B 4/22/08 cyclic 

profile

1 20.1 6.5 3 19,500 17.1 NA Figure 13

unmod

GenX

E 5/1/08 cyclic 

profile

1 13.3 1.8 2 13,100 17.5 NA Figure C1

modGenX 

with cat

O 4/2/10 cyclic 

profile

1 22 6.5 4.5 3000 19.4 NA Figure 14

SO1 with 

catmuffler 

and 

algorithm 

disabled

N 4/1/10 cyclic 

profile 

1 19.9 6.3 2 300 19.4 NA Figure 15

SO1 with 

catmuffler 

and 

algorithm 

enabled

L 3/25/10 cyclic 

profile 

1 21.3 8.4 0.5 420 20.5 Yes Not 

plotted 

due to 

instrument 

error.

SO1 with 

catmuffler 

and 

algorithm 

enabled

M 3/31/10 cyclic 

profile 

1 15.1 10.0 0.8 370 20.3 Yes Figure C2

Gen B P 4/6/10 cyclic

profile

1 34.0 9.0 2 5500 19.8 NA Figure C3

Gen B AU 7/9/10 cyclic 

profile

1 32.0 6.5 1 3200 20.0 NA Figure C4

Gen B AG 5/10/10 500 W 1 17.5 8.6 2 5100 20.0 NA Figure C5
101

Generator Test 

ID

Date Load 

Profile

Test 

house 

Config

Outdoor 

Temp 

(°C)

Wind 

speed 

(m/s)

Approx. 

Run 

Time (h)

Garage Peak 

CO 

Concentration 

(μL/L)

Lowest 

Garage O2

Concentration 

(%)

Automoatic 

shut-off

activated 

Notes

SO1 with 

noncat 

muffler 

and 

algorithm 

enabled

AP 5/21/10 500 W 1 23.7 6.9 0.2 270 20.9 Y Figure C6

SO1 with 

noncat 

muffler 

and 

algorithm 

enabled

AV 7/9/10 500 W 1 35.1 6.3 2 

(manually 

shut off) 

270 20.3 No Figure C7

Gen B AI 5/14/10 5500 

W

1 20.8 7.5 1.1 5500 19.6 NA Figure C8

SO1 with 

noncat 

muffler 

and 

algorithm 

disabled

AR 5/21/10 5500 

W 

1 32.8 6.7 4 920 19.0 NA Figure C9

SO1 with 

noncat 

muffler 

and 

algorithm 

disabled

AS 6/10/10 5500 

W 

1 27.6 8.5 4 890 19.0 NA Figure

C10

SO1 with 

noncat 

muffler 

AL 5/19/10 5500 

W 

1 16.9 7.9 0.7 

(manually 

shut off)

810 20.1 No Figure

C11
102

Generator Test 

ID

Date Load 

Profile

Test 

house 

Config

Outdoor 

Temp 

(°C)

Wind 

speed 

(m/s)

Approx. 

Run 

Time (h)

Garage Peak 

CO 

Concentration 

(μL/L)

Lowest 

Garage O2

Concentration 

(%)

Automoatic 

shut-off

activated 

Notes

and 

algorithm 

enabled

SO1 with 

noncat 

muffler 

and 

algorithm 

enabled

AM 5/20/10 5500 

W 

1 24.0 7.7 7 min 720 21.0 Y Figure 

C12

SO1 with 

noncat 

muffler 

and 

algorithm 

enabled

AN 5/20/10 2500 

W 

1 28.5 6.6 13 min 400 21.1 Y Figure 

C13

unmod 

GenX

F 5/6/08 cyclic 

profile

2 22.8 7.7 4 1,500 20.5 NA Figure 16

unmod 

GenX

H 5/14/08 cyclic 

profile

2 23.2 4.0 4 1,200 20.8 NA Figure 

C14

modGenX R 4/12/10 cyclic 

profile

2 19.9 6.7 4 30 20.7 NA Figure 17

SO1 with 

catmuffler 

and 

algorithm 

disabled

T 4/14/10 cyclic 

profile 

2 12.7 6.9 3 300 20.7 NA Figure 18

SO1 with 

catmuffler 

and 

S 4/13/10 cyclic 

profile 

2 10.5 6.4 0.9 210 21.0 Y Figure

C15
103

Generator Test 

ID

Date Load 

Profile

Test 

house 

Config

Outdoor 

Temp 

(°C)

Wind 

speed 

(m/s)

Approx. 

Run 

Time (h)

Garage Peak 

CO 

Concentration 

(μL/L)

Lowest 

Garage O2

Concentration 

(%)

Automoatic 

shut-off

activated 

Notes

algorithm 

enabled

Gen B Q 4/7/10 cyclic 

profile

2 30.0 7.7 4 480 21.0 NA Figure

C16

unmod 

GenX

I 5/15/08 cyclic 

profile

3 22.8 7.4 4 18,600 17.5 NA Figure 19

unmod 

GenX

A 4/18/08 cyclic 

profile

3 24.5 2.5 1.5 9,200 18.6 NA Figure 

C17

unmod 

GenX

C 4/24/08 cyclic 

profile

3 22.2 2.9 4 21,000 17.0 NA Figure 

C18

SO1 with 

noncat 

muffler 

and 

algorithm 

disabled

Z 5/5/10 cyclic 

profile 

3 28.3 6.7 4.75 630 19.5 NA Figure 20

SO1 with 

noncat 

muffler 

and 

algorithm 

enabled

AA 5/6/10 cyclic 

profile 

3 27.6 9.0 1.6 1000 20.0 Y Figure 

C19

unmod 

GenX

J 5/21/08 cyclic 

profile

4 18.2 9.6 2.25 21,300 16.0 NA Figure 21

SO1 with 

catmuffler 

and 

algorithm 

disabled

W 4/29/10 cyclic 

profile 

4 17.8 9.5 6 960 18.2 NA Figure 22
104

Generator Test 

ID

Date Load 

Profile

Test 

house 

Config

Outdoor 

Temp 

(°C)

Wind 

speed 

(m/s)

Approx. 

Run 

Time (h)

Garage Peak 

CO 

Concentration 

(μL/L)

Lowest 

Garage O2

Concentration 

(%)

Automoatic 

shut-off

activated 

Notes

SO1 with 

catmuffler 

and 

algorithm 

enabled

X 4/30/10 cyclic 

profile 

4 24.7 7.1 0.6 730 19.8 Y Figure 

C20

unmod 

GenX

D 4/30/08 cyclic 

profile

5 12.2 8.2 2 23,000 below 16 NA Figure 23

SO1 with 

noncat 

muffler 

and 

algorithm 

disabled

AH 5/13/10 cyclic 

profile 

5 15.6 6.5 5 2,300 17.8 NA Figure 24

SO1 with 

noncat 

muffler 

and 

algorithm 

enabled

AB 5/7/10 cyclic 

profile 

5 21.6 6.6 1 1500 19.2 Y Figure 

C21

SO1 with 

noncat 

muffler 

and 

algorithm 

enabled

AQ 5/21/10 cyclic 

profile 

5 27.7 6.7 0.8 660 19.4 Y Figure 

C22

unmod 

GenX

G 5/7/08 cyclic 

profile

6 25.1 7 2 1,100 20.5 NA Figure 25

SO1 with 

catmuffler 

U 4/22/10 cyclic 

profile

6 20.4 7.8 2 260 20.9 NA Figure 26
105

Generator Test 

ID

Date Load 

Profile

Test 

house 

Config

Outdoor 

Temp 

(°C)

Wind 

speed 

(m/s)

Approx. 

Run 

Time (h)

Garage Peak 

CO 

Concentration 

(μL/L)

Lowest 

Garage O2

Concentration 

(%)

Automoatic 

shut-off

activated 

Notes

and 

algorithm 

disabled

SO1 with 

catmuffler 

and 

algorithm 

enabled

Y 4/30/10 cyclic 

profile 

6 28.3 7.8 0.5 250 21 Y Figure 

C23

unmod 

GenX

K 5/23/08 cyclic 

profile 

(high 

to low)

7 13.8 7 >2 680 20.4 NA Figure 27

SO1 with 

noncat 

muffler 

and 

algorithm 

disabled

V 4/23/10 cyclic 

profile 

(high 

to low)

7 15.8 6.5 >2 430 20.9 NA Figure 28

SO1 with 

noncat 

muffler 

and 

algorithm 

enabled

AC 5/7/10 cyclic 

profile 

7 24.6 6.5 10 min 520 21 Y (but not 

confirmed 

that it was 

algorithm) 

Not 

plotted as 

test was 

only 10 

min

SO1 with 

noncat 

muffler 

and 

algorithm 

AD 5/7/10 cyclic 

profile 

7 24.1 7.1 6 min 340 21 Y Not 

plotted as 

test was 

only 6 min
106

Generator Test 

ID

Date Load 

Profile

Test 

house 

Config

Outdoor 

Temp 

(°C)

Wind 

speed 

(m/s)

Approx. 

Run 

Time (h)

Garage Peak 

CO 

Concentration 

(μL/L)

Lowest 

Garage O2

Concentration 

(%)

Automoatic 

shut-off

activated 

Notes

enabled

SO1 with 

noncat 

muffler 

and 

algorithm 

enabled

AW 7/9/10 cyclic 

(high 

to low) 

7 29.3 6.3 1 290 21.1 Y Figure 

C27

SO1 with 

noncat 

muffler 

and 

algorithm 

enabled

AO 5/20/10 cyclic 

profile 

7 30.6 6.4 1.1 110 20.9 Y Figure 

C25 

Gen B AJ 5/19/10 5.5 kW 8 15.8 7.1 3 190 21.1 NA Figure 

C26

SO1 with 

noncat 

muffler 

and 

algorithm 

enabled

AK 5/19/10 5.5 kW 8 16.9 8.2 1.4 200 21.1 Y Figure 

C27 
107

Table C2 House Configurations 

Test house Configuration Garage bay door Garage to house entry door HVAC fan

1 Closed Open 5 cm OFF

2 Open 0.6 m Closed Off

3 Closed Open 5 cm On

4 Closed Closed On

5 Closed Closed Off

6 Open 0.6 m Open 5 cm On

7 Open 0.6 m Open 5 cm Off

8 Fully Open Open 5 cm Off

Figures C1a, C1b, and C1c show the results for Test E, which was a 2 h test of unmod 

Gen X in Configuration 1 (garage bay door closed, garage access door to house open 

nominally 5 cm, and the house central HVAC fan off) with the cyclic load profile (see 

Table 1 in the body of the report). Since it was a two hour test, the hourly cyclic load 

profile in Table 1 was repeated two times. At the end of the second cycle, the generator 

was stopped, and the garage was mechanically vented. 

Figure C1a CO and O2 concentrations in the garage and measured load for Test E

(unmod Gen X, Configuration 1)

 

Figure C1a shows the concentration of CO in the garage reached a peak of over 

13,000 μL/L (note that μL/L are equivalent to the commonly used unit for CO 

concentration of ppmv) and the volume fraction of O2 dropped by 3.5 % to nearly 17.5 % 

when the generator was stopped. It also shows that in the first load cycle, the delivered 

electrical load was less than the load bank settings for the two highest loads in the cycle, 

4500 W and 5500 W, which were applied when the oxygen was below 19 %. As the 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

2000

4000

6000

8000

10000

12000

14000

0 30 60 90 120

CO (

% Oxygen

μL/L) Load (W)

Time (min)

N1 HI-CO N1 LO-CO Delivered Load O2
108

oxygen continued to drop in the subsequent load cycles, the delivered power for these 

load points decreased further. 

Figures C1b and C1c show the CO concentration in six rooms of the test house (see 

Figure 2 for room locations) as measured on the ‘ppm range’ (where the CO 

concentration plot plateaus at the instrument’s 2000 μL/L limit) and ‘high range’ CO 

instruments, respectively. The CO reached a peak concentration of over 6000 μL/L in the 

family room, with peak concentrations in the other rooms ranging from about 4000 μL/L

to 6000 μL/L. The results of Test E were similar to those of Test B, described in the body 

of the report, which was the same generator under the same house configuration. 

Figure C1b CO (ppm range) concentrations in the house for Test E 

(unmod Gen X, Configuration 1)

0

1000

2000

3000

4000

5000

6000

7000

8000

0 30 60 90 120

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
109

Figure C1c CO (high range) concentrations in the house for Test E (unmod Gen X, 

Configuration 1) 

Figures C2a and C2b show the results for Test M, which was a shut-off test of Gen 

SO1with the shutoff algorithm enabled under test house configuration 1 and the cyclic 

load. The algorithm shut off the generator after approximately 0.8 h. A natural decay 

period of about 0.5 h was included after the generator was stopped, followed by 

mechanical venting. The results shown in Figures C2a and C2b for Test M, prior to 

generator shut-off, are similar to those in Figure 15 for Test N in the body of the report (a 

test of the same generator and house configuration but without the shut-off activated).

0

1000

2000

3000

4000

5000

6000

7000

8000

0 30 60 90 120

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
110

Figure C2a CO and O2 concentrations in the garage and measured load for Test M (Gen 

SO1, Configuration 1) 

Figure C2b CO concentrations in the house for Test M (Gen SO1, Configuration 1) 

Figures C3a, C3b (the CO concentration exceeded the limit of the ppm range instrument), 

and C3c show the results for Test P, which was a two hour test of Gen B in Configuration 

1 with the cyclic load. Figure C3a shows the concentration of CO in the garage reached a 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

1000

2000

3000

4000

5000

0 15 30 45 60 75

% Oxygen

CO (μL/L), Load (W)

Time (min)

N3 CO Delivered Load N2 O2

0

50

100

150

200

0 20 40 60

CO (

μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
111

peak of over 5500 μL/L and the volume fraction of O2 in the garage dropped to about 

19.8 % when the generator was stopped. 

Figure C3a CO and O2 concentrations in the garage and measured load for Test P (Gen 

B, Configuration 1)

 

Figure C3b CO (ppm range) concentrations in the house for Test P 

(Gen B, Configuration 1)

12

14

16

18

20

22

0

1000

2000

3000

4000

5000

6000

0 30 60 90 120

% Oxygen

CO (μL/L), Load (W)

Time (min)

N2 HI-CO N3 CO Delivered Load O2

0

500

1000

1500

2000

2500

3000

3500

4000

0 30 60 90 120

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
112

Figure C3c CO (high range) concentrations in the house for Test P 

(Gen B, Configuration 1) 

Figures C4a and C4b show the results for Test AU, which was a one hour test of Gen B 

in Configuration 1 with the cyclic load. Figure C4a shows the concentration of CO in the 

garage reached a peak of over 3000 μL/L and the volume fraction of O2 in the garage 

dropped to about 20 % when the generator was stopped. 

Figure C4a CO and O2 concentrations in the garage and measured load for Test AU

(Gen B, Configuration 1)

 

0

500

1000

1500

2000

2500

3000

3500

4000

0 30 60 90 120

CO (

μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM

12

14

16

18

20

22

0

1000

2000

3000

4000

5000

0 30 60

% Oxygen

CO (μL/L), Load (W)

Time (min)

N2 HI-CO N3 CO Delivered Load O2
113

Figure C4b CO (ppm range) concentrations in the house for Test AU 

(Gen B, Configuration 1)

Figures C5a and C5b show the results for Test AG, which was a 2 h test of Gen B in 

Configuration 1 with a constant 500 W load. The concentration of CO in the garage 

reached a peak of over 5000 μL/L and the volume fraction of O2 in the garage dropped to 

about 20 % when the generator was stopped. 

Figure C5a CO and O2 concentrations in the garage and measured load for Test AG

(Gen B, Configuration 1)

 

0

100

200

300

400

500

600

700

800

900

1000

0 30 60

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

0 30 60 90 120

% Oxygen

CO (μL/L), Load (W)

Time (min)

Delivered Load N2 HI-CO O2
114

Figure C5b CO concentrations in the house for Test AG 

(Gen B, Configuration 1)

Figures C6a and C6b show the results for Test AP, which was a shut-off test of Gen 

SO1with the shutoff algorithm enabled under test house configuration 1 and a constant 

500 W load. The algorithm shut off the generator after approximately 10 min. A natural 

decay period of about 20 min was included after the generator was stopped, followed by 

mechanical venting. 

0

500

1000

1500

2000

2500

3000

0 30 60 90 120

CO (μL/L)

Run Time (min)

BR3 BR2 LR MBR KIT FAM
115

Figure C6a CO and O2 concentrations in the garage and measured load for Test AP (Gen 

SO1, Configuration 1) 

Figure C6b CO concentrations in the house for Test AP (Gen SO1, Configuration 1)

Figures C7a and C7b show the results for Test AV, which was also a shut-off test of Gen 

SO1with the shutoff algorithm enabled under test house configuration 1 and a constant 

500 W load. The algorithm did not shut off the generator so it was stopped manually after 

2 h. 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

0 30

% Oxygen

CO (μL/L), Load (W)

Time (min)

N3 CO Delivered Load O2

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
116

Figure C7a CO and O2 concentrations in the garage and measured load for Test AV

(Gen SO1, Configuration 1) 

Figure C7b CO concentrations in the house for Test AV (Gen SO1, Configuration 1)

Figures C8a and C8b show the results for Test AI, which was a 1.1 h test of Gen B in 

Configuration 1 with a constant 5500 W load. The concentration of CO in the garage 

reached a peak of about 5500 μL/L and the volume fraction of O2 in the garage dropped 

to about 19.6 % when the generator was stopped. 

Figure C8a CO and O2 concentrations in the garage and measured load for Test AI (Gen 

B, Configuration 1)

12

14

16

18

20

22

0

100

200

300

400

500

600

700

800

900

1000

0 30 60

% Oxygen

CO (μL/L), Load (W)

Time (min)

N3 CO Delivered Load O2

0

20

40

60

80

100

120

140

160

180

200

0 30 60

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
117

 

Figure C8b CO concentrations in the house for Test AI (Gen B, Configuration 1)

Figures C9a and C9b show the results for Test AR, which was a 4 h test of Gen SO1 with 

test house configuration 1 and a constant 5500 W load. During this test, the load bank 

overheated multiple times (resulting in the temporary sharp drops in load seen in Figure 

9a) and, following the test, the exhaust manifold was found to be loose, which may have 

impacted the test results. 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

0 30 60

% Oxygen

CO (μL/L), Load (W)

Time (min)

Delivered Load NOVA2 ppm HI-CO O2

0

500

1000

1500

2000

2500

3000

0 30 60

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
118

Figure C9a CO and O2 concentrations in the garage and measured load for Test AR (Gen 

SO1, Configuration 1) 

Figure C9b CO concentrations in the house for Test AR (Gen SO1, Configuration 1)

Figures C10a and C10b show the results for Test AS, which was also a 4 h test of Gen 

SO1 with test house configuration 1 and a constant 5500 W load. As with Test AR, the 

load bank overheated multiple times during this test. The results of Test AR were very 

similar to those of Test AS. 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

0 30 60 90 120 150 180 210 240

% Oxygen

CO (μL/L), Load (W)

Time (min)

N3 CO Delivered Load O2

0

100

200

300

400

500

600

700

800

900

1000

0 30 60 90 120 150 180 210 240

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
119

Figure C10a CO and O2 concentrations in the garage and measured load for Test AS

(Gen SO1, Configuration 1) 

Figure C10b CO concentrations in the house for Test AS (Gen SO1, Configuration 1) 

Figures C11a and C11b show the results for Test AL, which was a test of Gen SO1with 

the shutoff algorithm enabled with test house configuration 1 and a constant 5500 W 

load. The algorithm did not shut off the generator before it was manually stopped and the 

test terminated after about 40 min when a generator circuit breaker tripped. 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

0 30 60 90 120 150 180 210 240

% Oxygen

CO (μL/L), Load (W)

Time (min)

N3 CO Delivered Load O2

0

100

200

300

400

500

600

700

800

900

1000

0 30 60 90 120 150 180 210 240

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
120

Figure C11a CO and O2 concentrations in the garage and measured load for Test AL

(Gen SO1, Configuration 1) 

Figure C11b CO concentrations in the house for Test AL (Gen SO1, Configuration 1) 

Figures C12a and C12b show the results for Test AM, which was also a test of Gen 

SO1with the shutoff algorithm enabled under test house configuration 1 and a constant 

5500 W load. The algorithm shut off the generator after about 7 min, which was followed 

by a 2 h natural decay period before mechanical venting of the house. 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

0 10 20 30 40

% Oxygen

CO (μL/L), Load (W)

Time (min) Manual shut-off

N3 CO Delivered Load O2

0

50

100

150

200

250

300

350

400

450

500

0 10 20 30 40

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
121

Figure C12a CO and O2 concentrations in the garage and measured load for Test AM

(Gen SO1, Configuration 1) 

Figure C12b CO concentrations in the house for Test AM (Gen SO1, Configuration 1) 

Figures C13a and C13b show the results for Test AN, which was a test of Gen SO1with 

the shutoff algorithm enabled under test house configuration 1 and a constant 2500 W 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

0 20 40 60 80 100 120

% Oxygen

CO (μL/L), Load (W)

Time (min) Manual shut-off

N3 CO Delivered Load O2

0

50

100

150

200

250

300

350

400

450

500

0 20 40 60 80 100 120

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
122

load. The algorithm shut off the generator after about 13 min which was followed by a 

1 h natural decay period before mechanical venting of the house. 

Figure C13a CO and O2 concentrations in the garage and measured load for Test AN

(Gen SO1, Configuration 1) 

Figure C13b CO concentrations in the house for Test AN (Gen SO1, Configuration 1) 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

0 10 20 30 40 50 60 70 80

% Oxygen

CO (μL/L), Load (W)

Time (min) Manual shut-off

N3 CO Delivered Load O2

0

50

100

150

200

250

300

350

400

450

500

0 10 20 30 40 50 60 70 80

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
123

Figures C14a and C14b show the results for Test H, which was a 4 h test of unmod Gen 

X with Configuration 2 (garage bay door open, garage access door to house closed, and 

the house central HVAC fan off). The results of Test H were similar to those for Test F 

which was the same generator and house configuration. The peak concentration occurred 

during the 1500 W setting and rose slightly in each load cycle, reaching a maximum 

concentration near 1200 μL/L in the fourth load cycle. For this test, the garage was not 

instrumented with a low concentration CO analyzer, and the instrument uncertainty is 

large relative to measured concentrations below 500 μL/L. During the course of this test, 

with the garage bay door open, the oxygen level dipped only slightly, and the delivered 

electrical output was consistent during each cycle, largely meeting the load bank setting 

with the exception of the 5500 W setting. 

Figure C14a CO and O2 concentrations in the garage and measured load for Test H 

(unmod Gen X, Configuration 2) 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

1000

2000

3000

4000

5000

6000

0 30 60 90 120 150 180 210 240

CO (

% Oxygen

μL/L) Load (W)

Time (min)

N1 HI-CO N1 LO-CO Delivered Load O2
124

Figure C14b CO concentrations in the house for Test H (unmod Gen X, Configuration 2) 

Figures C15a and C15b show the results for Test S, which was a shut-off test of Gen 

SO1with the shutoff algorithm enabled under test house configuration 2 and the cyclic 

load. The algorithm shut off the generator after approximately 0.9 h. The results shown in 

Figure C15 for Test S, prior to generator shut-off, are similar to those in Figure 18 in the 

body of the report for Test T (a test of the same generator and house configuration but 

without the shut-off activated).

0

10

20

30

40

50

60

70

80

90

100

0 30 60 90 120 150 180 210 240

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
125

Figure C15a CO and O2 concentrations in the garage and measured load for Test S (Gen

SO1, Configuration 2) 

Figure C15b CO concentrations in the house for Test S (Gen SO1, Configuration 2)

Figures C16a and C16b show the results for Test Q, which was a 4 h test of Gen B with 

the house in Configuration 2 under the cyclic load. Figure C16a shows the concentration 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

0 10 20 30 40 50 60

% Oxygen

CO (μL/L), Load (W)

Time (min) 

N3 CO Delivered Load O2

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
126

of CO in the garage reached a peak of near 500 μL/L and the volume fraction of O2 in the 

garage did not measurably change when the generator was stopped. 

Figure C16a CO and O2 concentrations in the garage and measured load for Test Q (Gen 

B, Configuration 2)

 

Figure C16b CO (ppm range) concentrations in the house for Test Q 

(Gen B, Configuration 2) 

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 30 60 90 120 150 180 210 240

% Oxygen

CO (μL/L), Load (W)

Time (min)

N3 CO Delivered Load O2

0

10

20

30

40

50

60

70

80

90

100

0 30 60 90 120 150 180 210 240

CO (μL/L)

Time (min)

BR3 BR2 FAM MBR KIT LR
127

Figures C17a and C17b show the results for Test A, which was a 1.5 h test of unmod Gen 

X in Configuration 3 (garage bay door closed, garage access door to house open two 

inches, and the house central HVAC fan on). These conditions are the same as Test I and 

the results of Test A were similar to the first 1.5 h of Test I (see Figure 19 in the body of 

the report). After the generator was manually stopped, the garage and house were 

mechanically vented. Figure C17a shows that the concentration of CO in the garage 

reached a peak of about 9200 μL/L and the concentration of O2 in the garage dropped to 

18.6 % when the generator was stopped. It also shows that in the first load cycle the 

delivered electrical output was less than the load bank settings for the two highest loads 

in the load cycle, 4500 W and 5500 W, which were applied as the oxygen was 

approaching 19 %. 

Figure C17a CO and O2 concentrations in the garage and measured load for Test A

(unmod Gen X, Configuration 3)

17

17.5

18

18.5

19

19.5

20

20.5

21

21.5

22

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 30 60 90

CO (

% Oxygen

μL/L) Load (W)

Time (min)

N1 HI-CO N1 LO-CO Delivered Load O2
128

Figure C17b CO (high range) concentrations in the house for Test A 

(unmod Gen X, Configuration 3)

Figures C18a and C18b show the results for Test C, which was a 4 h test of unmod Gen 

X in Configuration 3 followed by a 1 h natural decay. These conditions are the same as 

Test I and the results of Test C were similar to those of Test I (see Figure 19 in the body 

of the report). Figure C18a shows that the concentration of CO in the garage reached a 

peak of about 21,000 μL/L and the concentration of O2 in the garage dropped to 17.0 % 

when the generator was stopped. It also shows that during the first load cycle the 

delivered electrical output was less than the load bank settings for the two highest loads 

in the cycle, 4500 W and 5500 W, which were applied as the oxygen was around 19 %. 

As the oxygen continued to drop in the subsequent load cycles, the delivered power for 

these two load points decreased further. 

0

1000

2000

3000

4000

0 30 60 90

CO (μL/L)

Time (min)

BR3 BR2 LR MBR KIT FAM
129

Figure C18a CO and O2 concentrations in the garage and measured load for Test C

(unmod Gen X, Configuration 3)

Figure C18b CO (high range) concentrations in the house for Test C 

(unmod Gen X, Configuration 3)

Figures C19a and C19b show the results for Test AA, which was a shut-off test of Gen 

SO1with the shutoff algorithm enabled with test house configuration 3 and the cyclic 

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load. The algorithm shut off the generator after approximately 1.6 h. A natural decay 

period of about 1 h was included after the generator was stopped, followed by mechanical 

venting. The results shown in Figure C19 for Test AA, prior to generator shut-off, are 

similar to those in Figure 20 (in the body of the report) for Test Z (a test of the same 

generator and house configuration but without the shut-off activated) although the CO 

concentration reached a higher level during the initial spike for Test AA. 

Figure C19a CO and O2 concentrations in the garage and measured load for Test AA

(Gen SO1, Configuration 3) 

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Figure C19b CO concentrations in the house for Test AA (Gen SO1, Configuration 3) 

Figures C20a and C20b show the results for Test X, which was a shut-off test of Gen 

SO1with the shutoff algorithm enabled under test house configuration 4 and the cyclic 

load. The algorithm shut off the generator after approximately 0.8 h. A natural decay 

period of about 1.2 h was included after the generator was stopped, followed by 

mechanical venting. The results shown in Figure C20 for Test X, prior to generator shutoff, are similar to those in Figure 20 for Test Z (a test of the same generator and house 

configuration but without the shut-off activated).

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Figure C20a CO and O2 concentrations in the garage and measured load for Test X (Gen 

SO1, Configuration 4) 

Figure C20b CO concentrations in the house for Test X (Gen SO1, Configuration 4)

Figures C21a and C21b show the results for Test AB, which was a shut-off test of Gen 

SO1with the shutoff algorithm enabled under test house configuration 5 and the cyclic 

load. The algorithm shut off the generator after approximately 1 h. A natural decay period 

of about 1.1 h was included after the generator was stopped, followed by mechanical 

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venting. The results shown in Figure C21 for Test AB, prior to generator shut-off, are 

similar to those in Figure 24 (in the body of the report) for Test AH (a test of the same 

generator and house configuration but without the shut-off activated) although the initial 

spike in CO concentration in the garage was larger for Test AB. 

Figure C21a CO and O2 concentrations in the garage and measured load for Test AB

(Gen SO1, Configuration 5) 

Figure C21b CO concentrations in the house for Test AB (Gen SO1, Configuration 5) 

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Figures C22a and C22b show the results for Test AQ, which was also a shut-off test of 

Gen SO1with the shutoff algorithm enabled under test house configuration 5 and the 

cyclic load. The algorithm shut off the generator after approximately 0.8 h. A natural 

decay period of about 1.5 h was included after the generator was stopped, followed by 

mechanical venting. Figure C22a shows no initial spike in CO concentration in the garage 

for Test AQ because Test AQ was a warm start while Test AB was a cold start.

Figure C22a CO and O2 concentrations in the garage and measured load for Test AQ 

(Gen SO1, Configuration 5) 

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Figure C22b CO concentrations in the house for Test AQ (Gen SO1, Configuration 5) 

Figures C23a and C23b show the results for Test Y, which was a shut-off test of Gen 

SO1with the shutoff algorithm enabled under test house configuration 6 and the cyclic 

load. The algorithm shut off the generator after approximately 0.5 h, followed by 

mechanical venting. The results shown in Figure C23 for Test Y, prior to generator shutoff, are very similar to those in Figure 26 (in the body of the report) for Test U (a test of 

the same generator and house configuration but without the shut-off activated).

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Figure C23a CO and O2 concentrations in the garage and measured load for Test Y (Gen 

SO1, Configuration 6) 

Figure C23b CO concentrations in the house for Test Y (Gen SO1, Configuration 6)

Figures C24a and C24b show the results for Test AO, which was a shut-off test of Gen 

SO1with the shutoff algorithm enabled under test house configuration 7 and the cyclic 

load. The algorithm shut off the generator after approximately 1.1 h, followed by 

mechanical venting. The results shown in Figure C24 for Test AO do not show an initial 

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spike in garage CO concentration like that in Figure 28 (in the body of the report) for 

Test V (a test of the same generator and house configuration but without the shut-off 

activated) but with a somewhat smaller initial spike in concentration.

Figure C24a CO and O2 concentrations in the garage and measured load for Test AW

(Gen SO1, Configuration 7) 

Figure C24b CO concentrations in the house for Test AW (Gen SO1, Configuration 7) 

Figures C25a and C25b show the results for Test AO, which was a shut-off test of Gen 

SO1with the shutoff algorithm enabled under test house configuration 7 and the cyclic 

load. The algorithm shut off the generator after approximately 1.1 h, followed by 

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mechanical venting. The results shown in Figure C25 for Test AO do not show an initial 

spike in garage CO concentration like that in Figure C24 for Test AW (a test of the same 

generator and house configuration but loaded in a high to low cyclic pattern), likely 

because Test AO was a warm start. 

Figure C25a CO and O2 concentrations in the garage and measured load for Test AO

(Gen SO1, Configuration 7) 

Figure C25b CO concentrations in the house for Test AO (Gen SO1, Configuration 7) 

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Figures C26a and C26b show the results for Test AJ, which was a 3 h test of Gen B with 

the house in Configuration 8 with a 5500 W load. Figure C26a shows the concentration 

of CO in the garage reached a peak near 200 μL/L and the volume fraction of O2 in the 

garage did not measurably change during the test. 

Figure C26a CO and O2 concentrations in the garage and measured load for Test AJ

(Gen B, Configuration 8)

 

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Figure C26b CO concentrations in the house for Test AJ (Gen B, Configuration 8) 

Figures C27a and C27b show the results for Test AK, which was a shut-off test of Gen 

SO1with the shutoff algorithm enabled under test house configuration 8 and a 5500 W 

load. The algorithm shut off the generator after approximately 1.4 h, followed by 

mechanical venting. 

Figure C27a CO and O2 concentrations in the garage and measured load for Test AK

(Gen SO1, Configuration 8) 

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Figure C27b CO concentrations in the house for Test AK (Gen SO1, Configuration 8)

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