Combustion Safety Simplified 

Test Protocol Field Study 

L. Brand, D. Cautley, D. Bohac, 

P. Francisco, L. Shen, and S. Gloss

Partnership for Advanced Residential Retrofit and 

NorthernSTAR

November 2015
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States 

government. Neither the United States government nor any agency thereof, nor any of their employees, 

subcontractors, or affiliated partners makes any warranty, express or implied, or assumes any legal 

liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, 

product, or process disclosed, or represents that its use would not infringe privately owned rights.

Reference herein to any specific commercial product, process, or service by trade name, trademark, 

manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or 

favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency 

thereof.

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iii

Combustion Safety Simplified Test Protocol

Field Study

Prepared for: 

The National Renewable Energy Laboratory 

On behalf of the U.S. Department of Energy’s Building America Program 

Office of Energy Efficiency and Renewable Energy 

15013 Denver West Parkway

Golden, CO 80401 

NREL Contract No. DE-AC36-08GO28308 

Prepared by: 

L. Brand, D. Cautley, D. Bohac, P. Francisco, L. Shen, and S. Gloss 

Partnership for Advanced Residential Retrofit and NorthernSTAR

Gas Technology Institute 

1700 S. Mt. Prospect Road

Des Plaines, IL 60018 

NREL Technical Monitor: Stacey Rothgeb 

Prepared under Subcontract No. KNDJ-0-40346-05 

November 2015 
iv

The work presented in this report does not represent 

performance of any product relative to regulated 

minimum efficiency requirements.

The laboratory and/or field sites used for this work are 

not certified rating test facilities. The conditions and 

methods under which products were characterized for 

this work differ from standard rating conditions, as 

described.

Because the methods and conditions differ, the reported 

results are not comparable to rated product performance 

and should only be used to estimate performance under 

the measured conditions.
v 

Contents

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................. vii

Acronyms.................................................................................................................................................. viii

Acknowledgments ..................................................................................................................................... ix

Executive Summary.................................................................................................................................... x

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

2 Approach............................................................................................................................................... 3

3 Test Procedures.................................................................................................................................... 5

3.1 Test Procedures in Effect at the Time of Project Initiation......................................................... 12

3.1.1 National Fire Protection Agency 54-2012 (National Fuel Gas Code) ........................... 12

3.1.2 Air Conditioning Contractors of America QH12-2011.................................................. 12

3.1.3 Building Performance Institute-101 ............................................................................... 13

3.1.4 Standard Work Specifications-2011............................................................................... 13

3.2 Current Test Procedures.............................................................................................................. 14

3.2.1 National Fire Protection Agency w2qA 54-2015 (National Fuel Gas Code)................. 14

3.2.2 Air Conditioning Contractors of America QH12-2014.................................................. 14

3.2.3 Building Performance Institute-1200-2015.................................................................... 15

3.2.4 Standard Work Specifications-2015............................................................................... 16

3.3 Overall Alignment and Partnership for Advanced Residential Retrofit/NorthernSTAR

Involvement................................................................................................................................. 16

4 Field Data Collection and Analysis................................................................................................... 18

4.1 Test Plan...................................................................................................................................... 18

4.1.1 Must-Fail/Must-Pass Criteria ......................................................................................... 18

4.1.2 Three-Step Process......................................................................................................... 18

4.2 Field-Test Site Screening and Selection—Sampling Strategy .................................................... 19

4.3 Field Research Methods.............................................................................................................. 19

4.3.1 Screening Procedures..................................................................................................... 19

4.3.2 Additional Data Collected Postscreening....................................................................... 23

4.3.3 Short-Term Testing and Inspection Methods................................................................. 24

4.3.4 Monitoring System and Installation ............................................................................... 25

4.4 Field Research Results and Analysis........................................................................................... 29

4.4.1 Recruitment.................................................................................................................... 29

4.4.2 Screening and Selection ................................................................................................. 32

4.4.3 House Characteristics..................................................................................................... 33

4.4.4 Short-Term Testing and Inspection................................................................................ 38

4.4.5 Long-Term Monitoring .................................................................................................. 49

5 Survey Results from State Weatherization Agencies on Combustion Safety Field Data 

Collection ............................................................................................................................................ 69

5.1 Development of the Survey Instrument....................................................................................... 69

5.2 Survey Part I—Overview of States Programs............................................................................. 70

5.3 Survey Part II—Homes that Fail Spillage Tests.......................................................................... 72

5.4 Discussion and Conclusion ......................................................................................................... 74

6 Conclusions ........................................................................................................................................ 76

References................................................................................................................................................. 79

Bibliography .............................................................................................................................................. 80

Appendix A. Overview of Short-Term Testing Protocol........................................................................ 81

Appendix B. Monitoring System Parts.................................................................................................... 85

Appendix C. House Data Sample Photos ............................................................................................... 86

Appendix D. Combustion Safety Survey ................................................................................................ 90

Appendix E Logistic Model Definition and Detailed Output................................................................. 92
vi 

List of Figures

Figure 1. Checking spillage on a water heater............................................................................................ 20

Figure 2. CO thresholds.............................................................................................................................. 21

Figure 3. Field-test site selection flowchart................................................................................................ 23

Figure 4. Schematic view of monitoring system sensor placement at water heater.................................... 27

Figure 5. Monitoring system main circuit board and valves (yellow tops) in enclosure, cover removed .. 28

Figure 6. Water heater draft hood with copper tube for CO2 sampling and thermocouple temperature wire 

showing .................................................................................................................................. 28

Figure 7. Custom wireless sensor module (black box) with pressure sensing tube placed in dryer exhaust 

duct......................................................................................................................................... 29

Figure 8. Orphaned water heater................................................................................................................. 34

Figure 9. Common vented water heater...................................................................................................... 34

Figure 10. House MN 01 exhaust fan house depressurization (in Pa)........................................................ 41

Figure 11. House MN 02 exhaust fan house depressurization (in Pa)........................................................ 41

Figure 12. House MN 03 exhaust fan house depressurization (in Pa)........................................................ 42

Figure 13. House MN 04 exhaust fan house depressurization (in Pa)........................................................ 42

Figure 14. House MN 05 exhaust fan house depressurization (in Pa)........................................................ 43

Figure 15. House MN 06 exhaust fan house depressurization (in Pa)........................................................ 43

Figure 16. House MN 07 exhaust fan house depressurization (in Pa)........................................................ 44

Figure 17. House MN 08 exhaust fan house depressurization (in Pa)........................................................ 44

Figure 18. House MN 09 exhaust fan house depressurization (in Pa)........................................................ 45

Figure 19. House MN 10 exhaust fan house depressurization (in Pa)........................................................ 45

Figure 20. House WI 01 exhaust fan house depressurization (in Pa) ......................................................... 46

Figure 21. Comparison of the BPI protocol with the STP for measuring CAZ depressurization (in Pa)... 47

Figure 22. Comparison of the comprehensive protocol with the STP for measuring CAZ depressurization 

(in Pa)..................................................................................................................................... 47

Figure 23. Comparison of the modified protocol with the STP for measuring CAZ depressurization (in 

Pa) .......................................................................................................................................... 48

Figure 24. Comparison of the comprehensive protocol with the BPI protocol for measuring CAZ 

depressurization (in Pa).......................................................................................................... 48

Figure 25. Average number of water heater burner cycles per day, by house ............................................ 51

Figure 26. Water heater burner runtime distribution, by house .................................................................. 51

Figure 27. Conditions during characteristic water heater burner cycle, House MN 01. Time shown in 

hours, minutes, and seconds. A brief increase in CO2 concentration at the start of the cycle is 

evidence of spillage, but the four spillage temperatures in the bottom panel show no 

discernable rise....................................................................................................................... 52

Figure 28. Fan runtime and CAZ door closed time, by house .................................................................... 54

Figure 29. Conditions during long-lasting spillage episode, House WI 01. Time shown in hours, minutes, 

and seconds. A sustained elevation of CO2 concentration and obvious rise in all four spillage 

temperatures (bottom panel) provide evidence of prolonged spillage. .................................. 56

Figure 30. Number of water heater cycles lasting at least as long as time displayed, in 2-minute bins.

Cycles with spillage are differentiated by color..................................................................... 57

Figure 31. Logistic model fits for probability of spillage at selected outdoor temperatures and by time into 

firing cycle, by house ............................................................................................................. 60

Figure 32. Conditions during downdrafting episode, House MN 03.......................................................... 63

Unless otherwise noted, all figures were created by the Partnership for Advanced Residential 

Retrofit and NorthernSTAR. 
vii 

List of Tables

Table 1. Combustion Safety Testing Requirements from Multiple Standards in 2012 and 2015................. 6

Table 2. Combustion Appliance Safety Test Procedures Differences and Similarities................................ 7

Table 3. Range of Building Types for the Four Climates........................................................................... 19

Table 4. Monitored Parameters and Sensors............................................................................................... 26

Table 5. House Screening Results............................................................................................................... 32

Table 6. Building Characteristics of Participating Houses ......................................................................... 33

Table 7. Mechanical Systems of Participating Houses............................................................................... 35

Table 8. Combustion Vent Configuration: Orphaned Water Heaters......................................................... 35

Table 9. Combustion Vent Connector Configuration: Common Vented Water Heaters............................ 36

Table 10. Common Vent Configuration: Common Vented Water Heaters................................................ 36

Table 11. Combustion Air Configuration ................................................................................................... 36

Table 12. Clothes Dryer and Kitchen Exhaust Fan Characteristics............................................................ 38

Table 13. Bathroom Exhaust Fan Characteristics....................................................................................... 38

Table 14. Scheduled Visits of Screening and Short-Term Testing Visits................................................... 39

Table 15. House Interior Door Positions .................................................................................................... 40

Table 16. Comparison of CAZ Pressure Measurements from Four WC Protocols for Participating Houses, 

in Pa (SD = standard deviation)............................................................................................. 46

Table 17. House CVEP Measurements....................................................................................................... 49

Table 18. Data Collection Dates by House ................................................................................................. 50

Table 19. Summary of Air Handlers, Exhaust Devices, and CAZ Door Monitoring ................................. 53

Table 20. Water Heater Spillage Time by House ....................................................................................... 57

Table 21. Summary of Coefficients (odds ratio) from Logistic Regression Predictors for Spillage (Model 

1) ............................................................................................................................................ 59

Table 22. Summary of Coefficients (odds ratio) from Logistic Regression Predictors (binary indicators of 

device operation) for Water Heater Spillage (Model 2)......................................................... 59

Table 23. Incidence and Duration of Downdrafting by House ................................................................... 62

Table 24. Incidence and Duration of Downdrafting Episodes Coinciding with Water Heater Burner 

Operation................................................................................................................................ 62

Table 25. Explanatory Factors for Development of Downdrafting ............................................................ 65

Table 26. Observed CO Concentrations in CAZ Airspace by House ......................................................... 67

Unless otherwise noted, all tables were created by the Partnership for Advanced Residential 

Retrofit and NorthernSTAR.
viii

Acronyms

ACCA Air Conditioning Contractors of America

ANSI American National Standards Institute

BPI Building Performance Institute

CAZ Combustion Appliance Zone

CEE Center for Energy and Environment

CO Carbon Monoxide

CO2 Carbon Dioxide

CVEP Cold Vent Establishment Pressure

DOE U.S. Department of Energy

LIWAP Low Income Weatherization Assistance Program

NFGC National Fuel Gas Code

NFPA National Fire Protection Association

NREL National Renewable Energy Laboratory

PARR Partnership for Advanced Residential Retrofit

RESNET Residential Energy Services Network

STP Simplified Test Procedure

SWS Standard Work Specifications

WAP Weatherization Assistance Program

WC Worst Case

Wx Weatherization Retrofit
ix

Acknowledgments

The authors would like to thank several key contributors to this significant effort. From the 

Minnesota Center for Energy and the Environment: Alex Haynor for his help installing the 

monitoring equipment in all the Minnesota locations, Kirk Kolehma for conducting the screening 

visits for the Minnesota houses and performing the short-term tests at all 11 monitored houses, 

and Jim Fitzgerald who provided guidance on test protocols from his vast experience and 

performed the appliance combustion vent and air sizing analysis. 

The researchers thank the survey participants who provided responses from Alaska, Arkansas, 

Colorado, Idaho, Illinois, Kansas, Minnesota, Missouri, New Hampshire, New Jersey, New 

Mexico, New York, Oregon, Texas, Virginia, Washington State, and Washington, D.C. The 

authors also thank all the weatherization agency professionals in the many states who looked for 

homes to participate in this study as well as those who responded to the project survey. The 

National Association for State Community Services Programs’ assistance in getting the word out

is gratefully acknowledged. The authors thank the staff of Lawrence Berkeley National 

Laboratory for their valuable contributions to the combustion safety research done by the 

Partnership for Advanced Residential Retrofit and NorthernSTAR teams during the past several 

years. A special thanks to the homeowners of the 11 participating homes for their patience and 

for the invaluable field data used in this project. 

Finally the authors offer a special thanks to Merry Sweeney from GTI who built this report with 

submissions from the many contributors in record time.
x 

Executive Summary

Combustion safety testing is an important step in the process of upgrading homes for energy 

efficiency. Field practitioners use several approaches based on published standards. Researchers 

have indicated that the test procedures in use are complex to implement in the field and provide 

too many false positives—i.e., too many failures that do not relate to long-term problems in the 

home. Failures often mean that money is diverted from energy-efficiency measures—or upgrades 

may not be made at all—if the program does not include remediation of safety issues. In this 

report the U.S. Department of Energy’s Building America research teams Partnership for 

Advanced Residential Retrofit and NorthernSTAR provide a simplified test procedure (STP) that 

is easier to implement and should produce fewer false positives. The report includes a survey of 

state weatherization agencies on combustion safety issues, details of a field data collection 

instrumentation package, summary of data collected over 7 months, data analysis, and results. 

The project provides several key results. State weatherization agencies do not generally track 

combustion safety failures. The data from those that do suggest limited actual evidence that 

combustion safety failures due to spillage from nondryer exhaust are common and that only a 

very small number of homes are subject to the failures. 

Based on sequential application of the tests in the field, the STP identifies problem houses as 

effectively as the worst-case procedures. 

The project team collected field data over a period of 7 months on 11 houses that indicated 

failures under the STP. Of these, 2 houses that demonstrated prolonged and excessive spillage 

were also the only 2 with venting systems that were out of compliance with the National Fuel 

Gas Code. The remaining homes experienced spillage that only occasionally extended beyond 

the first minute of operation. Combustion zone depressurization, outdoor temperature, and 

operation of individual fans all provide statistically significant predictors of the low level of 

spillage observed in these houses. 

The authors concluded that vent system inspection should be the primary element of any 

combustion safety evaluation, and auditors need appropriate training to recognize vent system 

deficiencies. Carbon monoxide monitoring is an important part of the test and should also be 

included. The STP is an improvement over the worst-case approaches, but it still overpredicts the 

occurrence of problem houses. More work is needed to establish a more-predictive short-term 

test method. 

These results are based on the 17 states that responded to the survey and the 11 houses 

instrumented in the project over 7 months. Technology transfer has already begun in the form of 

contributing many of the recommendations in the STP to the Building Performance Institute1200 standard that was published in the spring of 2015. 
1 

1 Introduction

Performing energy-efficiency upgrades in existing homes requires a number of steps to achieve 

the best performance and ensure continued occupant safety. The weatherization agency or home 

performance contractor will ensure safe operation at test-in before the work is done and again at 

test-out to confirm that the upgrades applied did not impact the safety of the home. Combustion 

safety is one of the elements evaluated by the agency or contractor.

Combustion safety is the general term for evaluating gas-fired water heaters, boilers, and 

furnaces for safe operation. Several approaches are used to: 

Inspect vents and heat exchangers for corrosion or cracking 

Assess whether the vent is installed according to code 

Measure draft hood spillage time 

Evaluate the carbon monoxide (CO) in both the appliance flue and in the occupied space. 

This report focuses on combustion safety test procedures in high-performance houses. 

High-performance homes follow the “seal tight and ventilate right” approach to air sealing 

before thermal envelope and equipment upgrades are applied. Sealing tight reduces the 

infiltration load on the heating and cooling system, but it also reduced the availability of 

combustion air. Unless appropriately sized combustion air openings are installed according to 

code, exhaust fans tend to depressurize the house, further reduce the availability of combustion 

air, and impede the formation of normal draft in vent systems. These fans are generally range 

hood exhausts, standard gas or electric clothes dryers, and, to a lesser extent, bath fans and 

unbalanced whole-house ventilation fans. Air handler blowers can and do create depressurization 

when they serve unbalanced areas such as bedrooms with supply but no return ducting (unless a 

transfer grille is used), when passing through space that does not reside fully within the pressure 

envelope (unconditioned space), and in some cases simply if the ductwork is not well sealed. 

Wood-burning fireplaces (which are beyond the scope of this study) may also have an impact on 

house depressurization levels. 

Field practitioners use several combustion safety test procedures, which are generally based on 

the training of the inspector. The Building Performance Institute (BPI), Residential Energy 

Services Network (RESNET), and the National Fuel Gas Code (NFGC) all provide combustion 

safety test procedures. Traditionally, the more commonly used procedures create worst-case 

(WC) conditions: draft hood spillage time is assessed when all exhaust fans are operating on high 

speed and all operable doors are adjusted such that the greatest negative pressure is developed 

near the gas appliance being tested. The inspector flags homes that fail the WC test; these homes 

may not be eligible for an energy-efficiency upgrade. Although the testing standards have 

recently moved away from WC conditions, few field data remain to correlate the test result to the 

actual combustion safety incidents in the home. In fact, the general assumption is that the 

combustion safety test is too severe and results in many false positives that are not supported by 

the actual operation of the house (Rapp et al. 2012; Cautley et al. 2012). The greatest needs are: 
2 

Hard data about field failures 

A simplified test procedure (STP) that is more predictive than the commonly used 

procedures of failure under real operating conditions in occupied homes 

Data from the field to verify the effectiveness of the proposed test procedure. 

The U.S. Department of Energy’s (DOE’s) Building America research team Partnership for 

Advanced Residential Retrofit (PARR) addressed combustion safety in a measure guideline: 

Combustion Safety for Natural Draft Appliances Using Indoor Air (Brand 2014). NorthernSTAR

produced a measure guideline: Combustion Safety for Natural Draft Appliances through 

Appliance Zone Isolation (Fitzgerald and Bohac 2014). These guidelines are based on current

codes and standards and building performance audit techniques that weatherization teams, gas 

utility inspectors, and building performance contractors use in the field. 

Improving the ability to identify combustion appliance hazards will allow DOE and Building 

America to conduct more home efficiency upgrades and make progress toward achieving the 

goal of 30% energy reduction in existing homes according to the 2009 International Energy 

Conservation Code standard.
3 

2 Approach

The approach to this project was to: 

Support an STP based on the field experience of the PARR and NorthernSTAR Building 

America teams. Specifically, the proposed STP is expected to: 

o Include a setup for depressurization testing with prescribed door positions. 

o Include the impact of leaky ducts in setting up depressurization testing (as part of 

evaluating the impact of the air handler operation). 

o Include the impacts of the dryer, range hood, whole-building ventilation system, 

and largest fan.

o Operate the largest fans at lower rates. 

o Use spillage test times that depend on the appliance and the season when the 

testing is being conducted. 

o Test for CO level in the appliance flue that exceeds the American National 

Standards Institute (ANSI) certification threshold limit for the type of appliance.

o Avoid draft pressure testing and failure criteria based on draft pressure. 

o Avoid the requirement that the technician drill a hole in the vent system.

Measure the house depressurization levels produced by a severe condition (must fail) and 

a less severe (must pass) condition in the field.

Collect and analyze continuous field data from homes that fail the most severe condition 

and pass the less severe condition. Use the field data to correlate the proposed test with 

conditions occurring in the houses. 

In parallel, survey state weatherization agencies to gather data about the frequency and 

severity of combustion safety failures and the approach to remediation. 

Participate in the 2015 update of the BPI-1200-S “Standard Practice for Basic Analysis of 

Buildings” to apply the concepts from this project. 

The scope of this project was limited to combustion safety for houses in which the appliances 

were installed within the pressure boundary of the space (includes basements unless isolated) 

that take air for combustion and dilution from inside the pressure boundary or from a 

combination of inside and outside the pressure boundary. 

This study also focused on nondryer exhausts and door positioning. Dryers and air handlers 

commonly operate at the same time as other combustion appliances, so combustion safety 

failures due to dryers or air handler operation were considered to be serious issues that should be 

addressed. Homes that failed due to either dryer or air handler operation were excluded from the 

study. However, failures due to all exhausts running at their highest flows and specific 

positioning of interior doors were considered less likely to all occur simultaneously and with 

other combustion appliance operation; therefore, this was the primary target scenario for this 

project.
4 

The following research questions were to be answered in this project: 

1. Does the STP accurately predict combustion safety hazards as determined by long-term 

monitoring? Does it predict hazards more accurately than the conventional protocol used 

by weatherization agencies? 

2. Is the STP practical for field implementation, and can it be improved? 

3. What is the incidence of spillage and combustion safety failures based on feedback from 

weatherization agencies and other field experience?
5 

3 Test Procedures

Combustion safety test procedures for existing buildings were highly varied at the time that the 

overall effort to simplify combustion safety testing started in earlier projects. Procedures that 

were simple were also inadequate for the range of conditions encountered in existing homes. 

Procedures that considered all the most vital potential problems were too complex. Even for 

metrics that were covered by multiple procedures, the values at which passage or failure was 

indicated were inconsistent. 

These standards have been substantially aligned, in part due to the contributions of PARR team 

members. This process has not always been simple or without controversy. However, at this time 

many (though not all) the simplifications proposed by the PARR and NorthernSTAR teams have 

been incorporated into these standards.

This section discusses four sources for combustion safety guidelines: 

NFPA 54 (the National Fuel Gas Code [NFGC]) 

Air Conditioning Contractors of America (ACCA) Standard QH12 

BPI Standards 101 and 1200 

The National Renewable Energy Laboratory’s (NREL’s) Standard Work Specifications 

(SWS) (NREL 2013). 

The section is divided into several parts: the standards as of PARR/NorthernSTAR project 

initiation, the standards today, and the involvement of PARR/NorthernSTAR team members in 

the evolution of these standards. Other standards, including derivatives implemented by specific 

programs, are not included Table 1 shows the various key requirements incorporated into each of 

these standards in 2012 and 2015. Table 2 shows the primary differences and similarities 

between combustion appliance safety test procedures. 

In Table 1, definitions of column headings are as follows: 

Org/Std-Yr: the organization, standard, and standard publication date 

Req.?: whether or not the organization requires that the testing be done 

Focus appliances: which appliances are covered by the standard

Combustion appliance zone (CAZ) door: whether the standard includes evaluation of the 

impact of the position of the door to the combustion appliance zone (Test) or has it in a 

specific position throughout (e.g., Closed) 

Other doors: whether the standard includes evaluation of the impact of positioning of 

other interior doors (Test each), does not require any evaluation (No), or specifies door 

positioning without further evaluation (Default) 

Air Handler: whether the standard sets maximum depressurization limits that indicate 

failure of the appliance regardless of actual spillage or draft problems

Spillage: allowable time to last spillage

Draft: whether a separate test for draft is conducted. 
6 

Table 1. Combustion Safety Testing Requirements from Multiple Standards in 2012 and 2015

Org/

Std-Yr Req.? Focus Appliances CAZ Door

Other 

Doors

Air 

Handlera Limits Spillage Draft

Requirements as of 2012

NFPA/NFGC-2012 No Existing furnaces and boilers Closed No No No 5 min No

ACCA/QH12-2011 Yes Existing appliances Test Test each No No 5 min No

BPI/101-2011 Yes Existing appliances Test Test each Yes Yes 1 min Yes

NREL/SWS-2011 Yes Existing appliances Test Test each Yes Yes 2 min No

Requirements as of 2015

NFPA/NFGC-2015 No Existing appliances Test No Yes No 5 min No

ACCA/QH12-2014 Yes Existing appliances Test Test each Yes No 5 min No

BPI/101-2015 Yes Existing appliances Test Default Yes No 2 or 5 minc No

NREL/SWS-2015b Yes Existing appliances Test Default Yes No 2 or 5 minc No a Separate operation of air handler to assess direction of pressure change; ACCA QH12-2011 did call for the air handler to be operated but at the same time 

as exhaust fans.

b Not formally published/available, but changes have been agreed to and are in the process of being implemented. c Time depends on whether the appliance is in “warm-vent” mode (2 minutes) or “cold-vent” mode (5 minutes).
7 

Table 2. Combustion Appliance Safety Test Procedures Differences and Similarities

ACCA

(ACCA QH12 2014)

NFGC 2015 Annex G

(NFPA 2015)

BPI

(BPI-1200-2015) STP

Occupant and Inspector 

Safety

Note: A CO alarm must 

sound within the 

following three 

concentrations and time 

windows to meet 

Underwriters 

Laboratories standards:

60–240 min for 70 

ppm

No less than 10 min 

for 150 ppm

4–15 min for 400 ppm

If ambient CO levels of 9 ppm 

are detected for more than 15 

min, the auditor shall have the 

discretion to stop all CO and 

depressurization testing.

If ambient CO levels of 25 

ppm are detected, the auditor 

shall stop all CO and 

depressurization testing.

If ambient CO is 70 ppm or 

greater, evacuate and call 

911.

If ambient CO is 30–70 

ppm, the inspector should 

recommend all CO sources 

be turned off and windows 

and doors opened.

30 ppm or lower, the 

inspection can continue.

Combustible gas of 20% 

lower explosive limit or 

higher, evacuate and call 

911.

Combustible gas below 20% 

lower explosive limit, 

inspection can continue.

If ambient CO is 70 ppm or greater, 

evacuate and notify appropriate 

emergency services.

If ambient CO is 36–69 ppm, the 

inspector should recommend all 

CO sources be turned off and 

windows and doors opened.

If ambient CO is 9–35 ppm, the 

inspector should advise occupants 

of the reading and recommend 

checking possible sources and 

opening windows and doors.

Combustible gas greater than 10% 

lower explosive limit, evacuate and 

notify appropriate emergency 

services.

No specific requirement

Detection of Gas Leaks 

from Piping or Fittings

Check all exposed gas piping 

in the building for leaks with 

the appropriate gas detector 

capable of measuring 20 ppm.

Confirm leaks with leakdetection fluid.

Mark the locations of leaks 

with a clearly visible tag and 

notify the owner.

Check leaks with 

noncorrosive leak detection 

fluid or 

Check leaks with 

combustible gas detector 

confirmed with 

noncorrosive leak detection 

fluid.

Test according to NFGC Ch. 8 and 

Annex C where called for by local 

jurisdiction or 

Check gas piping for leaks with 

combustible gas detector confirmed 

with noncorrosive leak detection 

fluid.

No specific requirement
8 

ACCA

(ACCA QH12 2014)

NFGC 2015 Annex G

(NFPA 2015)

BPI

(BPI-1200-2015) STP

Inspection of Appliance 

Condition while Off

Document whether the 

combustion appliance venting 

system shows evidence of the 

following: 

Blockages

Soot

Corrosion or oxidation

Improper support, slope,

and/or termination

Verify vent systems are 

compliant with NFGC 

chapters 11 and 12. Size and 

installation for other than 

natural draft and Category I 

appliances should be in 

compliance with the 

manufacturer’s instructions.

Inspect for damage, 

blockages, restrictions, 

leakage and corrosion.

Ensure plastic venting 

systems are free of sagging 

and are sloped upward.

Inspect internal components.

Inspect vent system to ensure that

the materials and horizontal pitch 

meet the manufacturers’ 

specifications.

Inspected for blockages, 

restrictions, leakage, and corrosion.

Inspect visible sections of 

combustion chamber and wiring.

No specific requirement

General Appliance 

Operation 

Document whether the 

combustion appliance venting 

system shows insufficient 

performance for the following: 

Blockages

Soot

Corrosion or oxidation

Improper support, slope,

and/or termination

Insufficient draft

Check proper ignition and 

ensure safety features are 

functioning properly.

Visually check flame 

appearance and ensure main 

gas burner is burning 

properly.

Verify appliance shuts off 

properly.

No specific requirement No specific requirement
9 

ACCA

(ACCA QH12 2014)

NFGC 2015 Annex G

(NFPA 2015)

BPI

(BPI-1200-2015) STP

Baseline Testing No specific requirement No specific requirement

Place all appliances in CAZ on 

standby mode.

Close fireplace dampers and doors 

and ensure that they are fully 

extinguished.

Close all exterior doors and 

windows.

Close interior doors between CAZ 

and rest of house.

Close interior doors to all rooms 

except for rooms with an exhaust 

fan, central forced-air system 

return.

Turn off mechanical forced-air 

heating and cooling systems.

No specific requirement
10 

ACCA

(ACCA QH12 2014)

NFGC 2015 Annex G

(NFPA 2015)

BPI

(BPI-1200-2015) STP

Preparing for WC 

Testing

Close all exterior windows, 

doors, and attic hatches.

Ensure that drain traps are

filled with water.

Turn on all indoor fans: 

bathroom exhaust, range hood, 

clothes dryer, powered attic 

fans (with the exception of a 

whole-house exhaust fan).

Turn on the air handler if the 

pressure differential in the 

CAZ becomes more negative 

with reference to the outdoors.

Open or close interior doors to 

the CAZ, rooms with exhaust 

fans, or other interior rooms to 

achieve the highest pressure 

differential in the CAZ with 

reference to the outdoors.

Makeup air systems, 

combustion air ducting, and 

ventilation systems are to 

remain as is.

Ensure fireplace damper is

closed or a simulator is

operating in the fireplace with 

the damper open.

Ensure the vent or flue is at 

room temperature.

Close all exterior building 

doors and windows.

Close all interior doors 

between the CAZ and the 

rest of the house.

Turn on exhaust fans 

(clothes dryer, range hoods, 

and bath exhaust, all at 

maximum speed).

Close fireplace damper and 

close doors, if present.

Turn on all exhaust to highest 

speed setting (clothes dryer—check 

and clean dryer filter and check for 

blockages at the external vent 

damper, range hoods, and other 

exhaust fans).

Turn on central forced-air system 

blowers—if it makes the CAZ 

more depressurized leave on, 

otherwise turn off.

Open interior CAZ doors—if it 

makes the CAZ more depressurized 

leave open, otherwise close.

Close all exterior doors, 

windows, and fireplace dampers.

Open all interior doors, and

leave open all combustion air 

openings to the outdoors.

In homes with central air 

handlers: close the door to any 

room that does not have either a 

return air duct or an exhaust fan.

Close the nearest door to the 

combustion appliances to

separate them from the rest of 

the house.

Do not turn off whole-building 

ventilation systems operating 

continuously. Disable any 

ventilation systems controlled 

for intermittent operation.

Do not disable any interlocked 

appliance makeup air systems.

Turn on any clothes dryer 

vented to the outside and kitchen 

ventilation to the max operating 

speed.

If the next-largest exhaust fan is 

a higher-speed operating exhaust 

ventilation system, turn it to the 

higher speed. Otherwise turn on 

the next-largest exhaust fan.

Turn on the air handler fan, if it 

makes CAZ more depressurized 

leave on, otherwise turn off.
11 

ACCA

(ACCA QH12 2014)

NFGC 2015 Annex G

(NFPA 2015)

BPI

(BPI-1200-2015) STP

WC Testing

Place the lowest Btu input 

appliance being tested into 

operation first and adjust the 

thermostat so the appliance 

operates continuously.

Test for spillage after 5 min of 

operation using a match, 

candle, or smoke.

If a draft is established within 

5 min the combustion 

appliance passes, otherwise 

the combustion appliance fails 

the test.

For additional fossil fuel 

appliances in the same room, 

turn on the next appliance 

being tested so it operates at 

the full input while the 

previous appliance continues 

to operate. 

Repeat testing for each 

additional appliance.

Or follow the 

methodology/procedure per 

the Authority having 

Jurisdiction.

Turn on appliance being 

inspected for continuous 

operation.

Verify all the appliances 

within the CAZ are on 

standby mode and ready to 

be turned on.

After 5 min check for 

spillage using smoke pen.

Immediately after smoke 

pen test turn on all other 

appliances within the CAZ 

to their maximum inputs and 

repeat spillage test.

Shut down all appliances to 

standby mode, wait 15 min.

Repeat steps for each 

appliance being inspected.

Additional inspections: 

determine if appliance 

venting is impacted by other 

door and air handler settings

There is also a list of 

appliance specific 

inspections.

Turn on appliance with the lowest

Btu capacity for continuous 

operation.

Measure spillage and CO in cold 

vent (except domestic water 

heaters) 

Test for spillage after burner has 

been on for 5 min.

Measure CO of undiluted flue gas 

at 5 min of burner operation.

Measure spillage and CO in 

domestic water heater and warm 

vents

Test for spillage at 2 min of burner 

operation.

Measure CO of undiluted flue gas 

taken at 5 min of main burner 

operation. 

Follow instructions for WC testing 

of multiple combustion appliances 

sharing a chimney and/or venting 

system. 

Test combustion appliances in 

order from lowest Btu to highest.

2 above methods to be used.

After spillage has been completed 

for the 1st appliance place the nextsmallest Btu appliance into 

operation leaving the 1st operating.

Retest the 1st appliance once the 2nd

appliance has been operating for 2 

min.

Test the 2nd appliance immediately 

after retesting 1st. 

Continue for all additional 

appliances sharing the same 

chimney and/or vent system.

Place the appliance being 

inspected in operation. The 

lowest-capacity appliance 

should be tested first. Follow 

lighting instructions for each 

appliance.

If the appliance being tested is a 

draft-hood-equipped furnace, 

turn all thermostats to heating 

mode so the appliance will 

operate continuously.

Test for spillage at the draft 

hood relief opening using a 

smoke pen at the following 

times:

o For furnaces in heating 

season and water heaters, test 

at 3 min. 

o For furnaces out of heating 

season, test at 5 min.

Measure CO in the appliance 

flue

Turn on all other fuel gas 

appliances within the CAZ, if 

the appliances are furnaces or 

boilers they should be set to 

continuous operation. 

Test for spillage at the appliance 

being tested. 

Turn off all appliances and 

continue onto next-lowest Btu. 
12 

3.1 Test Procedures in Effect at the Time of Project Initiation

3.1.1 National Fire Protection Agency 54-2012 (National Fuel Gas Code) 

NFPA 54 (also known as ANSI Z223.1) has been in use for years and has undergone periodic 

updates. Appendix G of the 2012 edition recommended—but did not require—a procedure for 

assessing the combustion safety of existing furnaces and boilers. It was not intended to cover 

water heaters. 

This standard considered the impact of exhaust devices such as dryers and exhaust fans. Doors 

between the CAZ and the remainder of the house were closed.

Other door positions were not considered. Any impact on pressures due to air handler operation 

was also not considered. This includes pressure impacts of duct leakage and from insufficient 

return air pathways in homes with central returns.

Spillage in atmospherically vented appliances was assessed at 5 minutes of burner operation. 

This timing is consistent with measurements that are intended to be conducted on newly installed 

appliances with cold vents. (NFPA 54 covers installation of new appliances.) 

Draft was measured as part of this procedure. Also, no limits were specified for maximum 

depressurization of the CAZ. 

3.1.2 Air Conditioning Contractors of America QH12-2011

ACCA, in cooperation with the Residential Energy Services Network, published its first audit 

standard for existing residential buildings in 2011. This standard, referred to as QH12, included 

requirements for combustion safety testing. Unlike NFPA 54, QH12 required that the testing be 

conducted. It also covered water heaters, furnaces, and boilers. 

Compliance with the combustion safety testing requirements of QH12-2011 could be achieved 

through multiple means. Several test procedures from other organizations were referenced. 

Additionally, QH12-2011 provided a test method in an appendix. Because the test procedures 

from other organizations were allowable, it is not possible to state that QH12-2011 necessarily 

required a specific setup or set of tests. The following discussion focuses on the contents of 

Appendix A of QH12-2011. Because compliance with other standards was allowed, any 

differences between QH12-2011 and these standards were also permissible as long as the other 

procedure was followed in its entirety.

As with all the standards discussed in this section, QH12-2011 considered the impact of 

intentional exhausts, including exhaust fans and dryers. 

QH12-2011 also considered the impact of interior door positioning. Each interior door, including 

the one to the CAZ, was supposed to be positioned to produce the greatest negative pressure in 

the CAZ whether that position was open or closed (as determined by testing). 

The air handler was to be turned on at the same time as all exhaust fans for the QH12-2011 

procedure. Whether the air handler pressurized or depressurized the CAZ was not considered. 

This means that door closure impacts would be identified, but if the air handler positively 
13 

pressurized the CAZ (e.g., supply leaks in a basement) a problem from, say, a dryer, could have 

been masked. 

Spillage was to be tested at 5 minutes of burner operation. This is consistent with NFPA 54. In 

an effort to ensure that the flue was in “cold-vent” conditions, QH12-2011 specified that the 

auditor should ensure that the flue was at room temperature.

No tests for draft or limits for maximum depressurization were included in this procedure.

The primary differences between combustion safety procedures in QH12-2011 and NFPA 54-

2012 were that QH12 included the operation of the air handler fan, requirement for testing, and 

allowing other procedures to be used. 

3.1.3 Building Performance Institute-101

BPI-101 was the Building Analyst standard. It has since been superseded by BPI-1200-2015. 

BPI-101 required that combustion safety testing be conducted. It also covered water heaters, 

furnaces, and boilers. 

As with all the standards discussed in this section, BPI-101 considered the impacts of intentional 

exhausts, including exhaust fans and dryers. 

BPI-101 considered the impact of interior door positioning. Each interior door, including the one 

to the CAZ, was supposed to be positioned to produce the greatest negative pressure in the CAZ 

whether that position was open or closed (as determined by testing). 

In BPI-101 the air handler was to be turned on separately from other exhaust fans. This means 

that door closure impacts and the direct impact of the air handler on the CAZ were considered. 

Spillage was to be tested at 1 minute of burner operation. Draft in the flue was to be measured; 

passage failure was based on comparing the draft pressure to values in a table that depended on 

outdoor temperature. BPI-101 also included limits for maximum depressurization. 

The draft measurement and maximum depressurization limits provided additional failure criteria 

relative to NFPA 54-2012 and ACCA QH12-2011. The separate operation of the air handler 

differentiated BPI-101 as a standard that would fully consider the effects of the air handler on the 

CAZ.

3.1.4 Standard Work Specifications-2011 

The SWS were developed by NREL. As a federally developed document, the standards of other 

organizations were frequently referenced instead of developing independent procedures. Initially, 

the SWS specifically referred to BPI-101. Eventually that was changed, and many of the 

requirements for which tests needed to be performed were explicitly laid out when the SWS was 

first finalized.

The SWS did not provide extensive details for how to do testing. It stated that depressurization 

testing should include the effects of doors, duct leakage, air handler operation, and door position, 

implying the same set of requirements as BPI-101. 
14 

The SWS had requirements for spillage and maximum depressurization limits but no requirement 

to measure draft. Spillage was based on 2 minutes, putting the SWS between BPI-101 and the 

requirements of NFPA 54-2012 and ACCA QH12-2011. Depressurization limits were 

inconsistent within the SWS but were closely aligned with BPI-101. 

3.2 Current Test Procedures

This section describes the states of the above standards after their most recent revisions. Many of 

these changes are in line with the recommendations of the PARR/NorthernSTAR STP. Specific 

recommendations of this STP include:

Not testing for draft 

Not failing units solely because they exceed the maximum depressurization limits

Positioning doors based on supply registers, return grilles, and exhaust fans rather than 

from door-by-door pressure testing 

Spillage assessed based on a “warm-vent/cold-vent” decision tree

Operating large exhaust fans at lower flow rates. 

3.2.1 National Fire Protection Agency w2qA 54-2015 (National Fuel Gas Code) 

NFPA 54 has had some significant updates to Appendix G. It now covers water heaters, though 

such testing is still only recommended, not required. 

In this edition the position of the door between the CAZ and the remainder of the house is now 

considered. Positions of other interior doors are still not considered. 

A major change in this edition is that the impact on pressures due to air handler operation is now 

considered. Air handler operation has multiple potential impacts, and accounting for these 

impacts is a significant improvement. One of these, the impacts due to interior door closures, 

however, is not considered in the current edition of the code. 

Spillage in atmospherically vented appliances is still assessed at 5 minutes of burner operation. 

There continue to be no measurement of draft or limits for maximum depressurization of the 

CAZ.

3.2.2 Air Conditioning Contractors of America QH12-2014

ACCA updated its QH12 standard in 2014. Specific references to other organizations’ standards 

were removed, though discretion was given to the Authority Having Jurisdiction to choose 

another procedure. 

Exhaust fans and the impact of interior door positioning are unchanged from the previous 

version. Each interior door, including the one to the CAZ, is supposed to be positioned to 

produce the greatest negative pressure in the CAZ whether that position was open or closed (as 

determined by testing). 
15 

The air handler is to be turned on separately from other fans to determine whether it pressurizes 

or depressurizes the CAZ. This is a major advancement of QH12 and was one reason other 

organizations’ methods were removed for the 2014 edition. 

Spillage continues to be assessed at 5 minutes of burner operation. The language about ensuring 

that the flue was in “cold-vent” conditions was also retained. 

There continue to be no measurement of draft or limits for maximum depressurization of the 

CAZ.

3.2.3 Building Performance Institute-1200-2015

BPI-1200-2015 was published as an ANSI standard in spring 2015. BPI-1200 requires that 

combustion safety testing be conducted. It continues to cover water heaters, furnaces, and 

boilers.

As with all the standards discussed in this section, BPI-1200 considers the impact of intentional 

exhausts, including exhaust fans and dryers. 

The impact of interior door positioning is considered by BPI-1200 but not through a one-by-one 

door evaluation as before. The procedure that has been adopted is very similar to that in the STP 

proposed by PARR and NorthernSTAR. Doors to rooms that contain exhaust fans or return grilles 

are left open. Doors to rooms with only supply registers are closed. 

In BPI-1200 the air handler continues to be turned on separately from other exhaust fans. This 

means that door closure impacts and the direct impact of the air handler on the CAZ are 

considered. 

In BPI-1200, spillage testing now takes a multipath approach. Specifically, the amount of time 

allowed for spillage to stop depends on whether the appliance is in “cold-vent” or “warm-vent” 

mode. Water heaters are assumed to be in warm-vent mode at all times, because water heaters 

fire throughout the year. Furnaces are assumed to be in warm-vent mode if the thermostat is set 

to “HEAT” and in cold-vent mode if the thermostat is set to any other setting. Even though flues 

may be cold even if the appliance is in “warm-vent” mode, the thinking was that expecting 

auditors to accurately measure flue temperatures or to wait long enough to make sure the vent is 

cold is not reasonable. 

For spillage, appliances in “warm-vent” mode are to be tested at 2 minutes of burner operation. 

This is longer than the 1-minute period in BPI-101, but shorter than the test period for NFPA 54 

or ACCA QH12. Appliances in “cold-vent” mode are to be tested at 5 minutes of burner 

operation, which is aligned with NFPA 54 and ACCA QH12. Both aim to have appliances tested 

in “cold-vent” mode. 

Draft in the flue no longer needs to be measured. BPI-1200 also removed limits for maximum 

depressurization.
16 

3.2.4 Standard Work Specifications-2015

The SWS undergo periodic maintenance. The maintenance committee met in April 2014 to 

address comments submitted through public review. However, many of the comments about 

combustion safety were postponed until all the standards that were discussed above were revised. 

These revisions were completed after BPI-1200-2015 was published. The committee 

subsequently drafted changes to the combustion safety section of the SWS to bring it into 

alignment with BPI-1200-2015. As of this writing these changes had not yet been formally 

published; however, the committee has agreed upon the changes to be made. 

3.3 Overall Alignment and Partnership for Advanced Residential 

Retrofit/NorthernSTAR Involvement 

Table 1 shows that the standards have been substantially aligned. As of 2012, there were 

differences about whether testing was required, door positioning, operation of the air handler, 

allowable time to last spillage, draft testing, and maximum depressurization limits. In 2015, the 

only areas of real difference are in whether testing is required (all except NFPA 54 require it) 

and positioning of interior doors. Spillage also is somewhat different; NFPA 54 and ACCA 

QH12 use cold-vent limits and BPI-1200 and the SWS use a two-path approach. Otherwise, the 

procedures are all effectively the same. The only major recommendation from the STP that has 

not been adopted in any of these standards is to operate large exhaust fans at lower flow rates. 

Removing draft testing and maximum depressurization limits and simplifying interior door 

positioning that is in BPI-1200-2015 and the SWS are consistent with the proposed STP from 

PARR/NorthernSTAR. The two-path spillage testing requirements of 2 minutes for warm-vent 

mode and 5 minutes for cold-vent mode also appear in the STP. Thus, even while the Building 

America project was underway, some organizations were adopting the recommendations of the 

project.

Although not discussed previously, CO limits have also been brought into alignment and are 

consistent with the Building America project. BPI-1200-2015 was permitted to use the table of 

CO limits used in NFPA 54-2015, and these appear in the published standard. ACCA QH12 

refers to the NFPA 54-2015 table. Therefore, all these organizations are using the same CO 

limits for flue gases. However, requirements continue to differ for various levels of ambient 

(e.g., indoor air) CO. 

PARR team members have been involved in some capacity with each of these standards and the 

resulting alignment. 

Paul Francisco provided public review comments to both NFPA 54 and ACCA QH12 

during their revision processes. In both cases, comments on original drafts included 

recommendations to require air handlers to be operated separately from other fans to 

assess their impact on CAZ pressures. In both cases, these comments were accepted and 

contributed to both standards.

Paul Francisco and Larry Brand were members of BPI Working Group-7, which was

convened to make recommendations to the BPI-1200 committee about combustion safety 

testing. Paul Francisco was the originator of the “warm-vent/cold-vent” two-path 

strategy.
17 

Paul Francisco is also on the maintenance committee for the SWS and drafted the 

changes to align the SWS with BPI-1200. 

NorthernSTAR team members have also been involved in some capacity with each of these 

standards and the resulting alignment.

Rebecca Olson has served as chair of the BPI Standards Technical Committee since 

2013. She helped guide the adoption of the new BPI-1200--Standard Practice for Basic 

Analysis of Buildings and was one of the major drivers of Working Group-7. 

Jim Fitzgerald was on the BPI-1200 Working Group-7, and the BPI Standards Technical 

Committee from the beginning through comment resolution and voting on BPI-1200. 
18 

4 Field Data Collection and Analysis

4.1 Test Plan

4.1.1 Must-Fail/Must-Pass Criteria

The project objectives were to identify (1) if homes that passed the STP had a very low 

probability of developing spillage events, and (2) how many homes that failed the STP were 

safe, as defined by having no significant spillage events. Consequently, houses selected for the 

project would be those that failed a stringent combustion safety test but might pass under less 

stringent conditions. The following must-fail/must-pass criteria were defined:

For the must-fail criterion, the house was placed in the conditions specified by the STP. 

The steps required to meet these conditions were: 

o Close the door nearest to the combustion appliance that separates the appliance 

from the main body of the house. 

o Close the interior doors of all rooms except for rooms with an exhaust fan and 

rooms with a central forced-air system return. 

o Turn on the clothes dryer. 

o Turn on the kitchen exhaust fan to its highest speed. 

o Turn on the next largest capacity exhaust fan. 

o Check that the air handler fan on (if it depressurizes the CAZ).

Water heaters failed this test if spillage continued for more than 2 minutes after the 

burner was activated. Space heating systems failed if spillage continued for more than 5 

minutes. The house conditions for the must-pass criterion were the same as for the mustfail conditions, with the following exceptions: 

o Switch the kitchen exhaust fan to its lowest speed (if available). 

o Turn off all other exhaust fans. 

The spillage time requirements for the must-pass conditions were the same as those for 

the must-fail conditions. 

4.1.2 Three-Step Process

A three-step process was followed to identify candidate houses, select those of interest, and 

collect data over a heating, shoulder, and cooling season. 

1. Screening. This step was used to identify houses that met the must-fail/must-pass 

criteria. These tests typically took 1.5–2 hours to perform. Houses that passed this 

requirement moved on to the second step. 

2. Short-term testing. Houses in this step underwent a full day of evaluation to: 

A. Confirm the findings from the first step. 

B. Ensure that the combustion appliances were installed and operating correctly. 
19 

C. Record detailed pressure, temperature, spillage, and performance data. Houses 

that passed the requirements in this step moved on to the last step. 

3. Long-term monitoring. Houses that passed the initial steps were monitored over 3 to 6 

months to determine the circumstances under which combustion safety events occurred. 

A full instrumentation package was installed in each house and data were downloaded 

remotely. Each homeowner signed a site participation agreement.

4.2 Field-Test Site Screening and Selection—Sampling Strategy

Sample size for this test was initially determined by the cost of monitoring and data analysis. It 

was originally proposed that in each location/climate up to 100 houses would be screened to 

select 6 to 10 qualified houses for the long-term study. Locations were selected to obtain 

extremes in temperature and a range of building types. The project plan was for NorthernSTAR to 

test and instrument 10 houses in Minnesota (cold climate) and 6 houses in Phoenix, Arizona (hot 

climate). PARR was to test and instrument 6 houses in southern Illinois (moderate climate) and 6 

houses in Atlanta, Georgia (mixed climate). Table 3 shows the mix of expected building 

construction types. The columns are sorted with highest frequency at the top. Because of the 

difficulty finding houses that met the criteria, only 11 houses were qualified—all in cold

northern climates. See Section 4.4. 

Table 3. Range of Building Types for the Four Climates

Minnesota 

(Minneapolis)

Southern 

Illinois

Arizona 

(Phoenix)

Georgia 

(Atlanta)

Building Typea

Basement Basement Slab Basement

Crawl space Slab Crawl space Slab

Crawl space Basement

Possible Appliance 

Location 

Basement Basement Attic Basement

Inside closet Inside closet Inside closet Attic

Crawl space Basement

a www.census.gov/construction/chars/completed.html 

4.3 Field Research Methods

4.3.1 Screening Procedures

The screening procedure was used to identify houses for short-term testing and long-term 

monitoring. Building technicians performed the must-fail/must-pass screening tests. A 

NorthernSTAR building technician conducted most of the tests on houses in Minnesota, which 

were located in the Twin Cities metropolitan area. PARR and NorthernSTAR staff trained local 

weatherization building technicians to conduct the tests on houses in their area. Sections A 

through E describe the screening test procedures. To be included in the monitoring section of the 

study, a house had to fail the Section C spillage test when set up according to the must-fail 

conditions in Sections A and B. Those conditions were equivalent to those specified for the STP. 

The house also needed to pass the spillage requirement for the must-pass conditions specified by 

Section D. 
20 

A. Set Up Depressurization Conditions

1. Close all exterior doors, windows, and fireplace dampers. Open all interior doors. Leave 

open all combustion air openings to the outdoors. 

2. Close the door to any room with a supply register that does not have either an exhaust fan 

or a central air handler return air duct (independent of a transfer grille, jumper duct, or 

other mechanism for providing a return air pathway).

3. Close the nearest operable door to the combustion appliances to separate them from the 

rest of the house.

4. Do not turn off a whole-building ventilation system that is operating continuously. 

Disable supply or balanced ventilation systems that are controlled for intermittent 

operation. Do not turn on whole-house (summer) exhaust fans.

5. Do not disable any interlocked appliance makeup air systems. 

6. Turn on any clothes dryers that are vented to the outside.

B. Set Up Must-Fail Test Conditions 

1. Turn on any kitchen exhaust ventilation systems and operate at maximum speed. 

2. If the next-largest exhaust fan is a higher speed of an operating exhaust ventilation 

system, turn it to the higher speed. If not, turn on the next-largest exhaust fan. 

3. Turn on the air handler fan and measure the indoor-to-outdoor pressure difference. If the 

air handler operation depressurizes the CAZ relative to the outdoors, leave the air handler 

on. If not, turn off the air handler. 

C. Perform Must-Fail Combustion Safety Spillage and Carbon Monoxide Tests

1. Place the appliance being inspected in operation. Test the lowest-capacity appliance first. 

Follow the lighting instructions for each appliance. If the appliance being tested is a 

draft-hood-equipped furnace, turn all thermostats to heating mode so the appliance will 

operate continuously. 

2. Use smoke to test for spillage at the draft hood relief opening of the appliance being 

tested. The test duration is specified by: 

A. For furnaces in heating season and water heaters, test at 2 minutes and 5 minutes. 

Use the 2-minute result for pass/fail decisions.

B. For furnaces out of heating season, test at 5 minutes. 

Figure 1. Checking spillage on a water heater
21 

Figure 2. CO thresholds 

3. Measure CO levels in the appliance flue. These levels should not exceed the ANSI 

certification requirement for the type of appliance (see Figure 2). If the appliance fails 

this test: 

A. Record the failure. 

B. Notify the homeowner that the appliance should be evaluated by a qualified 

service technician. 

C. Exclude this house from the study. This test may be repeated after the appliance is 

serviced.

= 

20.9

20.9 – × 

= ×

(Excerpted from NFGC (NFPA 54) 2015 Annex G with permission from the American 

Gas Association)

Appliance Threshold Limit

Central Furnace (all categories) 400 ppm1air free2,3

Floor Furnace 400 ppm air free

Gravity Furnace 400 ppm air free

Wall Furnace (BIV) 200 ppm air free

Wall Furnace (Direct Vent) 400 ppm air free

Water Heater 200 ppm air free

1 Parts per million

2 Air free emission levels are based on a mathematical equation (involving carbon 

monoxide and oxygen or carbon dioxide readings) to convert an actual diluted flue gas 

carbon monoxide testing sample to an undiluted air free flue gas carbon monoxide level 

utilized in the appliance certification standards. For natural gas or propane, using

as-measured CO ppm and O2 percentage:

Where:

COAFppm = Carbon monoxide, air-free ppm

COppm = As-measured combustion gas carbon monoxide ppm

O2 = Percentage of oxygen in combustion gas, as a percentage

3 An alternate method of calculating the CO air free when access to an oxygen meter is 

not available:

Where:

UCO2 = Ultimate concentration of carbon dioxide for the fuel being burned in percent 

for natural gas (12.2 percent) and propane (14.0 percent)

CO2 = Measured concentration of carbon dioxide in combustion products in percent

CO = Measured concentration of carbon monoxide in combustion products in percent
22 

4. Turn on all other fuel gas appliances within the same room. If the other appliances are 

furnaces or boilers, they should be set to operate continuously. Furnace fans should be 

running. 

5. Use smoke to test for spillage at the draft hood relief opening of the appliance being 

tested.

6. Turn off all combustion appliances and set up the next appliance for testing. 

7. If any combustion appliance fails the spillage tests when operating alone or with another 

appliance, record the failures and continue to the next section. If all appliances pass the 

spillage tests, return the house to original conditions. This house does not qualify for the 

monitoring section of the project. 

D. Set Up Must-Pass Conditions and Perform Spillage Test

1. Repeat the depressurization conditions with the kitchen fan set to low speed and the next 

largest exhaust fan kept off. 

2. Return to depressurization conditions created in Set Up Depressurization Conditions: 

Step 1. 

3. Turn on any kitchen exhaust ventilation system and operate it at the lowest speed.

4. Turn off the next-largest exhaust fan, if one is present. 

5. Turn on the air handler fan and measure the indoor-to-outdoor pressure difference. If air 

handler operation further depressurizes the CAZ relative to the outdoors, leave the air 

handler on. Otherwise turn off the air handler. 

6. Use smoke to test for spillage at the draft hood relief opening of the appliance being 

tested.

7. If the combustion appliance fails spillage, this house does not qualify for the monitoring 

section of the project, and the homeowner should be notified that remediation is 

necessary. If the combustion appliance passes the spillage criteria, this house qualifies for 

monitoring. 

E. Return the Site to Its Original Condition

1. Return doors, windows, exhaust fans, fireplace dampers, and all fuel-gas-burning 

appliances to their previous conditions of use before leaving the site. 
23 

Figure 3. Field-test site selection flowchart 

4.3.2 Additional Data Collected Postscreening

If a house met the screening criteria, the technician collected additional site data to help 

determine compliance with current combustion safety codes and guidelines and to see if the 

house was suitable for further study. The following information was collected for each candidate 

house that was selected for further study: 
24 

1. Natural gas input and output capacities from appliance labels

2. Location of each appliance and source of combustion air (sealed mechanical closet open 

to the outdoors, basement with no combustion air from outdoors, garage, etc.) 

3. Area of intentional combustion air openings to the outdoors, size, and types of grilles or 

louvers in doors and for outside openings, as well as the duct length and diameter (if a 

duct is used) 

4. Volume of the room where each appliance is installed (except for rooms outside the 

pressure boundary); basements are inside the pressure boundary 

5. Vent system configuration, including data about vent connector and common vent (or 

chimney) diameters and lengths 

6. Blower door house envelope air leakage test results

7. Duct leakage results (if test was performed)

8. List of all exhaust fans including attic exhaust fans (at roof), kitchen exhaust fans, 

ventilation fans, bathroom ventilation fans, and dryers. The location and measured and/or 

rated fan air flow rate should be noted and a photo taken of the name plate or the make 

and model if possible. 

9. Floor area and ceiling heights for the NFGC Standard Method if combustion air was from 

indoors only or from both indoors and outdoors 

10. Measured baseline and induced depressurization under the must-fail condition (see 

Section 2.3.1) 

11. Pass/fail results of the appliance spillage tests under must-pass and must-fail conditions. 

The project team reviewed the above information from candidate houses and then evaluated the 

screening information to determine if the houses met the screening criteria to be selected for the 

study. 

4.3.3 Short-Term Testing and Inspection Methods

For houses that passed the field-test screening, project staff conducted more extensive short-term 

tests, which achieved two objectives: 

Confirmed that the house met the criteria for long-term monitoring 

Provided the data needed to evaluate options for an STP. 

For the short-term test procedure, the project staff did not conduct a comprehensive safety test 

(e.g., gas leak detection). Instead, the tests focused on combustion venting performance and 

documenting conditions related to venting performance. Additional sections of the PARR and 

NorthernSTAR Measure Guideline combustion safety test procedures were conducted as time 

allowed and as deemed necessary after a brief visual inspection of the equipment. A detailed 

protocol for screening and selecting and the short-term and long-term tests was developed and 

communicated to the weatherization teams that participated in the project. Appendix A includes 

an overview of the short-term testing protocol for this project.
25 

Safe work practices were followed to protect the health of the project staff and occupants. If the 

results of this inspection identified a baseline house CO level that exceeded the 35 ppm NREL 

SWS Limit (https://sws.nrel.gov/spec/202011), the homeowner was notified and the inspection 

stopped until the situation was corrected. 

Houses were included in the long-term monitoring section of the study if they failed the must-fail 

test and passed the must-pass test. While monitoring the system installation, the must-fail/mustpass test was performed using the 2-minute criteria as a check of the repeatability of the tests. 

Two houses did not pass this retest. For the project, the screening visit test results were used to 

qualify the houses to be included in the study. 

4.3.4 Monitoring System and Installation

The project team designed and assembled a monitoring system specifically for the project. The 

system generally made use of commercially available sensors selected for suitability across the 

ranges and conditions expected (Table 4). Appendix B includes more information about system 

components. 
26 

Table 4. Monitored Parameters and Sensors

Measurement Sensor Used Use of This Measurement

Temperature in Water Heater (or Furnace) 

Flue Type K thermocouple Determination of water heater (or 

furnace) burner firing status

Temperature in Water Heater (or Furnace) 

Vent Type K thermocouple Detection of downdrafting 

Temperature at Four Locations Around 

Water Heater (or Furnace) Draft Hood Type K thermocouple Temperature in combustion products 

spillage zone

Temperatures in CAZ (High and Low 

Positions) and Outdoors Type K thermocouple 

Pressure Difference, Water Heater (or 

Furnace) Vent to CAZ

Differential pressure transducer, 

digital output

Determination of normal operating 

conditions, detection of downdrafting

Pressure Difference, Outdoors to CAZa Differential pressure transducer, 

digital output

Characterization of CAZ operating 

conditions

Current Draw of Air Handler 20A current transducer, analog 

output Air handler fan operating status

Pressure Induced by Fan Operationb Custom wireless sensor module Exhaust fan operating status

Carbon Dioxide (CO2) Concentration above 

Water Heater (or Furnace) Draft Hood

Nondispersive infrared CO2

sensor, analog output

Detection of combustion products 

spillage

CO2 Concentration in CAZ away from 

Appliances Same Background CO2 level

CO near Ceiling in CAZ Electrochemical sensor Detection of elevated CO levels a

 The CAZ pressure difference used in this report is outdoor pressure with respect to CAZ pressure, and CAZ depressurization appears as a positive value.

b

 Wired current sensors and wireless pressure sensing modules were used to monitor clothes dryers and other exhaust devices, depending on accessibility for 

wiring. 
27 

The locations of the sensors associated with the water heaters are shown in Figure 4. 

Figure 4. Schematic view of monitoring system sensor placement at water heater

A single sensor was used to measure pressure. It was mounted in the main system enclosure, 

with valves that allowed sequential measurement of differential pressures and a periodic 

rezeroing. Rezeroing reduces the effects of drift over time and with varying temperature, 

common in pressure measurement in the range of a few Pascals. The pressure measurement 

valving array was operated on a 3-second cycle, with a 1-second allowance for signal settling 

after the valves were reset and 2 seconds of data collection at each measurement location. The 

digital-output pressure sensor was queried 25 times during each second of measurement (though 

even this was not adequate to filter out significant wind-induced noise, as discussed later). 

Fractional-inch plastic tubing carried pressure signals from the measurement locations to the 

enclosure. 

Similarly, a single CO2 sensor was mounted in the enclosure, and a pump drew samples 

sequentially from each measurement location through a set of valves. The concentration of CO2

around the appliances compared to room level was used as the primary metric to identify 

spillage. A common sensor allowed more accurate measurement of differences in concentration, 

independent of absolute sensor accuracy. The initial design placed the CO2 sampling pumps 

inside the main system enclosures, but comments from homeowners led to a decision to place 

them in separate noise-reducing enclosures that were mounted near the main system. CO2

valving was operated on a 20-second cycle, with a 10-second purge period followed by 10 

seconds of data collection at each sampling point. The CO2 sensor output was captured once 

each second during active measurement.
28 

The team used thermocouples for all temperature sensors, based on their durability and ability to 

withstand the temperature ranges expected. Spillage temperature probes were mounted at four 

positions spaced around the circumference of the water heater draft hood, about 1⁄4 in. below the 

edge and 1⁄4 in. beyond the circumference of the lower edge of the draft hood, where combustion 

spillage products would be expected to heat them. The thermocouples were constructed of 24-

gauge wire. The response time for moving air was a few seconds. 

The system was based around a Beaglebone Black single-board computer, which offers a lowcost, fully programmable platform that allows essentially unlimited measurement and control 

options. A micro secure digital card provided storage capacity for programs and data. A customprinted circuit board included a number of integrated circuits that provided amplification (of 

thermocouple and CO sensor output), analog-to-digital converters, and other components. The 

team ran custom software, written in Python, on the Beaglebone computers. All parameters were 

sampled once per second, except the oversampling of differential pressures as mentioned earlier. 

Data were recorded at 1-minute intervals when no burner was firing, and at 1-second intervals 

when a burner was firing (and for a cooldown period of 2 to 3 minutes after firing). To preserve 

pump life, CO2 data were collected for the first 15 minutes of every burner cycle; additional 

background readings were taken every 4 hours if no burner operation was detected. 

The computer, printed circuit board, wiring connections, valves for pressure signals, and CO2

sampling were mounted in an 8- × 16- × 4-in. enclosure with mounting hardware to permit 

attachment to framing or other surfaces (Figure 5 and Figure 6). Figure 5 shows the wiring 

connectors along the lower edge of the circuit board and the CO2 sampling pump in a separate 

gray box below the right side of the enclosure. The uninterruptible power supply backup power 

unit is also shown.

Figure 5. Monitoring system main circuit board 

and valves (yellow tops) in enclosure, cover 

removed 

Figure 6. Water heater draft hood with copper 

tube for CO2 sampling and thermocouple 

temperature wire showing

The project team developed a wireless sensor module specifically for the project. The module is 

based around a commercially available ZigBee wireless transceiver with onboard analog-todigital conversion capability. The module includes a pressure sensor and amplifier circuit and 
29 

was designed to be placed near an exhaust fan, with a flexible tube placed within the fan housing 

or exhaust duct so the sensor would see an elevated (positive or negative) pressure when the 

device was operating. The modules were programmed to read and transmit a pressure signal at 

15-second intervals and are operated by three AA batteries (Figure 7).

Figure 7. Custom wireless sensor module (black box) with pressure sensing tube placed in dryer 

exhaust duct 

Communications for Internet-based remote data downloads were provided using one of the 

following: 

Direct cabled connection to a home Internet router

Powerline carrier connection to a home internet router

Wi-Fi link to a cellular modem; this was generally placed on an above-grade floor (a 

“Mi-Fi” arrangement). 

4.4 Field Research Results and Analysis

4.4.1 Recruitment

Originally this project was designed to study the implications of a simplified combustion safety 

test protocol in four areas of the country (cold, moderate, hot-humid, and hot-dry) with different 

weather conditions and construction types. The initial target locations within these four climates 

were the Twin Cities Metro area of Minneapolis-St. Paul, Minnesota (cold climate); southern 

Illinois (moderate climate); Atlanta, Georgia (hot-humid climate); and Phoenix, Arizona (hot-dry 

climate). The project plan was for NorthernSTAR to test and instrument 10 houses in Minnesota 

and 6 houses in Phoenix, Arizona, and for PARR to test and instrument 6 houses in southern 

Illinois and 6 houses in Atlanta, Georgia. 

In Minnesota contact with the state grantee for DOE’s low-income weatherization assistance 

program (WAP) was made through NorthernSTAR team member Center for Energy and 

Environment (CEE). In Illinois contact with the state WAP grantee was made through PARR 
30 

team member Indoor Climate Research & Training (ICRT), which is also a WAP training center. 

For the other two locations contact was made with WAP training centers that ICRT located in 

each state. The training center in Phoenix was interested and willing to participate. The training 

center in Atlanta stated that a recently publicized incident made participation impossible at that 

time. Therefore, efforts were redirected to contacting the WAP training center in Little Rock, 

Arkansas, which in turn put ICRT in touch with the state WAP grantee. 

Multiple states expressed concerns about the workflow and liability of the project. The extended 

monitoring period would require that retrofits be completed EXCEPT for addressing combustion 

safety issues, which could interrupt workflow. From a liability perspective, states were 

concerned that if an incident occurred in a home in which a combustion safety hazard had been 

identified they could be deemed culpable. These states were willing to collaborate on the project 

if the project team could show support from DOE’s WAP. The small number of homes to be 

tested in each location was such that the workflow issue was deemed manageable.

ICRT approached DOE about the project, and DOE sent an email to target states encouraging 

their participation. In addition to the states previously mentioned, DOE recommended 

Mississippi as another hot-humid state that could be a promising partner. Therefore, the email 

was also sent to Mississippi and ICRT reached out to the state grantee.

Despite these efforts, finding eligible homes proved elusive. Therefore, additional contacts were 

made to expand the number of states seeking homes. ICRT reached out to WAP state managers 

or training centers in Texas, New Mexico, Idaho, Oregon, and Washington State. ICRT 

contacted Conservation Services Group, which administers utility programs in a number of 

states. All were willing to look for eligible homes. Conservation Services Group focused on New 

York. NorthernSTAR also contacted the WAP state manager in Wisconsin.

In the end, the team reached out to 13 states, and almost all expressed willingness to collaborate. 

However, more than 600 homes were reviewed for eligibility according to project requirements, 

and only about 10 were identified. These were almost entirely in Minnesota. Homes were often 

owned by friends and families of project team members and were not WAP clients. This led to 

the administration of a survey, discussed in Section 5. The details follow about each state that the 

project team contacted.

Arizona: Agencies in Arizona expressed interest in the project; however, no houses 

could be recruited. The Phoenix area low-income housing stock typically has water 

heaters in the garages, and the heating systems are either gas packs or induced draft. 

These houses do not fit the project criteria. The agency suggested that we contact the 

Flagstaff WAP provider, which, because of the altitude, might be able to find houses that 

would qualify for the project but needed approval from the state of Arizona to proceed. 

Unfortunately, communications with the state grantee delayed approval until June 2015, 

and the timing and logistics ruled out proceeding with Arizona.

Arkansas: The state WAP grantee required DOE approval to look for homes. The WAP

reported reviewing several hundred homes throughout the state but was unable to locate 

any that met the project criteria.
31 

Georgia. The WAP training center in Atlanta was interested but stated that a recent 

incident in the state made participation impossible.

Idaho: The state WAP program was initially very optimistic that it could find homes that 

met the criteria. Once approached the WAP supported the project quickly and readily 

began reviewing homes. However, no homes that failed combustion safety testing met the 

project criteria.

Illinois: Several agencies in southern Illinois were recruited to look for homes. Two 

agreed to review homes. However, no homes were located that fit the project 

requirements. Agencies reported that cases in which homes failed spillage were almost 

entirely due to air handler operation; dryers were the cause of others.

Minnesota: Although discussion began in fall 2014, official permission from the state to 

the three WAP providers that had been recruited was delayed until February 2015. The 

agencies eventually provided three referrals. The project team scheduled these houses for 

screening tests.

A reason for the dearth of referrals from the WAP providers is likely related to the fact that a 

backdrafting water heater was their primary requirement for identification. The agencies singled 

out houses with orphaned water heaters as candidates. Any houses they worked on that required 

furnace replacement were not included in the search, because these needed immediate work. 

Because the mechanical contractor would be in the house at this time, remedial work was 

performed on the orphaned water heater as needed. 

Because of the delays in the team’s recruitment efforts with the WAP agencies, an approach was 

taken in fall 2014 to find participants through friends and family. CEE inquired among staff and 

partnering groups if their own houses or those of people they knew might fit the requirements. 

Fourteen candidates were identified from these efforts and screening tests were scheduled as they 

were identified.

Mississippi: The state WAP grantee referred ICRT to a specific agency. This agency 

reviewed its files and found that very few homes in the last 3 years would have 

potentially been eligible. Eleven homes were identified as potential candidates but none 

qualified. In the end, Mississippi chose not to participate further. 

New Mexico: The WAP training center and state WAP grantee were both very interested 

in the project; however, no houses could be recruited. As with Arizona, the housing stock 

typically has water heaters in the garages, and the heating systems are either gas packs or 

induced draft. These houses do not fit the project criteria. One home was identified as a 

possible candidate due to spillage, but further discussions revealed that failure was due to 

air handler operation.

Oregon: The state WAP grantee agreed to recruit homes for the project. However, no 

homes were located that fit the project requirements.

Texas: The state WAP grantee indicated it would be agreeable. However, the program 

director stated that the housing stock probably would not meet the project criteria.
32 

Washington: The WAP training center expressed willingness to approach the state 

agencies. However, the program has a heavy focus on all-electric homes, and no leads 

were generated.

Wisconsin: Through friends and family efforts, a participant who is a staff person at 

Seventhwave was recruited in Madison, Wisconsin. Discussions also took place with the 

Wisconsin Weatherization Program, which chose not to participate in the project. 

Conservation Services Group (New York): This is the one substantial program that was 

contacted that was not a part of DOE’s WAP program. As with most state WAP grantees, 

Conservation Services Group expressed a willingness to look for homes for the project, 

focusing on New York, but was unable to find suitable candidates. 

4.4.2 Screening and Selection

From December 2014 to June 2015, 18 houses in Minnesota were identified as candidates for the 

study. Six were owned by CEE or Seventhwave staff. Eight were referrals by CEE staff or 

partners of CEE. Four were referred by local Low Income WAP (LIWAP) providers. Table 5

lists the houses that were screened in Minnesota and the results of the screening tests. 

Table 5. House Screening Results

Category Date 

Evaluated

Eligible/ 

Ineligible Reason Failed

Friend 12/1/2014 Ineligible Venting issue

Family 12/3/2014 Eligible

Family 12/5/2014 Eligible

Friend 2/1/2015 Ineligible Did not meet must-pass criterion at time of 

screening

Family 2/16/2015 Eligible

Family 2/20/2015 Eligible

LIWAP 2/28/2015 Ineligible Did not meet must-fail criterion at time of screening

LIWAP 3/13/2015 Ineligible Did not meet exhaust equipment requirements

Family 3/24/2015 Ineligible Did not meet must-fail criterion at time of screening

Friend 3/29/2015 Ineligible Did not meet must-fail criterion at time of screening

Friend 4/6/2015 Eligible

Friend 4/6/2015 Eligible

LIWAP 4/13/2015 Eligible

Friend 4/28/2015 Ineligible Did not meet must-pass criterion at time of 

screening

Family 4/30/2015 Eligible

Friend 5/4/2015 Ineligible Did not meet must-fail criterion at time of screening

Friend 5/5/2015 Eligible

LIWAP 5/6/2015 Eligible

The screening test took 1.5–2 hours and was performed by a CEE field technician. The 

screenings for the LIWAP houses were completed by a CEE field technician and LIWAP staff. 

These screenings were used as training for LIWAP staff members so they could perform the 

screenings themselves; however, too few LIWAP houses were available for the process to 
33 

transition over to them. An eleventh house located in Madison, Wisconsin, was added to the 

project. It is owned and occupied by a Seventhwave staff person, and he performed the screening 

test on the house. 

For all the houses that passed the screening criteria, the water heater was the combustion 

appliance that met the must-fail/must-pass spillage criteria and was instrumented for long-term 

monitoring. For the smoke test, a pass was given if the visible smoke was completely drawn into 

the draft hood. 

Appendix C includes photos from the selected 11 homes. 

4.4.3 House Characteristics

Table 6 summarizes the house characteristics. The houses are fairly representative of the older 

housing stock in the metropolitan area: smaller one-story ranches and story-and-a-half houses. 

Both styles include basements. None of the houses were overly tight. The envelope air leakage 

ranged from 3.9 to 11.1 ACH50 with an average of 6.15 ACH50. Six of the houses had exterior 

masonry flues; these all had metal liners.

Table 6. Building Characteristics of Participating Houses

House Type

Air 

Leakage 

(CFM50/ 

ACH50)

Foundation 

Type

Floor 

Area

Type of 

Flue

Garage 

Location 

MN01 Story-and-ahalf 1,680/5.80 Basement 

(conditioned) 2,174 Exterior masonry

Attached

MN02 Story-and-ahalf 1,373/4.48 

Basement with 

partial crawl 

space 

(conditioned)

2,300 Interior 

masonry 

Detached

MN03 Story-and-ahalf 1,415/4.68 Basement 

(conditioned) 2,269 Exterior masonry

Detached

MN04 Story-and-ahalf 1,277/7.07 Basement 

(conditioned) 1,354 Interior masonry

Detached

MN05 Story-and-ahalf 2,370/7.25 

Basement with 

partial crawl 

space 

(conditioned)

2,453 Exterior 

masonry 

Detached

MN06 Ranch 995/3.89 Basement 

(conditioned) 1,920 Exterior masonry

Detached

MN07 Ranch 1,061/4.08 Basement 

(conditioned) 1,950 B-vent Detached

MN08 Story-and-ahalf 2,633/11.14 Basement 

(conditioned) 1,772 Exterior masonry

Attached

MN09 Story-and-ahalf 2,279/6.51 Basement 

(conditioned) 2,624 Interior masonry

Attached
34 

House Type

Air 

Leakage 

(CFM50/ 

ACH50)

Foundation 

Type

Floor 

Area

Type of 

Flue

Garage 

Location 

MN10 Ranch 1,153/4.16 Basement 

(conditioned) 2,080 B-vent Detached

WI01 Story-and-ahalf 3,009/8.55 Basement 

(conditioned) 2,639 Exterior masonry

Attached

All but one of the houses had forced-air heating systems. All the houses with central air 

conditioning had those systems connected in common with their forced-air heating systems. All 

the water heaters were natural draft, and 7 of the 11 houses had orphaned water heaters (water 

heaters that were common vented before the other appliances were removed from the vent). The 

furnaces on the common vented systems were either natural draft or induced draft. Figure 8

shows the combustion vent system of an orphaned water heater; Figure 9 shows a common 

vented water heater with the other vent exiting from the heating system. 

Table 7 lists the components of the mechanical systems present in each house.

Figure 8. Orphaned water heater Figure 9. Common vented water heater

Testing was performed to measure the cold vent establishment pressure (CVEP). The CEVP is 

the highest level of CAZ depressurization for which the appliance can overcome a cold stack and 

establish a draft up the flue. This provides an appliance- and house-specific prediction of the 

level of CAZ depressurization necessary to cause depressurization-induced combustion gas 

spillage. Instead of a “generic” depressurization guideline for an appliance type (e.g., 5 Pa for 

natural draft appliances), the measured CVEP is specific to the appliance location, combustion 

vent configuration, and other factors that impact the ability of an appliance to draft properly 

under depressurization conditions (Table 8 through Table 11). For example, a water heater can 

more easily establish a draft when the vent connector runs straight up from the draft hood to the 

roof termination than when an elbow is attached to the draft hood followed by a 4-ft horizontal 

run before it connects to a vertical section. An appliance can more easily establish draft when the 

vertical section is closer to the draft hood.
35 

Table 7. Mechanical Systems of Participating Houses

House Heating 

System Type

Water Heater 

Type

Water Heater 

Venting

Air 

Conditioning

Whole-House 

Ventilation

MN01 Hydronic, 

direct vent Natural draft Orphaned Window ERV

MN02 Forced air, 

direct vent Natural draft Orphaned Central N/A

MN03 Forced air, 

induced draft Natural draft Common vented Central N/A

MN04 Forced air, 

natural Natural draft Common vented Window Continuous exhaust only

MN05 Forced air, 

induced draft Natural draft Common vented Central N/A

MN06 Forced air, 

direct vent Natural draft Orphaned Central N/A

MN07 Forced air, 

direct vent Natural draft Orphaned Central N/A

MN08 Forced air, 

induced draft Natural draft Common vented Window N/A

MN09 Forced air, 

direct vent Natural draft Orphaned Window N/A

MN10 Forced air, 

direct vent Natural draft Orphaned Central N/A

WI01 Forced air, 

direct vent Natural draft Orphaned Central N/A

Table 8. Combustion Vent Configuration: Orphaned Water Heaters

House Input 

(Btu/h)

Diameter

(in.) 

Rise, 

above 

DD (ft)

Vent 

Height 

(ft)

Total 

Lateral 

(ft)

All 

Elbows 

Offsets

Table 

Limit/ 

Inputa

Vent 

Meets 

Code

MN 01 39,600 4 1.5 27 5 3 136% Yes

MN 02 39,500 3 0.5 23 4 2.5 119% Yes

MN 06 28,000 4 1.0 20 2 3 248% Yes

MN 07 37,000 4 1.5 18 4 2.5 238% Yes

MN 09 40,000 4 0.3 20 1 1 217% Yes

MN 10 40,000 4 1.5 18 1 2 185% Yes

WI 01 38,000 3 0.3 25 20 6 N/A No

a Ratio of NFPA 54 vent maximum capacity with actual input rate.
36 

Table 9. Combustion Vent Connector Configuration: Common Vented Water Heaters

House

WH 

Input 

(Btu/h)

Type Diam 

(in.) 

Rise, 

Total 

(ft)

Rise, 

above 

DD 

(ft)

Run 

(ft)

90 

Elbow 

Saddle

Table 

Limit/ 

Input

Connector 

Meets 

Code

MN 

03 39,706 SW 4 1 0.5 4 2 Wye 138% Yes

MN 

04 33,000 SW 3 2 0.5 6 4 Stacked 

tee

75% No

MN 

05 40,000 SW 4 1 1 1 1 B-vent 

Wye 138% Yes

MN 

08 40,000 SW 4 2 2 10 3 Stacked 

tee

137% Yes

Table 10. Common Vent Configuration: Common Vented Water Heaters

House

Total Input 

(Btu/h)

Other 

Appliance

Type

Dia

meter

(in.) 

Height (ft)

Liner 

Offset

Lateral (ft)

Cap

Table 

Limit/

Input

Common 

Vent Meets 

Code

MN 

03 105,706 ID 

furnace SW 6 24 Yes 2 Listed 124% Yes

MN 

04 128,000 Nat 

furnace SW 5 30 No 0 Listed 105% Yes

MN 

05 110,000 ID 

furnace SW 4 24 No 1 None 106% Yes

MN 

08 106,000 ID 

furnace

SW 

offset 5 26 Yes N/A Listed 113% Yes

Table 11. Combustion Air Configuration

House

CAZ Allowance Outdoor Air Allowance

Combined 

Allowance 

(Btu/h) 

Appl 

Input 

(Btu/h) 

% of 

Code 

Req

Meets 

Code 

CAZ 

Vol 

(ft3

) 

Allowed 

Input 

(Btu/h) 

Outdoor 

Comb 

Air

(in.

2

)

Free 

Air 

(in.

2

) 

Allowed 

Input 

(Btu/h) 

MN 

01 720 14,400 12.6 9.45 28,350 42,750 39,600 108% Yes

MN 

02 5,460 109,200 19.6 14.7 44,100 153,300 40,000 383% Yes

MN 

03 11,040 220,800 19.6 14.7 44,100 264,900 40,000 662% Yes

MN 

04 7,100 142,000 0 0 0 142,000 140,000 101% Yes
37 

House

CAZ Allowance Outdoor Air Allowance

Combined 

Allowance 

(Btu/h) 

Appl 

Input 

(Btu/h) 

% of 

Code 

Req

Meets 

Code 

CAZ 

Vol 

(ft3

) 

Allowed 

Input 

(Btu/h) 

Outdoor 

Comb 

Air

(in.

2

)

Free 

Air 

(in.

2

) 

Allowed 

Input 

(Btu/h) 

MN

05 6,720 134,400 19.6 14.7 44,100 178,500 110,000 162% Yes

MN 

06 210 4,200 0 0 0 4,200 28,000 15% No

MN 

07 1,260 25,200 19.6 14.7 44,100 69,300 40,000 173% Yes

MN 

08 6,200 124,000 19.6 14.7 44,100 168,100 106,000 159% Yes

MN 

09 960 19,200 19.6 14.7 44,100 63,300 40,000 158% Yes

MN 

10 7,500 150,000 19.8 14.85 44,550 194,550 40,000 486% Yes

WI 

01 6,662 133,240 0 0 0 133,240 38,000 351% Yes

The CVEP test procedure is described in Appendix A. In brief, a blower door creates a level of 

depressurization that is expected to generate backdrafting, the appliance is fired, and the blower 

door flow is gradually reduced until the appliance drafts properly. A reversal of positive to 

negative vent pressure is often used to indicate when draft is established. This test has not been 

used extensively in the laboratory or in the field. Additional experience is necessary to better 

understand the repeatability of the results and usefulness for predicting combustion spillage 

issues. Table 17 (in Section 4.4.4) lists the outdoor test conditions, the measured CVEP, and the 

measured CAZ depressurization using the simplified protocol. The CVEP varied widely with a 

low value of –1.7 Pa and high of –7.4 Pa. Some of the variation could be due to greater 

uncertainty for windy conditions when brief wind gusts can “trip” a backdrafting situation into 

positive venting. However, the CVEP ranged from –2.6 Pa to 6 Pa for the four houses tested 

when the wind speed was lower than 10 mph. This suggests that the appliance vent configuration 

and other factors have a significant impact on the resistance to depressurization induced spillage 

and that a single value for a depressurization guideline for safe venting may incorrectly identify a 

large fraction of passes or failures. 

Because the selection criteria required that these houses failed spillage for the simplified 

depressurization conditions, the measured depressurization for the simplified protocol is 

expected to be larger than the CVEP. That was true for 7 of the 10 houses and for 2 of the other 3 

houses the depressurization was within 1.1 Pa of the CVEP. Further work is necessary to better 

understand the test repeatability, protocol changes to improve repeatability, and vent system or 

other factors that influence the CVEP.

The exhaust appliances in the houses were the clothes dryer (located in the basement), an exhaust 

fan (typically multispeed), and at least one bathroom exhaust fan. Table 12 and Table 13 show 

the flow rates of the exhaust fans located in each house. Exhaust flow rates were directly 

measured using a flow hood whenever possible. Otherwise they were deduced from pressure 
38 

measurements. Houses MN 07 and MN 10 had a bathroom ceiling exhaust fan configured to run 

continuously as a whole-house ventilation system with a flow rate of 30 CFM and 72 CFM, 

respectively. A third house (MN 04) had a kitchen fan that ran continuously at a low speed with 

a flow rate of 30 CFM. 

Table 12. Clothes Dryer and Kitchen Exhaust Fan Characteristics

House

Clothes Dryer Kitchen Exhaust Fan

Location Flow Rate

(CFM) Type Flow Rate(CFM)

MN 01 Basement 125 Through wall 230/80

MN 02 Basement 95 Roof termination 159/N/A

MN 03 Basement 135 Roof termination 183/93

MN 04 Basement 130 Through wall 272/N/A

MN 05 Basement 175 Through wall 261/202

MN 06 Basement 87 Through wall 270/95

MN 07 Basement 87 Roof termination 167/122

MN 08 Basement 84 Roof termination 250/178

MN 09 Basement 154 Roof termination 276/220

MN 10 Basement 86 Roof termination 121/20

WI 01 Basement 90 N/A N/A

Table 13. Bathroom Exhaust Fan Characteristics

House

Bathroom Exhaust Fan(s)

One Two

Exhausted to: Flow Rate

(CFM) Exhausted to: Flow Rate(CFM)

MN 01 Through wall 30 Through wall 21

MN 02 Roof termination 70 Through wall 20

MN 03 Through wall 65 Roof termination 50

MN 04 Roof termination 44 N/A N/A

MN 05 Roof termination 39 N/A N/A

MN 06 Through wall 62 Roof termination 68

MN 07 Roof termination 108 Through wall 26

MN 08 Roof termination 59 N/A N/A

MN 09 Roof termination 62 N/A 33

MN 10 Roof termination 130 Through wall 72

WI 01 Through wall 46 N/A N/A

4.4.4 Short-Term Testing and Inspection

For the 11 houses that met the screening criteria, short-term testing was scheduled on the same 

day that instrumentation was installed in the houses (Table 14). In some cases it was also 

repeated after monitoring equipment was added. This work was done by a two-person team, one 

to perform the short-term testing and the other to do the instrument installation. The work took a 
39 

full day to complete, including data collection. The short-term testing took place during both 

winter and spring, from the middle of February 2015 to the middle of May 2015. 

Table 14. Scheduled Visits of Screening and Short-Term Testing Visits

House Screening Date Short-Term Testing Date

MN 01 12/3/2014 2/15/2015

MN 02 12/5/2014 2/16/2015

MN 03 2/16/2015 2/23/2015

MN 04 2/20/2015 2/27/2015

MN 05 4/6/2015 4/9/2015

MN 06 4/6/2015 4/17/2015

MN 07 4/13/2015 4/24/2015

MN 08 4/30/2015 5/7/2015

MN 09 5/5/2015 5/12/2015

MN 10 5/6/2015 5/19/2015

WI 01 3/25/2015 4/4/2015

In performing the pressure measurements for the short-term testing, interior door positions were

concerns, particularly with forced-air systems. Table 15 provides the door positions for each 

participating house. 
40 

Table 15. House Interior Door Positions

House CAZ Door Location CAZ Door 

Position BPI

Additional Doors Closeda: 

BPI

Additional Doors Closedb: 

Pressure

MN 01 Mechanical room Open

2-2nd-floor bedrooms, 1st-

floor bedroom, 1st-floor 

bath, basement office

2-2nd-floor bedrooms, 1st-floor

bedroom, 1st-floor bath, 

basement office

MN 02 Top of basement stairs Open 2nd floor, main-floor office, 

basement bedroom

2nd floor, main-floor office, 

basement bedroom

MN 03 N/A N/A None 2nd floor, 2 main-floor bedrooms, 

hall to bedrooms

MN 04 N/A N/A 2 -2nd-floor bedrooms 2-2nd-floor bedrooms

MN 05 Top of basement stairs Open Main-floor bath Main-floor bath, main-floor hall

MN 06 Mechanical room Open None 2 main-floor bedrooms

MN 07 Utility room Closed None None

MN 08 Top of basement stairs Open Main-floor bath Main-floor bath, 2nd floor,

2 main-floor bedrooms

MN 09 Utility room Open None 2 basement bedrooms,

basement bath

MN 10 Top of basement stairs Closed Master bedroom Master bedroom

WI 01 Top of basement stairs Closed Bonus room, 2nd-floor bath Bonus room, 2nd-floor bath,

2nd-floor east bedroom a Interior doors closed based on BPI protocol. b Interior doors closed based on assessment of pressure difference across door. 
41 

Figure 10 through Figure 20 show the house depressurization that resulted from exhaust fan 

operation for each house operating individually and in concert. The error bars indicate the 

uncertainty of the change in house pressure due to wind-driven variations in the measured 

pressure. The uncertainties were computed using the square root of the sum of the squares of the 

standard deviations of the 1-second measurements for the baseline and fan-on periods. The 

uncertainties were minimized by using outdoor pressure references at all four sides of the houses 

and a weighted average of three 30-second baseline and three 30-second fan-on periods. See 

Appendix A for further details. 

Figure 10. House MN 01 exhaust fan house depressurization (in Pa)

Figure 11. House MN 02 exhaust fan house depressurization (in Pa)

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.0

Kitchen

Bath1

Bath2

Dryer

All

House Pressure (in Pa)

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.0

Kitchen

Bath1

Bath2

Dryer

All

House Pressure (in Pa)
42 

Figure 12. House MN 03 exhaust fan house depressurization (in Pa)

Figure 13. House MN 04 exhaust fan house depressurization (in Pa)

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.0

Kitchen

Bath1

Bath2

Dryer

All

House Pressure (in Pa)
43 

Figure 14. House MN 05 exhaust fan house depressurization (in Pa)

Figure 15. House MN 06 exhaust fan house depressurization (in Pa)

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.0

Kitchen

Bath1

Bath2

Dryer

All

House Pressure (in Pa)
44 

Figure 16. House MN 07 exhaust fan house depressurization (in Pa)

Figure 17. House MN 08 exhaust fan house depressurization (in Pa)

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.0

Kitchen

Bath1

Bath2

Dryer

All

House Pressure (in Pa)

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.0

Kitchen

Bath1

Bath2

Dryer

All

House Pressure (in Pa)
45 

Figure 18. House MN 09 exhaust fan house depressurization (in Pa)

Figure 19. House MN 10 exhaust fan house depressurization (in Pa)

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.0

Kitchen

Bath1

Bath2

Dryer

All

House Pressure (in Pa)

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.0

Kitchen

Bath1

Bath2

Dryer

All

House Pressure (in Pa)
46 

Figure 20. House WI 01 exhaust fan house depressurization (in Pa)

Four WC test protocols were performed and pressure measurements were taken to compare the 

protocols: 

The BPI protocol 

A comprehensive protocol that modifies the BPI protocol by determining the interior 

door positions based on pressure 

The STP proposed by this project 

A modified STP again with the interior door positions based on pressure. 

Table 16 shows the CAZ pressure measurements recorded for each protocol for 10 of the 

participating houses. The pressure measurements for house MN 10 could not be performed by 

the time of the writing of this report.

Table 16. Comparison of CAZ Pressure Measurements from Four WC Protocols for

Participating Houses, in Pa (SD = standard deviation)

House BPI Comprehensive STP Modified STP

SD SD SD SD

MN 01 –4.0 0.1 –4.0 0.1 –3.6 0.2 –3.8 0.2

MN 02 –5.4 0.2 –5.4 0.2 –4.1 0.1 –4.1 0.2

MN 03 –7.8 0.1 –7.8 0.1 –6.8 0.2 –6.7 0.0

MN 04 –9.7 0.1 –9.8 0.4 –10.0 0.3 –9.8 0.5

MN 05 –3.7 0.1 –5.0 0.1 –3.5 0.1 –5.0 0.3

MN 06 –13.1 0.4 –13.7 0.7 –10.1 0.4 –10.1 0.3

MN 07 –13.4 0.1 –13.4 0.1 –11.1 0.1 –11.1 0.1

MN 08 –2.6 0.1 –2.3 0.1 –2.1 0.1 –2.3 0.1

MN 09 –4.7 0.2 –5.2 0.1 –5.0 0.1 –5.1 0.2

WI 01 –1.6 0.2 –1.9 0.1 –1.7 0.1 –1.9 0.1

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.0

Kitchen

Bath1

Bath2

Dryer

All

House Pressure (in Pa)
47 

To look at the effectiveness of the STP, the pressure measurements from the STP were plotted 

separately versus the BPI, comprehensive, and modified STP protocols, respectively. These are 

shown in Figure 21 through Figure 23. 

Figure 21. Comparison of the BPI protocol with the STP for measuring CAZ depressurization (in 

Pa)

Figure 22. Comparison of the comprehensive protocol with the STP for measuring CAZ 

depressurization (in Pa)

BPI Depressurization (in Pa)

Simplified Depressurization (in Pa)

Comprehensive Depressurization (in Pa)

Simplified Depressurization (in Pa)
48 

Figure 23. Comparison of the modified protocol with the STP for measuring CAZ depressurization 

(in Pa)

The results of the STP approach and the BPI protocol show good agreement with the exception 

of two houses: MN 06 and MN 07. The modified STP protocol did not have significantly 

different results than the STP protocol, suggesting that the prescribed door position approach is 

adequate. A final check on using the prescribed door position approach versus the pressure 

approach is provided by comparing the comprehensive protocol results with the BPI protocol 

measurements. Again the results agree well.

Figure 24. Comparison of the comprehensive protocol with the

BPI protocol for measuring CAZ depressurization (in Pa)

Modified Depressurization (in Pa)

Simplified Depressurization (in Pa)

Comprehensive Depressurization (in Pa)

BPI Depressurization (in Pa)
49 

Finally, testing was performed to measure the CVEP. The CEVP is the highest level of CAZ 

depressurization for which the appliance can overcome a cold stack and establish a draft up the 

flue. The CVEP test procedure is described in Appendix A. Table 17 lists the outdoor test 

conditions, the measured CVEP, and the measured CAZ depressurization using the STP. The 

CVEP varied widely with a low value of –1.7 Pa and high of –7.4 Pa. This suggests that the 

appliance vent configuration and other factors may have significant impacts on the resistance to 

depressurization-induced spillage and that a single value for a depressurization guideline for safe 

venting may not be appropriate. Because these houses all failed spillage for the simplified 

depressurization conditions, the measured depressurization for the simplified protocol is 

expected to be lower than the CVEP. That was true for 7 of the 10 houses. Further work is 

necessary to better understand the repeatability and factors that influence the CVEP. House WI 

01 continued to spill after the backdrafting was established during the test. All the other houses 

re-established a draft after the test was performed.

Table 17. House CVEP Measurements

House

Outside 

Temperature

(°F)

Wind Speed 

(mph)

CAZ Base 

Pressure

(Pa)

CVEPa Simplified 

Depressurization 

(Pa)

MN 01 35 23 –2.3 –4.4 –3.6

MN 02 27 18 –2.4 –7.4 –4.1

MN 03 56 7 –1.3 –2.6 –6.8

MN 04 14 8 –3.1 –6.0 –10.0

MN 05 37 10 –2.3 –3.4 –3.5

MN 06 74 6 –0.9 –3.1 –10.1

MN 07 46 15 –0.8 –4.9 –11.1

MN 08 70 15 –0.1 –1.9 –2.1

MN 09 54 16 –1.4 –6.1 –5.0

MN 10 43 7 –2.4 –4.8 N/A

WI 01 80 11 –0.2 –1.7 –1.9 a CAZ depressurization when appliance vent pressure transitions from positive to negative

4.4.5 Long-Term Monitoring

The project team gathered monitoring data starting January 16, 2015, and with changes in the 

monitoring system and software, began gathering usable data on February 9. Data collected 

through August 17 were used for the analysis in this report. Communication was lost with House 

MN 10 near the end of June; because of an extended absence by the homeowner, data after that 

date were not recovered in time to be included in the analysis Table 18 shows the sampling dates 

for each home. Houses in the upper Midwest may generally be closed up more tightly in the 

winter (and are thus more susceptible to depressurization from exhaust fan or air handler 

operation), draft force is reduced in warm weather, and data collection across a range of outdoor 

temperatures allows more complete characterization of spillage under normal operating 

conditions. 
50 

Table 18. Data Collection Dates by House 

House First Data Used Last Data Used Number of Days 

of Data Used

MN 01 2/10/2015 8/17/2005 188

MN 02 2/ 9/2015 8/17/2005 189

MN 03 2/23/2015 8/17/2005 175

MN 04 2/27/2015 8/17/2005 171

MN 05 4/9/2015 8/17/2005 130

MN 06 4/17/2015 8/17/2005 122

MN 07 4/24/2015 8/17/2005 115

MN 08 5/7/2015 8/17/2005 102

MN 09 5/12/2015 8/17/2005 97

MN 10 5/19/2015 6/29/2015 41

WI 01 2/18/2015 8/17/2005 180

The monitoring system generally performed well. A limited number of spurious data points 

appeared in values coming from the onboard analog-to-digital converters and were filtered out of 

the data set. Additionally, several sensor-specific issues were identified: 

Outdoor temperature values showed large, unexpected variations over periods of minutes 

at several sites. The installation team used the clothes dryer exhaust duct as a conduit for 

the outdoor sensor wire and tubing, and exposure to heated air from the dryers is the 

likely cause of the problem. The team ultimately chose to use airport weather data for 

Minneapolis and Madison as a replacement for on-site temperature data. 

Zone pressure measurements sometimes exhibited very high variability from one second 

to the next, much larger than would be expected from indoor-outdoor measurements 

averaged over 1 second or longer. CARB and NorthernSTAR concluded this is due to the 

influence of wind pressure on the outdoor termination of the pressure measurement 

system in combination with the responsiveness of the pressure sensor and lack of any 

damping in the measurement system. High variability in measured zone pressure 

correlated well with wind speed (from airport weather data). The team could not tell with 

certainty whether the wind-induced scatter introduced a significant bias and decided to 

filter out pressure readings that showed more than 11 Pa variation over a 1-minute period 

and to record these periods as “windy.” This affected about 11% of zone pressure data 

and less than 0.3% of water-heater-vent pressure data. The zone pressure data values used 

in this report represent the outdoor pressure relative to the indoor (CAZ) pressure, so a 

depressurized CAZ has a positive value. 

Fan status data from wireless modules proved spotty in some cases, and data from several 

devices were unusable. Wireless sensor modules were dislodged from the intended 

mounting position in at least three cases, with an extended loss of data from a kitchen 

exhaust fan in one case. 

A single atmospherically vented furnace with a draft diverter was included in the study 

(at House MN 04). This furnace was a convection design (lacking a blower) and had an 

unusually large-diameter concentric draft diverter. Clearly well out of the mainstream of 
51 

current technology, it was excluded from analysis. The water heater at this house is 

included in the results. 

4.4.5.1 Monitoring Results—Characteristic System Operation

4.4.5.1.1 Water Heater Operation

The water heaters in the houses studied went through 2 to 5 cycles in a typical day, with the 

exception of House MN 08, which saw only 1.1 cycles per day on average (Figure 25). Water 

heater burner operation nearly always lasted for 5 minutes or longer, but characteristic runtimes 

varied greatly across houses (Figure 26). House MN 04 had a particularly large fraction of cycles 

that lasted more than 30 minutes. 

Figure 25. Average number of water heater burner cycles per day, by house

Figure 26. Water heater burner runtime distribution, by house 

0 1 2 3 4 5

W ater he ater cycles per day

MN_01 MN_02 MN _03 MN _04 MN_05 MN_06 MN _07 MN _08 MN_09 MN_10 W I_01

0

1 0

2 0

3 0

4 0

0

1 0

2 0

3 0

4 0

0

1 0

2 0

3 0

4 0

0 1 0 20 30 >30

0 1 0 20 3 0 >30 0 10 2 0 30 >30 0 1 0 20 3 0 >30

MN _01 MN_02 MN_03 MN_04

MN _05 MN_06 MN_07 MN_08

MN _09 MN_10 W I_ 0 1

P e rc e n t o f cy c le s

C y c le le n g th (m in u te s )

(2-m in ute bins )
52 

Burners in conventional residential gas water heaters operate when a thermostat senses a water 

temperature lower than a threshold setting and opens the main gas valve. The gas is ignited by a 

standing pilot, and combustion products begin to flow up the internal flue, transferring heat to 

the water through the burner compartment and flue wall. In normal operation, the combustion 

products begin to flow across the draft hood gap (though some spillage of combustion products 

often occurs), then mix with additional air introduced at the draft hood and flow into the venting 

system and vertical chimney. Heating of the vent system increases draft and flow rate until a 

quasi-steady-state condition is reached. When the burner is off, the flue temperature and vent 

temperature typically remain elevated much higher than room temperature, heated by the pilot 

light and warm storage tank. Figure 27 shows measured conditions during a typical burner cycle. 

Figure 27. Conditions during characteristic water heater burner cycle, House MN 01.

Time shown in hours, minutes, and seconds. A brief increase in CO2 concentration at the

start of the cycle is evidence of spillage, but the four spillage temperatures in the

bottom panel show no discernable rise. 

0

250

500

13:00:00 13:30:00 14:00:00 14:30:00

Burner Vent

Burner and Vent temperature (F)

0

2500

5000

13:00:00 13:30:00 14:00:00 14:30:00

Vent Zone

Vent and zone CO2 (ppm)

-4

0

4

13:00:00 13:30:00 14:00:00 14:30:00

Zone Vent

Vent and zone pressure (Pa)

50

100

150

200

13:00:00 13:30:00 14:00:00 14:30:00

Spill temperatures (F)

(May 23, 2015; times are UTC)
53 

Vent pressure is typically very slightly positive during the off cycle (about –0.7 Pa in the case 

shown), and drops (to about –5 Pa in this case) as the venting system heats up and flow 

increases. The zone pressure is influenced by many factors and may run positive or negative 

during firing cycles. 

4.4.5.1.2 Fan Operation

Table 19 summarizes the monitoring of devices expected to affect CAZ depressurization and 

spillage. 

Table 19. Summary of Air Handlers, Exhaust Devices, and CAZ Door Monitoring

House Air 

Handler Dryer Kitchen Exhaust Bath Fan 1 Bath Fan 2 CAZ Door 

MN 01 Not 

presenta Y Y No datab –c Y 

MN 02 Y Y No data Y Y Y

MN 03 Y Y Y Y Y Not present

MN 04 Not 

present Y Y 

No data, 

probably 

used rarely

– No data 

MN 05 Y Y Y Y – Y

MN 06 Y Y Y Y – Y

MN 07 Y Y Y Y – Y

MN 08 Y Y Y No data – Y

MN 09 Y Y Y No data Y Y

MN 10 No data Y Y Y 

No data, 

probably 

used rarely

Not present 

WI 01 Y Y Not present Y Not present Y a Not present means the item did not exist in that house. a No data means the item was monitored but the data were unusable. c

 “–” means the item was either not present or not monitored. 

The observed operating status of air handlers, exhaust fans, and CAZ doors is summarized in 

Figure 28 (N/A means fan does not exist or data are unusable). The secondary red shading on 

each bar indicates the average number of minutes per day during which the device operated (or 

door was closed) with the water heater burner operating simultaneously.
54 

Figure 28. Fan runtime and CAZ door closed time, by house 

4.4.5.2 Monitoring Results—Spillage 

4.4.5.2.1 Identifying Spillage

The monitoring system design included two methods for identifying spillage: temperature 

sensing around the draft hood and CO2 sampling above the draft hood. Review of data collected 

makes it clear that CO2 measurement is a far more sensitive and robust means of identifying 

spillage. 

When spillage is significant and lasts much longer than 1 minute, the CO2 signal and the spill 

temperature sensor rise are usually large and unambiguous, and either could be used to identify 

such spillage. Brief spillage episodes on startup present a very different picture, however. CO2

spikes were frequently found during the first 20 seconds of water heater burner operation, which 

disappear before the end of the first minute, demonstrating brief spillage on startup. This 

phenomenon is apparent in Figure 27). The temperature rise around the draft hood during these 

brief spillage episodes, however, is frequently small and very uneven; one or more temperatures 

fall while one or more rise. Spill temperature sensor data are further complicated by a tendency 

for the temperatures to rise later in each burner cycle, after spillage has ended, likely driven by 

radiation from the heated flue. The result is that spill temperatures present a complicated picture 

and would be difficult to interpret reliably. 

A CO2 elevation of 500 ppm higher than the CAZ background level was used as an indicator of 

spillage. Background CO2 levels can vary, especially during prolonged spillage episodes and 

N A N A N A NA NA NA

N A N A N A NA N A NA N A NA N A NA N A NA

0

5 0

100

150

200

0

2 0

4 0

6 0

8 0

0

5 0

100

0

2 0

4 0

6 0

0

5

1 0

1 5

0

500

1,000

1,500

MN_01

M N_02MN_03

M N_04MN_05

M N_06

MN_07M N_08

MN_09

M N_10

WI_01

M N_01

M N_02

M N_03M N_04

M N_05M N_06

M N_07

M N_08M N_09

M N_10

W I_01

M N_01M N_02

M N_03

M N_04M N_05

MN_06M N_07

MN_08

M N_09MN_10

W I_01

Air ha ndler Dryer Kitch en e xha us t fan

Ba th e xha ust fan #1 Ba th exhau st fa n # 2 CAZ door

T otal C oinc ident w ith

W H o perati on

M ean m inutes per day
55 

occasionally at other times for unknown reasons; thus, the increase over the background 

concentration is a more robust indicator than any fixed level.1

 

As discussed earlier, CO2 measurements were made sequentially at the water heater draft hood, 

furnace draft hood,2 and room background on a 20-second cycle, with 10 usable values recorded 

in each cycle. This results in an effective resolution of water heater spillage data of 40 to 60 

seconds. In this report, spillage data are summarized in terms of the number of minutes the 

spillage lasted. 

Figure 29 shows a typical pattern associated with a persistent spillage episode. Observed CO2

concentrations start near 5,000 ppm and slowly dropped to near CAZ background level, 

indicating an end of spillage about the time the burner cycle ends. Vent pressure is negative 

throughout the period displayed (consistent with normal flow up the vent) and drops slightly 

during burner operation, indicating developing draft. The four spillage temperatures spaced 

around the draft hood show a distinct rise, confirming spillage early in the burner cycle, followed 

by a drop indicating the end of spillage. 

 

1 The team arrived at 500 ppm by informally examining CO2 concentrations, spillage temperatures, and vent 

pressures across a number of burner cycles and confirmed this value as a useful indicator of a change point by 

inspecting the distribution of CO2 elevation higher than background levels, which exhibited a “knee” at around 500 

ppm elevation. CO2 concentration, however, was often observed at around 4,500–5,000 ppm during spillage on 

startup and generally recovered quickly to very near the room background level when spillage ended. The 

identification of spillage is not particularly sensitive to the parts per million elevation selected. 2 The extra measurement was required at the one house with a furnace but included at some other houses.
56 

Figure 29. Conditions during long-lasting spillage episode, House WI 01. Time shown in hours, 

minutes, and seconds. A sustained elevation of CO2 concentration and obvious rise in all four 

spillage temperatures (bottom panel) provide evidence of prolonged spillage.

0

250

500

14:00:00 14:30:00 15:00:00

Burner Vent

Burner and Vent temperature (F)

0

2500

5000

14:00:00 14:30:00 15:00:00

Vent Zone

Vent and zone CO2 (ppm)

-4

0

4

8

14:00:00 14:30:00 15:00:00

Zone Vent

Vent and zone pressure (Pa)

50

100

150

200

14:00:00 14:30:00 15:00:00

Spill temperatures (F)

(Feb 20, 2015; times are UTC)
57 

4.4.5.2.2 Spillage Observed by House

The number and duration of spillage episodes varied dramatically across the houses studied. 

Houses MN 04 and WI 01 showed a far higher total time and percent of operating time in 

spillage than the other houses (Table 20). 

Table 20. Water Heater Spillage Time by House 

House Total Hours Burner 

Operating Time

Hours of 

Spillage

Percent of Operating 

Time in Spillage

MN 01 243.6 4.1 1.7

MN 02 134.7 0.7 0.5

MN 03 203.2 9.9 4.9

MN 04 226.6 105.5 46.6

MN 05 189.6 1.4 0.7

MN 06 176.2 1.1 0.6

MN 07 117.1 0.5 0.4

MN 08 34.7 0.4 1.1

MN 09 100.7 0.3 0.3

MN 10 39.0 0.2 0.6

WI 01 123.1 112.6 91.4

Except for the two spillage-prone houses, most of spillage identified occurred in the first minute 

of operation. This first-minute effect was especially pronounced at Houses MN 01 and MN 03 

(see Figure 30, with highlighting showing the number of cycles in which spillage was observed). 

Figure 30. Number of water heater cycles lasting at least as long as time displayed, in 2-minute 

bins. Cycles with spillage are differentiated by color. 
58 

4.4.5.3 Monitoring Results—Predictors of Spillage

The reasons for the far higher incidence of spillage at two houses are clear. House MN 04 had a 

vent system that was undersized for the capacity of the water heater, and WI 01 had a major 

venting defect in the form of a 5-in. connector that had once been the common vent attachment 

point for a furnace that was incompletely closed off. 

Inspection of the data suggests that some of the suspected drivers of spillage do indeed influence 

its frequency. To investigate the causal factors quantitatively, we used logistic regression to 

model the probability of spillage as a function of factors thought to contribute to it.3 These 

factors include:

Time into the firing cycle: As discussed, many water heaters spill on initial startup, 

before full draft is established. The project team created a binary indicator to represent 

the first minute of operation in each firing cycle to capture this phenomenon. 

Outdoor temperature: A spillage is more likely when the outside temperature is 

warm—and the stack effect is thus weak—than when the outside temperature is cold and 

stack forces are higher.

CAZ depressurization: The more the CAZ is depressurized relative to the outside, the 

more likely the water heater is to spill.

Operation of exhaust devices: Exhaust fans and clothes dryers depressurize the house 

when operated and thus can contribute to spillage. Air handler operation may 

depressurize the CAZ in specific houses. 

Closure of the door to the CAZ: Isolating the CAZ may lead to additional 

depressurization that could contribute to spillage. 

The last two factors manifested as an increase in CAZ depressurization. The project team 

therefore examined two models of spillage: (1) a model that used CAZ pressure directly as a 

predictor of spillage and (2) a model that substituted binary indicators of operation of various 

devices in place of the direct CAZ-depressurization measurements.

Table 21 summarizes the results of the first model, which examines how the first three factors 

above contribute to spillage.4 The fitted coefficients are expressed in terms of odds ratios: an 

odds ratio greater than unity indicates that an increase in the factor tends to increase the 

likelihood of spillage, and an odds ratio below unity means that in increase in the factor tends to 

decrease the likelihood of spillage.

As Table 22 shows, the coefficients are almost all greater than unity and highly significant, 

which indicates that, as expected, these factors tend to increase the likelihood of spillage. 

 

3 Logistic regression considers the probability of a binary outcome (spillage or no spillage) as a function of the 

possible causal factors. The team used a multiple regression method that models the effects of all independent 

parameters (“causal factors”) simultaneously, assigning each a coefficient that represents its contribution to 

increased spillage. 4 Appendix E includes complete descriptions of the model specifications and output.
59 

Table 21. Summary of Coefficients (odds ratio) from

Logistic Regression Predictors for Spillage (Model 1)

House

1st Minute of 

Operation

(Binary) 

Outdoor 

Temperature

(°F)

CAZ

Depressurization 

Relative to 

Outside

(Pa)§

MN 01 1,005.01 *** 1.10 *** 1.46 ***

MN 02 213.78 *** 1.23 *** 4.29 ***

MN 03 171.39 *** 1.07 *** 1.65 ***

MN 04 0.65 * 1.10 *** 1.21 *

MN 05 15.61 *** 1.06 *** 2.36 ***

MN 06 3.69 *** 1.10 *** 1.39 ***

MN 07 31.48 *** 1.03 2.32 ***

MN 08 244.16 *** 1.27 *** N/A†

MN 09 13.81 *** 1.09 * 2.79 ***

MN 10 396.99 *** 1.13 ** 2.74 ***

WI 01 N/A‡ 1.13 *** 1.07 *

Statistically significant at ***: a 99% confidence level; **: a 95% confidence level; *: a 90% confidence level.

†Omitted due to missing zone pressure data.

‡Omitted because spillage occurred throughout the first minute for nearly every firing cycle.

§CAZ pressures are defined here as outdoor pressure with respect to CAZ pressure, and a depressurized CAZ appears as a 

positive number.

Table 22. Summary of Coefficients (odds ratio) from Logistic Regression Predictors

(binary indicators of device operation) for Water Heater Spillage (Model 2) 

House Dryer Kitchen Fan Bath Fan 1 Bath Fan 2 Air Handler Door

MN 01 3.17a *** 2.91 *** ND ND ND NV

MN 02 15.03 *** ND 1.78 19.17 *** 1.93 NV

MN 03 3.28 ** 27.07 *** 2.40 *** 0.76 2.37 ** NV

MN 04 1.90 AS ND ND ND NV

MN 05 2.18 ** NS NV ND 1.04 1.95

MN 06 NS NV 2.75 ** ND 1.79 ND

MN 07 NS 13.10 *** NS ND 16.09 ** NV

MN 08 NV NV ND ND NV NV

MN 09 4.21 NS NS ND 9.E+04 *** 0.06 **

MN 10 ND ND 0.55 ND NS NV

WI 01 1.73 ND NV ND 0.91 1.05 a Bold text indicates statistical significance at a 90% or higher level.

Statistically significant at ***: a 99% confidence level; **: a 95% confidence level; *: a 90% confidence level.

Key:

ND = no data

NV = no variation (always on or always off)

AS = water heater always spilled when in operation

NS = water heater never spilled when in operation

The relative magnitudes of the odds ratios are also informative. The very large fitted odds ratios 

for some of the houses for the first minute of firing show that spillage is fairly common in the 
60 

first minute of operation but rare later. For example, the odds of spillage in the first minute at 

House MN 01 are 1,000 times higher than those of spillage in any subsequent minute. 

The seemingly more modest odds ratios observed for outdoor temperature and CAZ

depressurization are a bit trickier to interpret, partly because they represent the increase in the 

likelihood of spillage per unit increase in the predictor. For example, for House MN 01, the 

fitted odds ratio for outdoor temperature is 1.10. This means that on average, each Fahrenheit 

degree increase in outdoor temperature is associated with a 10% increase in the odds of spillage 

Extending this to seasonal changes, if typical winter and summer temperatures are 25°F and 

70°F, respectively, the odds of spillage will be about 50 times higher in summer than in winter. 

The odds of spillage may thus increase significantly if one examines them across a wide 

temperature or zone-pressure span.

However, the regression results are in terms of an odds ratio. If the odds of spillage are very low 

to begin with, spillage may still be rare even in the presence of the factor, even if it is a 

statistically significant predictor. The individual elements of the logistic regression are directly 

additive in predicting the combined effects of multiple factors. For example, the odds ratio 

associated with first-minute operation at House MN 05 is 15.61, and a similar odds ratio can be 

associated with a change in CAZ depressurization (outdoor pressure with respect to CAZ

pressure) of 6.6 Pa (because 15.61/2.36 = 6.6). This implies that, all other conditions being equal, 

the system has similar odds of spillage in the first minute with a CAZ depressurization of zero 

compared to a second minute with a CAZ depressurization of 6.6 Pa. 

Figure 31 shows the predicted probability of spillage with increasing outdoor temperature. 

Although temperature clearly has a significant effect, the overall probability of spillage at any 

time beyond the first minute remains extremely low for all houses except MN 04 and WI 01. 

Figure 31. Logistic model fits for probability of spillage at selected

outdoor temperatures and by time into firing cycle, by house 

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0F 20F 40F 60F 80F 0F 20F 40F 60F 80F 0F 20F 40F 60F 80F 0F 20F 40F 60F 80F

0F 20F 40F 60F 80F 0F 20F 40F 60F 80F 0F 20F 40F 60F 80F 0F 20F 40F 60F 80F

0F 20F 40F 60F 80F 0F 20F 40F 60F 80F 0F 20F 40F 60F 80F

MN_01 MN_02 MN_03 MN_04

MN_05 MN_06 MN_07 MN_08

MN_09 MN_10 WI_01

1st min. of operation beyond 1st min.

Probability of spillage

Estimated probabilities from logistic regression model
61 

Table 22 summarizes the device-related results for the model of spillage that includes binary 

indicators for device operation instead of CAZ pressure (Model 2). Some devices were not 

present in the homes, and others could not be included in the model because they were either 

always on or (more likely) always off over the course of the monitoring. But many of the 

remaining devices showed statistically significant indications of contributing to the likelihood of 

spillage.5 Clothes dryers significantly increased the probability of spillage in at least four houses, 

and kitchen exhaust fans and air handlers affected spillage in at least three houses. The CAZ 

door seemed to significantly reduced spillage when closed at house MN 09, perhaps a result of 

confounding between variables; e.g., if the door was more often open when the clothes dryer was

running. Air handlers may affect spillage by directly pressurizing or depressurizing the CAZ, 

depending on the distribution of supply-side and return-side leakage in the CAZ, the rest of the 

home, and to outside the building envelope, and may also indirectly affect spillage through 

heating or cooling, which affects the overall building stack effect and pressure balances. Again, 

even though these results are statistically strong in many cases, the overall impact of, say, dryer 

operation in a specific home is generally to increase a very low probability of spillage to a 

greater, but still quite low, probability. 

4.4.5.4 Monitoring Results—Downdrafting

Downdrafting is the term commonly used to describe full reversal of flow in the appliance 

venting system when the appliance is off (backdrafting is the term used for full reversal of flow 

when the appliance is operating). The team used a combination of positive vent pressure of at 

least 0.2 Pa and lower than typical vent temperature in the water heater vent as an indicator of 

downdrafting.6

The data summary by 1-minute periods includes about 2.04 million observations (about 1,400 

days’ worth). Of these, the team identified 5,377 minutes (about 0.03%) with apparent 

downdrafting (Table 23). The median duration of downdrafting was 18 minutes; some episodes 

lasted as long as 6 hours. 

Downdrafting often ends when a water heater burner fires, because some quantity of the hotter 

combustion products enters the vent system and starts to develop a normal draft. However, this is 

not always the case, and downdrafting can persist into or through burner cycles. Table 24

summarizes downdrafting that occurred during burner cycles in the test houses. Downdrafting 

during a burner operating cycle is characterized by a vent temperature that remains lower than 

normal, positive vent pressure, spillage zone temperatures that are elevated far above those seen 

during normal operation or modest spillage, and very high CO2 levels that last as long as both 

downdrafting occurs and burners operate (Figure 32). 

 

5

 For computational reasons, devices whose operation is either always or never associated with spillage also cannot 

be included in the model. The kitchen fan at House MN 04 was the only case in which the water heater always 

spilled when a device operated. However, this fan only operated on three occasions, and the water heater for the site 

spilled much of the time, so it is difficult to assess how much the fan contributed to spillage.

6 The specific rule selected for vent temperature indicating downdrafting during burner-off periods was at least 10°F 

lower than an average temperature calculated for daily periods using observations when the burner was off and had 

remained off for at least 2 minutes. For burner-on periods, a vent temperature of no more than 20°F higher than the 

same daily off-cycle average was used.
62 

Table 23. Incidence and Duration of Downdrafting by House 

House

Number 

of 

Episodes

Number of 

Minutes in 

Downdrafting

Percent of 

Operating Time in 

Downdrafting

Median 

Duration 

Maximum 

Duration 

MN 01 34 283 0.10% 7 44

MN 02 27 140 0.06% 6 63

MN 03 71 791 0.32% 10 92

MN 04 48 1,038 0.42% 96 383

MN 05 1 1 0.00% 1 1

MN 06 0 0.00%

MN 07 142 1,670 1.02% 16 102

MN 08 0 0.00%

MN 09 1 6 0.00% 3.5 6

MN 10 18 406 0.70% 28 137

WI 01 105 1,042 0.54% 24 211

Total 447 5,377 0.26% 18 383

Table 24. Incidence and Duration of Downdrafting Episodes

Coinciding with Water Heater Burner Operation

House

Number 

of 

Episodes

Median 

Duration 

(min)

Maximum 

Duration 

(min)

MN 01 2 16.5 17

MN 02 3 6 7

MN 03 32 14.5 33

MN 04 4 6.5 8

MN 05 0 – –

MN 06 0 – –

MN 07 14 4.5 31

MN 08 0 – –

MN 09 6 3.5 6

MN 10 1 9 9

WI 01 0 – –

Total 62 7 33
63 

Figure 32. Conditions during downdrafting episode, House MN 03 

0

250

500

Jul-29 00:00:00 Jul-29 00:30:00 Jul-29 01:00:00 Jul-29 01:30:00

Burner Vent

Burner and Vent temperature (F)

0

2500

5000

Jul-29 00:00:00 Jul-29 00:30:00 Jul-29 01:00:00 Jul-29 01:30:00

Vent Zone

Vent and zone CO2 (ppm)

-4

0

4

8

Jul-29 00:00:00 Jul-29 00:30:00 Jul-29 01:00:00 Jul-29 01:30:00

Zone Vent

Vent and zone pressure (Pa)

100

200

300

Jul-29 00:00:00 Jul-29 00:30:00 Jul-29 01:00:00 Jul-29 01:30:00

Spill temperatures (F)

Air handler

Dryer

Kitchen fan

1

2

3

Jul-29 00:00:00 Jul-29 00:30:00 Jul-29 01:00:00 Jul-29 01:30:00

(No operation for bath fans)

Exhaust-device operation

(Times are UTC)
64 

In general, CAZ depressurization appears to be a factor in the development of downdrafting, as 

expected; the average zone pressure during downdrafting was 2.3 Pa lower than outdoor pressure 

across all houses, compared to 0.9 Pa higher than outdoors when downdrafting is not occurring. 

Downdrafting clearly depends on some confluence of factors including the temperature and 

density of air in the vent system, outdoor temperature, and CAZ depressurization. One general 

statement that can be made is that the column of air in the vent system during downdrafting is 

probably always cooler and denser than during normal operation, because heat from the pilot 

light and water tank is absent. Combined with a modest CAZ depressurization (e.g., through air 

handler operation, exhaust fan operation, or building stack effect), the downward flow of air, 

once established, may tend to be self-maintaining. This suggests that the start of a downdrafting 

episode is the key to understanding the phenomenon, and the further analysis focused on the first 

minute of each downdrafting episode. 

Table 25 shows the difference in a number of measured parameters for the first minute of each 

downdrafting episode compared to all periods when downdrafting was absent. For outdoor 

temperature and CAZ pressure, the value shown is the difference in average measured values 

between downdrafting and nondowndrafting periods. For the variables that describe fan 

operation, the value shown is the difference in the average status across the periods. Results for 

the four houses that had zero or one downdrafting episode are excluded. 

This analysis strongly supports the obvious idea that CAZ depressurization was a factor in 

development of downdrafting at all homes, with outdoor less CAZ pressure 0.55 to 1.74 Pa 

higher at the time of downdraft formation than at other times. The air handler and individual 

exhaust fans played a significant role in some houses. The incidence of air handler operation at 

House MN 02, for example, was 50 percentage points higher during downdrafting episodes than 

at other times, and the incidence of kitchen fan operation at House MN 01 was 85 percentage

points higher during downdrafting. As noted earlier, air handler operation may affect CAZ 

pressures directly due to unbalanced leakage and indirectly by introducing warmer or cooler air 

into the building, changing stack effect pressure balances. 

CAZ door position, bathfan2, and wind speed were also considered but are not shown here. 

Average wind speed was about 2 mph higher at the start of downdrafting than at other times at 

several houses and may be a factor. Windiness as indicated by CAZ pressure variation did not 

appear to be a factor at any house. 
65 

Table 25. Explanatory Factors for Development of Downdrafting

House Downdraft 

Condition

# of 

Observations

Outdoor 

Temp. (F)

CAZ Pressure 

(PA)a

Air Handler 

Statusb

Dryer 

Status

Kitchen Fan 

Statusb

Bath Fan 

1 Statusb

MN 01

No downdraft 270,268 55 1.06 Not present 0.06 0.06 ND

1st minute 34 61 2.56 0.35 0.91

increase 6 1.50 0.30 0.85

MN 02

No downdraft 221,400 61 0.70 0.13 0.02 ND 0.04

1st minute 27 81 1.25 0.63 1.00 0.22

increase 20 0.55 0.50 0.98 0.18

MN 03

No downdraft 250,798 58 1.05 0.11 0.01 0.01 0.02

1st minute 71 76 1.72 0.45 0.23 0.35 0.43

increase 18 0.67 0.34 0.22 0.35 0.41

MN 04

No downdraft 245,103 59 1.93 Not present 0.04 0.00 ND

1st minute 48 74 2.82 0.54 0.10

increase 15 0.89 0.50 0.10

MN 05

No downdraft 186,746 66 0.53 0.09 0.05 0.000576 0.00

1st minute 1

increase

MN 06

No downdraft 175,679 67 0.49 0.04 0.01 0.00 0.00

1st minute 0

increase

MN 07

No downdraft 163,956 68 0.42 0.09 0.01 0.06 0.04

1st minute 142 75 2.16 0.78 0.02 0.31 0.13

increase 7 1.74 0.69 0.01 0.25 0.09

MN 08

No downdraft 144,099 69 0.02 0.00 0.01 0.01 ND

1st minute 0

increase

MN 09

No downdraft 139,529 70 0.41 0.01 0.04 0.00 0.00

1st minute 1

increase

MN 10

No downdraft 58,294 67 0.95 0.01 0.03

1st minute 18 72 2.16 0.00 0.11

increase 5 1.21 -0.01 0.09
66 

House Downdraft 

Condition

# of 

Observations

Outdoor 

Temp. (F)

CAZ Pressure 

(PA)a

Air Handler 

Statusb

Dryer 

Status

Kitchen Fan 

Statusb

Bath Fan 

1 Statusb

WI 01 

No downdraft 191,383 63 0.85 0.02 0.01 Not present 0.00

1st minute 105 81 2.16 0.36 0.34 0.00

increase 19 1.31 0.33 0.33 0.00

a CAZ pressure defined here as outdoor pressure relative to CAZ pressure. A depressurized CAZ shows a POSITIVE value.

b Status means the fraction of observations when the device was operating. 

ND: No data available. 

Not present: Device not present in this house. 
67 

4.4.5.4.1 Carbon Monoxide Accumulation 

Observed CO levels remained low in all the houses studied; the maximum observed 1-minute 

average value across all houses was 7 ppm. Though downdrafting is thought to trigger CO

production in some cases, the maximum values observed during downdrafting were the same as 

or lower than those observed during nondowndrafting conditions in the houses studied (Table 

26). The modest elevated levels of CO observed did appear to be related to water heater 

combustion; no evidence of other sources was found. 

Table 26. Observed CO Concentrations in CAZ Airspace by House 

House

Number of 1-Min 

Avg Observations 

Exceeding 3 ppm

Max Observed 1-Min Avg 

under Nondowndrafting

Conditions (ppm)

Max Observed 1-Min 

Avg under Downdrafting 

Conditions (ppm)

MN 01 0 1 1

MN 02 0 3 1

MN 03 20 7 5

MN 04 1,196 6 6

MN 06 17 1 1

MN 07 0 4 3

MN 08 0 3 1

MN 09 0 2 1

WI 01 24 3 1

4.4.5.5 Field Data Conclusions

Of the water heaters evaluated, two spilled combustion products in a majority of burner cycles, 

often for prolonged periods. Each system also had significant venting defects (undersizing or an 

inappropriate opening), which could be easily identified by a trained technician using visual 

inspection and venting code information only. Screening of homes for combustion safety should 

explicitly include vent inspection and sizing evaluation. 

Of the remaining nine systems, some demonstrated frequent spillage within the first minute of 

operation, but none showed a tendency toward frequent spillage beyond the first minute. For 

these nine systems, the maximum overall runtime spent in spillage was about 5%, with most 

below 1%. 

The largest influence on water heater combustion spillage was the specific house (systems and 

installation); temporary operating conditions such as exhaust fan operation and outdoor 

temperature played a lesser role. 

Considering the houses that did not have venting defects, operation of clothes dryers and 

individual exhaust fans had a statistically significant effect on the incidence of spillage in 

individual houses, though this generally appeared in the context of a low overall incidence of 

spillage; i.e., increasing the probability of spillage from one low number to another low number. 

Net CAZ depressurization and outdoor temperature also provide statistically significant 

predictors of spillages. 
68 

Stable downdrafting is a real phenomenon and can persist through burner cycles. Downdrafting 

during burner operation in the houses studied represented a small fraction of overall spillage. 

However, because downdrafting may lead to CO formation and release into living space, the 

conditions leading to development of downdraft should be further explored, and tests for 

susceptibility to downdrafting should be further developed. 

No significant accumulation of CO was found in any of the houses tested.
69 

5 Survey Results from State Weatherization Agencies 

on Combustion Safety Field Data Collection

As discussed previously, the recruitment of field-test houses was less successful than expected. 

States were willing to help but had a very difficult time finding eligible houses based on the 

screening criteria.

The difficulty in finding houses was surprising. The original work by Rapp et al. (2012) 

suggested that a significant frequency of homes failed combustion safety testing but were 

unlikely to have problems in practice, and these were the homes that this project was targeting. 

(The RAPP report was updated in 2015 [Rapp 2015].) Additionally, weatherization field staff 

had been reporting anecdotally that many homes failed combustion safety testing and needed to 

be addressed, especially once the WAP requirement for ASHRAE Standard 62.2 compliance was 

put into effect. This project targeted one type of potential failure: the result of nondryer exhausts. 

Failures due to dryers or air handlers were not included because dryer and air handler flow rates 

are not easily adjustable and were considered to be more likely to occur in practice.

The project team had assumed that locating suitable test homes would not be difficult. Given the 

challenge, the team sought additional sources that would support the notion that combustion 

safety failures due to exhaust-fan-induced depressurization was in fact known to be common. 

The first step was to inquire whether the National WAP Evaluation had this information. In that 

project, agencies across the country were asked to complete surveys detailing their operations. 

However, questions about combustion safety failure rates were not a part of these surveys. This 

lack of data indicated that it was not known whether these types of combustion safety failures 

were common, and a survey of this type was vital to understand the prevalence of combustion 

safety failures and to put the test home recruitment challenge into context. 

5.1 Development of the Survey Instrument

A survey was developed in May 2015 to be sent out nationally. The aims were to understand 

states’ methods for conducting WC depressurization tests, to quantify the prevalence of 

combustion safety failures, to understand the causes of these failures, and to assess the 

magnitude of resource allocation to measures implemented due to concerns of spillage from 

depressurization. Appendix D includes the National Weatherization Survey about Homes that 

Exceed Worst-Case Depressurization Limits and Fail Spillage. 

PARR approached the National Association for State Community Services Programs to seek 

assistance in distributing the survey. This organization has strong connections with programs in 

every state and agreed to support these efforts. The survey was distributed in June 2015 to all 

state weatherization programs. Weatherization team leads or program managers were asked to 

complete the survey. Responses were requested to be submitted to ICRT via email or mail no 

later than mid-July. 

The survey contained 16 questions divided into two sections. The first section included general 

questions about state weatherization program data: how many homes were weatherized within a 

program year, what is the primary fuel used in homes, etc. States were also asked to describe 

their WC draft testing and spillage testing procedure. The second section pertained to homes that 

fail spillage tests pre- and postweatherization, including questions that differentiate remediation 
70 

from expected and observed spillage events, as well as spillage that occurs from air handler use 

versus from exhausts, including dryers. 

These were differentiated because the PARR/NorthernSTAR study sought homes that failed 

spillage due to exhaust vents such as a bath or kitchen fan but not due to air handler operation. 

ICRT received 19 survey responses from 16 states and Washington D.C. The majority of states 

responded at the state program manager level. Because the United States has about 900 local 

action agencies, the National Association for State Community Services Programs did not ask 

these agencies to reply. 

Responses were received from Alaska, Arkansas, Colorado, Idaho, Illinois, Kansas, Minnesota, 

Missouri, New Hampshire, New Jersey, New Mexico, New York, Oregon, Texas, Virginia, 

Washington state and Washington D.C. Regionally, these responses covered the West, 

Northwest, Midwest, Southwest, Mid-Atlantic, and Northeastern states. No states from the hot 

and humid areas of the Southeast responded. 

Of note, Arkansas referred its survey to local agencies, and three agency responses were 

submitted to ICRT. Alaska also consulted with one of its largest local agencies to provide 

additional data from Anchorage. Minnesota’s response is based on a residential program 

sponsored by utilities, managed by CEE (a NorthernSTAR team member). 

In this report some states are specifically referenced to provide clarification or to provide 

examples; however, the researchers looked to aggregate the responses to provide a higher-level 

review of the data. 

For simplicity, in the discussion below Washington, D.C. is referred to as a state.

5.2 Survey Part I—Overview of States Programs

Survey Questions #1, #2, and #3: Please specify the last program year you have data for, how 

many homes were weatherized in the last program year, and what percentage of homes 

weatherized use either natural gas or fuel oil as the primary heating fuel. 

Eighteen respondents (including the three from Arkansas) provided data for 1 year and one state 

reported on 15 months. Nine states reported for the 2014–2015 fiscal year. Five states reported 

for the 2013–2014 fiscal year. Three states reported on calendar year 2014.

In total, the responses represent 30,385 homes or units in which an energy assessment and 

weatherization measures were completed. Of these 28,575 homes (94%) were weatherized using 

federal DOE WAP funds; the remainder were from the non-WAP Minnesota program. 

One state did not respond to any question after answering how many homes were weatherized. 

This means that only 30,172 homes were covered in the response to question #3 about the 

prevalence of natural gas or fuel oil. Of these, 23,442 homes (78%) were reported to use natural 

gas, fuel oil, or propane as their primary heating fuel. Natural gas was the most commonly 

reported fuel followed by fuel oil and propane. 
71 

Survey Questions #4 and #5: What percentage of homes have natural draft appliances within the 

pressure boundary of the home? What percentage of homes have natural draft appliances outside 

the pressure boundaries? 

Seven of 17 states responded that they either do not track this information in their databases or 

do not collect this information. Fewer than half of homes, 44% (14,395 of 30,385), are therefore 

excluded from this question. 

Of the remaining 10 states, 6 provided only an estimate for the number of natural draft 

appliances in the pressure boundary. These 6 states estimated that of 3,448 homes with natural 

gas or fuel oil, 1,415(41%) have natural draft appliances within the pressure boundaries. 

The remaining four states provided exact numbers. One state reported separately for water 

heaters and furnaces, with 4,382 natural gas water heaters (85.5%) and 1,883 (36.8%) furnaces 

located within the pressure boundary of 5,124 homes that have natural gas or fuel oil as their 

primary heating source. Given that most homes with natural draft furnaces also have natural draft 

water heaters, the 85.5% value is likely a good approximation of the total percentage of homes 

that have natural draft appliances within the pressure boundaries. With this assumption, the four 

states with exact numbers reported 6,952 homes had natural draft appliances within the pressure 

boundary of 8,588 homes that had natural gas, propane, or fuel oil (81%). One of these four 

states was the state that reported only on a non-WAP program. 

Alaska’s response was unique. It does not track these numbers in its database but explained that 

all the homes encountered by the weatherization program have space heating and water heaters 

inside the thermal boundaries. However, some are within the conditioned garage spaces, which 

may have combustion air inlets, so these would be considered outside the pressure boundaries. 

Survey Questions #6, #7, and #8: What is the test procedure your state uses to address 

combustion safety? Does spillage testing occur when a house exceeds WC depressurization 

limits? At how many minutes does the weatherization retrofit (Wx) assessor check for spillage 

under WC depressurization conditions? 

One of 17 states responded that it does not collect this information pertaining to all three 

questions. All the remaining states provided information for each question. 

The majority of states use BPI Building Analyst procedures. One state indicated that it is using 

the draft BPI-1200 test procedure with one exception on the limit for sealed combustion 

appliances. The state allows negative pressure of only –20 Pa rather than –50 Pa due to impact 

on heating systems. The final BPI-1200 test procedure does not include depressurization limits. 

Two states reported that they are currently using BPI Technical Standards for Building Analyst 

Professional but anticipate alignment with the ANSI/BPI-1200 procedures in PY 2015 or PY 

2016. One of those states indicated that it was using the BPI Technical Standards for the 

Building Analyst Professional Version 2/20/05mda. Two states indicated they are using BPI 

Building Analyst Standards. Three states reported that they use BPI technical standards for the 

Building Analyst Professional Version 1/4/12. Another three states reported they use BPI test 

procedures, BPI/QCI standards, or BPI combustion safety procedure for vented appliances; these 

states did not report a version. 
72 

Illinois, Oregon, and Kansas reported that their standards and procedures could be found in their 

state field guides or procedure manuals. Two states described their combustion safety test 

procedures within their survey responses without referring to a guide or to BPI standards. 

In terms of spillage testing, 2 of 17 states responded that they do not collect this information or 

the information is unknown. Fifteen of 17 states responded that they complete spillage testing on 

natural draft appliances. One of the Arkansas agencies responded that it “almost never” conducts 

spillage testing. Kansas reported that it tested spillage at natural conditions but was not testing 

WC depressurization limits. Effective July 1, 2015, Kansas began testing spillage at WC but 

would allow a 2-minute spillage test.

The majority of respondents (11) checked for spillage at 1 minute, which was the requirement of 

the BPI standards until 2015. Three states tested at 2 minutes and two states tested at 3 minutes. 

Question #9 asked about how assessors test for spillage when the CAZ has insufficient space for 

the assessor to be in the space with the door closed. This was an open-ended question that 

elicited a range of responses. The responses were split about evenly between states that would 

test with the door partially open and states that would use other means (e.g., measure CAZ 

pressure or leave a mirror by the draft diverter) to estimate spillage without the need to be in the 

CAZ.

5.3 Survey Part II—Homes that Fail Spillage Tests

Questions #10–#16 are about spillage test outcomes from the applicable homes. For these 

questions, eight states reported that they do not collect the data, the data are not tracked, or the 

data are not applicable. These eight states represent 66% (20,171 of 30,385) of homes reported 

on in the study. 

Of the remaining nine states, four provided data using precise numbers, and five states provided 

estimated data. For the four states that provided precise numbers, 5.6% of homes reported (1,707 

of 30,385) are represented. For the five states that provided estimated data, 28% of homes (8,507 

of 30,385) reported are represented. 

Estimated data and precise data are discussed separately below.

Survey Question #10: How many homes in the last program year received any remediation 

measures to alleviate expected post-Wx spillage due to approaching or exceeding WC

depressurization limits during pre-Wx assessments?

Four states provided precise data when answering question #10. They reported that 4.3% (74 of 

1,707) homes received remediation to alleviate expected post-Wx spillage. 

Five states provided estimated data that indicated about 6% (about 513 of 8,507) homes received 

remediation measures due to expected spillage. In this group, one state reported that it did not 

track the data but estimated very few. No value was assigned to “very few” to calculate the 

estimated number reported.
73 

The non-WAP Minnesota program reported that it expected 436 of 1,800 homes to receive 

measures due to expected post-Wx spillage. This figure was not accounted in either group 

because the Minnesota program does not complete combustion safety measures through its

retrofit program. The homeowners are ultimately responsible for addressing combustion safety 

issues in their homes. It is unknown why such a large percentage of these homes were expected 

to have spillage problems relative to the WAP homes. Perhaps the Minnesota program tends to 

achieve greater airtightness than is typical in the WAP program. 

Survey Question #11: How many homes in the last program year received any remediation 

measures to alleviate observed spillage due to depressurization (e.g., not from a blocked flue)? 

Four states provided precise data for question #11 and reported that 5.4% of homes (about 92 of 

1,707) homes received remediation measures due to observed spillage during a weatherization 

post-test. 

Five states provided estimated data that indicated about 16% (1,351 of 8,507) homes received 

remediation measures to alleviate observed spillage. In this group, one state reported that it did 

not track the data but estimated very few of 1,186 weatherized homes. No value was assigned to 

“very few” in calculating the estimated number. 

A second state in this group reported that the observed spillage occurred approximately 3% of 

the time, and that state’s agencies report that most spillage is due to improper flue sizing or a 

crushed roof cap, not an exhaust fan or air handler. This resulted in approximately 10 homes for 

the state of 839 weatherized homes. These 10 homes were included in the estimate data. 

In this group, Alaska reported that 40%–50% of homes with either natural gas or fuel oil as a 

primary heat source were remediated due to observed spillage. On average, 810 of 2,000 

weatherized homes were estimated to have received remediation due to this condition. 

The non-WAP Minnesota program reported that 111 of 1,488 water heaters failed WC testing 

and 43 of 1,076 gas furnaces failed WC testing. Within these failures, 33 homes had both 

appliances fail WC testing.

Survey Question #12: For homes that failed a spillage test, how many of those homes failed due 

to operation of the air handler (i.e., air handler return leaks)? 

Four states provided precise data for question #12 and reported that 4% (about 73 homes of 

1,707) homes failed spillage tests due to operation of an air handler. 

Four states provided estimated data that indicated 6.5% (374 homes of 5,757) weatherized failed 

spillage due to operation of an air handler. 

Survey Question #13: When homes failed a spillage test, how many of those homes failed due to 

operation of appliances that exhaust air (i.e., kitchen or bath ventilation, dryers)?

Three states provided precise data for question #13. Of 967 homes weatherized in this group, 39 

(4%) failed due to operation of appliances that exhaust air. 
74 

Four states estimated spillage failure due to exhaust and reported that 1,043 of 5,757 weatherized 

homes (18%) failed spillage due to exhaust appliances. Of these states, the numbers of failures 

reported were 260, 196, 585, and one. Alaska reported 585 failures due to appliances that 

exhaust air, but stated that half of these are failures mostly occurring from dryers. With Alaska 

removed, the rate of failures due to exhaust from states that provided estimated values dropped to 

12%. 

Survey Question #14: How many homes receiving remediation for expected or observed spillage 

post-Wx had a new appliance installed? 

Three states reported precisely that 31 of 967 weatherized units had new appliances installed 

(3.2%). 

Excluding Alaska, four states estimated that 150 of 6,507 total remediated homes had new 

appliances installed (2.3%). 

Based on reported figures, Alaska estimated 60% of homes that were remediated due to spillage 

received new appliances (702 of 1,170 remediated homes). 

Survey Question #15: How many homes receiving remediation for expected or observed spillage 

post-Wx had a power vent kit installed? 

Four states estimated that 17 out of 4,987 weatherized homes received power vent kits. Three 

states reported that no power vent kits were installed in 4,131 weatherized homes. 

Survey Question #16: How many homes did not receive weatherization services (or were 

deferred) due to combustion safety failures? 

Six states responded that no homes were deferred due to combustion safety failures. One state 

indicated that it does not defer homes due to combustion safety failures except for cracked heat 

exchangers. One Arkansas agency reported that six homes were deferred due to combustion 

safety failures. These were the only deferrals reported.

5.4 Discussion and Conclusion

The survey revealed a number of important findings. The most striking result is how few 

respondents actually had precise data about combustion safety failures. Of 17 states, only four 

had precise data of this type, and these four states represented fewer than 6% of the homes in the 

sample. Of the remaining 13 states, it would have been fair to simply state that none collect this 

information; however, the team tried to summarize estimated values when available.

A second important finding is that of the homes that failed spillage tests in the four states that 

reported precise numbers, the failures were split about evenly between being due to exhausts and 

being due to air handler operation (about 4% each). This is critical, because failures due to air 

handler operation are not the type that the field-testing for the STP project was intended to 

address. Further, although clearly some failures reported were due to exhausts, the survey did not 

attempt to distinguish between failures due to dryers or other exhausts. However, some states did 

volunteer that many failures due to exhausts were from dryers, meaning that the homes with 
75 

failures of a type that would have qualified for the field-testing would have been noticeably 

fewer than 4%. 

It is also of interest that the states that did not track this information but attempted to provide 

estimates indicated higher failure rates than those states with precise numbers. Given how few 

states reported precise numbers, they cannot be considered to constitute a representative sample, 

so other states may well have higher failure rates, but clearly this is speculative. In the absence of 

states recording these data, a survey targeting individual agencies would be required to improve 

the team’s detailed understanding of this issue. However, given the difficulty of finding eligible 

homes for field-testing, it may be considered sufficient that the few data that are available tend to 

concur with the conclusion that relatively few homes that fail for the reasons targeted by the 

project.

The primary takeaway is that little actual evidence shows that combustion safety failures due to 

spillage from nondryer exhaust are common. This is counter to the anecdotal opinions and 

assumptions that were made by a number of entities before the field-testing project was 

conducted. The few hard data that detail actual failures suggest that only a very small number of 

homes are subject to these failures; therefore, the actual cost to programs for correcting these 

failures is also expected to be low. 
76 

6 Conclusions

The conclusions of this project cover a wide array of topics related to combustion safety: state 

weatherization team reporting of combustion safety events, the development of an STP, 

evaluation of the procedure in the field compared to other methods in use, and field-test data and 

analysis.

Survey results from 17 state weatherization agencies are presented. These agencies do not 

generally track combustion safety failures. The data from those that do suggest that little actual 

evidence shows that combustion safety failures due to spillage from nondryer exhaust are 

common and that only a very small number of homes are subject to the failures. 

Field evaluations of the STP and the traditional and comprehensive BPI test procedures produced 

similar results—the same houses failed both tests. The authors conclude that the STP is 

appropriate for combustion safety assessment; that is, it is as effective at identifying problem 

houses as the WC procedures in use. 

According to the field data from 11 houses monitored for 7 months, the 2 houses that failed the 

STP and showed significant persistent flue gas spillage events also had vent systems that violated 

the NFGC sizing guidelines. This finding suggests that energy auditors and any personnel 

charged with combustion safety evaluation should be trained to perform inspections specifically

to identify violations of the current venting standards. The fact that only homes with clear 

venting code violations experienced significant spillage in normal operation also suggests that 

correcting venting system defects may prevent excessive spillage and avoid the need for higher 

cost system replacement in many cases. 

None of the remaining houses (i.e., the houses with code-compliant vent systems) in the field 

study experienced significant spillage beyond the first minute of operation, yet all failed the STP, 

suggesting that this procedure (as well as traditional procedures) is conservative; i.e., it creates 

false positive spillage test results that do not correlate with spillage under normal operating 

conditions. A lower exhaust fan level is likely more appropriate for the test, though this was not 

evaluated. 

In the houses with code-compliant venting, field data show that the operation of clothes dryers 

and exhaust fans (especially kitchen fans) is often statistically associated with spillage. The low 

incidence of spillage after the first minute of operation means, however, that the effect of fan 

operation is generally to increase the probability of spillage from one very low number to a 

higher but still low number. Test data show that kitchen fan airflow rates in the houses studied 

are on the same order of magnitude as dryer airflow, and kitchen fans are probably as likely as 

dryers to be triggers for spillage (although dryer operation appears more likely to overlap with 

burner cycling than kitchen fan operation). 

An investigation into whether exhaust fan (dryer, range hood) operation in proximity to an 

atmospheric water heater is a hazard or is a method of exhausting spilled flue gases is 

appropriate, especially in light of the fact that no elevated CO readings were recorded. If the fan 

is the cause for the spillage, once the fan stops the root cause for the spillage is removed.
77 

When CAZ net depressurization is considered (instead of the operation of individual fans) as a 

predictor of spillage, it showed a clear and statistically significant effect in most of the houses 

tested. A higher level of depressurization (defined here as CAZ pressure with respect to outdoor 

pressure) always correlated with a higher probability of spillage. 

Backdrafting (or persistent reversal of flow in the vent system) occurred in some homes, 

indicating that the buoyancy of the flue gases was insufficient to overcome the flow of cold air 

down the vent. Similar to spillage in general, the start of a backdrafting episode appears to be 

much more likely when exhaust fans are operating and when the CAZ is under greater 

depressurization. Backdrafting may be associated with CO production, and although the team did 

not find elevated CO levels, this fact suggests a need for more investigation into the causes and 

effects of backdrafting. 

Elevated CO levels at the Underwriters Laboratories alarm limit of 70 ppm were not detected in 

the test, even during spillage events. Appliance CO levels that exceeded the ANSI certification 

limit in the NFGC were also not detected, and CO levels observed during normal operation 

remained low in all homes. 

Finally, a primary conclusion related to audit practices is that the main cause of persistent 

failures in this project was improperly sized vents. Auditors should be trained in the proper 

sizing of vent systems to be able to resolve the failures that relate to serious and persistent 

combustion safety issues. From the weatherization survey results, another root cause of the 

failures is most commonly related to depressurization associated with poorly sealed ductwork 

that should be resolved during the upgrade. Appliances should be replaced only if a failure 

occurs at test-out, or if the flue gas CO reading exceeds the requirement in the NFGC. The STP 

in this report should provide an easier route to performing the test. Because the BPI-1200-2015 

standard was developed based on input from the authors, the combustion safety provisions in that 

standard are recommended as the STP until further research can be done.

Responses to research questions:

Q1: Does the STP accurately predict combustion safety hazards as determined by long-term 

monitoring? Does it predict hazards more accurately than the conventional protocol used by 

weatherization agencies? 

A1: The STP was demonstrated to be as effective as the WC methods using trial and error to 

select door positions. Long-term monitoring revealed that houses that fail the STP either show a 

pattern of persistent failures or show no pattern of failures. Persistent failures would have been 

detected by a careful inspection of the vent sizing in the small number of samples in this study. 

Q2: Is the STP practical for field implementation, and can it be improved?

A2: The team was able to train field weatherization agencies on the STP with little difficulty. 

The results of the field test show that if a vent inspection and remediation are done before the 

STP, the exhaust fan levels in the STP could be lowered. 

Q3: What is the incidence of spillage and combustion safety failures based on feedback from 

weatherization agencies and other field experience?
78 

A3: State weatherization agencies do not generally track combustion safety failures. The data 

from those that do suggest that little actual evidence shows that combustion safety failures due to 

spillage from nondryer exhaust are common and that only a very small number of homes are 

subject to the failures. In the field study, 2 of the 11 houses that failed the STP had persistent 

spillage that required remediation. 
79 

References

Brand, L. 2014. Combustion Safety for Natural Draft Appliances Using Indoor Air (Subcontract 

Report, NREL/SR-5500-61326). Golden, CO: National Renewable Energy Laboratory. Accessed 

October 19, 2015: www.nrel.gov/docs/fy14osti/61326.pdf?gathStatIcon=true. 

Cautley, D., J. Viner, M. Lord, and M. Pearce. 2012. Test Methods and Protocols for 

Environmental and Safety Hazards Associated with Home Energy Retrofits (Subcontract Report, 

DOE/GO-102012-3752). Golden, CO: National Renewable Energy Laboratory, 

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/test_methods_and_pr

otocols.pdf 

Fitzgerald, J., and D. Bohac. 2014. Combustion Safety for Natural Draft Appliances through 

Appliance Zone Isolation (Subcontract Report, NREL/SR-5500-61295). Golden, CO: National 

Renewable Energy Laboratory. Accessed October 19, 

2015: www.nrel.gov/docs/fy14osti/61295.pdf?gathStatIcon=true. 

IFGC. 2013. International Fuel Gas Code, International Code Council 

NREL. 2013. “Standard Work Specifications for Home Energy Upgrades.” Accessed October 

19, 2015: www.nrel.gov/tech_deployment/buildings_sws.html. 

NFPA 54. 2012. National Fuel Gas Code. Boston, MA: National Fire Protection Association, 

American Gas Association. 

Rapp, V.H., B.C. Singer, J.C. Stratton, and C.P. Wray. 2012. Assessment of Literature Related to 

Combustion Appliance Venting Systems. Berkeley, CA: Lawrence Berkeley National Laboratory, 

Berkeley, CA. June 2012. LBNL-5798E. 

Rapp, V.H., B.D. Less, B.C. Singer, J.C. Stratton, and C.P. Wray. 2015. Assessment of Literature 

Related to Combustion Appliance Venting Systems. Berkeley, CA: Lawrence Berkeley National 

Laboratory, LBNL-176805. 
80 

Bibliography

ASTM. 1998. “Standard Guide for Assessing Depressurization-Induced Backdrafting and 

Spillage from Vented Combustion Appliances.” 

BPI 1200 Basic Analysis of Buildings. 2012.Building Performance Institute, Malta New York. 

Brand, L., D. Cautley, and D. Bohac. 2014. Test Plan: Combustion Safety. Golden, CO: National 

Renewable Energy Laboratory. 

Building Performance Institute. 2015. “Standard Practice for Basic Analysis of Buildings”, 

ANSI/BPI-1200-S-2015. 

Cummings, J., and C. Withers. 2008. “Problems Related to Air Handler Leakage.” ASHRAE 

Journal, January 2008. American Society of Heating, Refrigerating and Air-Conditioning 

Engineers, Inc.,

DOE. n.d. “Venting Issues I Gas-Fired Water 

Heaters.” www1.eere.energy.gov/buildings/appliance_standards/residential/pdfs/d-3.pdf. 

Grimsrud, D T., M.D. Koontz, and N.L. Nagda. 1996. Field Protocol for Determining 

Depressurization-Induced Backdrafting and Spillage from Vented Residential Gas Appliances, 

Technical Report 96/0303. Chicago, IL: Gas Research Institute. 

Hnatov, M. 2012. “Non-Fire Carbon Monoxide Deaths Associated with the Use of Consumer 

Products 2009 Annual Estimates.” U.S. Consumer Products Safety Commission. 

IRC. 2013. International Residential Code for One and Two Family Dwellings. International 

Code Council. 

Koontz, M.D., and N.L. Nagda. 2002. “Depressurization-Induced Backdrafting and Spillage: 

Implications of Results from North American Field Studies.” ASHRAE Transactions, 108.

NFPA 211. 2012. Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning 

Appliances. National Fire Protection Association. 

Proctor, J., R. Chitwood, and B.A. Wilcox. 2011. “Efficiency Characteristics and Opportunities 

for New California Homes.” California Energy Commission. Publication number: CEC 500-

2012-062. 

RESNET. 2010. Interim Guidelines for Combustion Appliance Testing and Writing Work 

Scope. www.resnet.us/standards/RESNET_interim_guidelines_for_combustion_safety_scope_of

_work.pdf.

U.S. Census Bureau. Characteristics of New Single-Family Houses Completed, Type of 

Foundation. www.census.gov/construction/chars/completed.html.
81 

Appendix A. Overview of Short-Term Testing Protocol

Record the outdoor temperature, wind speed, and wind direction reported by the nearest weather 

station. 

1. Measure the house air leakage by performing a multipoint envelope air leakage test in 

accordance with ASTM E779. Close all exterior doors, windows, and fireplace dampers. 

Conduct only the depressurization section of the test (do not perform the pressurization 

test). The minimum induced pressure difference shall be no greater than 10 Pa. 

2. Measure the flow rate of all exhaust fans, heat recovery ventilators, energy recovery 

ventilators, dryer vents, and any other appliance that moves air into or out of the house. 

3. Measure the change in pressure as a result of appliances (including appliance air 

handlers). For each CAZ within the pressure boundary of the house, use an appropriate 

application (e.g., The Energy Conservatory’s Teclog3) to record the house pressure at the 

CAZ with respect to outside at 1-second intervals. 

4. To gauge the accuracy of the pressure measurements, record the results for three 30-

second measurement periods for the baseline conditions and for three 30-second periods 

under the test or other conditions. Each period is alternated so each baseline measurement 

period is followed by a test condition, as shown below: 

A. Baseline

B. Test condition (e.g., fan on) 

C. Baseline

D. Test condition 

E. Baseline

F. Test condition. 

5. The first series of pressure test conditions examines the impact of the individual exhaust 

appliances. Depending on the number of multispeed fans in the house, as many as 10 

tests will be performed. With all the interior doors open, the exhaust fans are tested in the 

following order; the last test is conducted with all the fans on: 

A. Kitchen fan low

B. Kitchen fan high

C. Bath fan 1 low 

D. Bath fan 1 high 

E. Bath fan 2 low 

F. Bath fan 2 high 

G. Dryer

H. Dryer, with CAZ door closed (this can be the door of the mechanical room or the 

basement door if the combustion appliances are in an open basement)

I. Any other fan high 
82 

J. All fans on high speed (do not need to wait 30 seconds between fans). 

The second series of test conditions investigates the impact of the air handler operation (if 

applicable) and the position of the CAZ door. For this series of tests, the baseline condition has 

the air handler off and doors and fans in the specified state for each defined test condition shown 

below. The air handler is on for the test condition. Below are the four conditions for this series of 

tests:

1. Air handler on/off. All interior doors open, all exhaust fans off. 

2. Air handler on/off. CAZ door closed, other interior doors open, all exhaust fans off.

3. Air handler on/off. CAZ door closed, other interior doors closed EXCEPT rooms with an 

exhaust fan or air handler return (BPI protocol), ALL exhaust fans on. 

4. CAZ door closed/open. Select air handler on or off, whichever gave more negative 

pressure in previous condition, other interior doors closed EXCEPT rooms with an 

exhaust fan or air handler return (BPI protocol), ALL exhaust fans on. 

The final series of pressure conditions investigates the test conditions of various combustion 

safety test protocols. For these tests, measure the pressure differences for the following four fan, 

door, and air handler combinations: 

1. BPI: All exhaust fans on; other interior doors as per BPI protocol, air handler on/off 

based on which produces most negative CAZ pressure (CAZ door closed); CAZ door 

open/closed based on which produces most negative CAZ pressure 

2. Comprehensive WC: All exhaust fans on, interior doors (including CAZ) set to WC 

conditions as indicated by smoke movement or pressure across door, air handler on or off 

depending on which condition produces most negative CAZ pressure 

3. STP: Dryer, kitchen fan high, next largest fan high; CAZ door closed, other interior 

doors as per BPI protocol, air handler on/off based on whichever produces most negative 

CAZ pressure

4. Modified STP: Dryer, kitchen fan high, next-largest fan high; interior doors (including 

CAZ) set to WC conditions as indicated by smoke movement or pressure across door, air 

handler on or off depending on which condition produces most negative CAZ pressure 

(same as comprehensive WC except only dryer, kitchen and next largest exhaust fans) 

5. Visually inspect the venting system for proper size and horizontal pitch and determine 

that there is no blockage, restriction, leakage, corrosion, or other deficiencies that could 

cause an unsafe condition (NFPA 54 Chapter 13; IFGC 2013, Chapter 5). For masonry 

chimneys determine if it has a metal liner, if required. Take and record the following 

basic measurements and check them against the tables in the NFGC and International 

Fuel Gas Code to determine whether the vent system is properly sized: 

A. Type of vent connector and common vent materials 

B. The vertical common vent/chimney diameter and approximate height, noting 

offsets
83 

C. The common vent manifold diameter and horizontal length (if any)

D. Each appliance vent connector diameter, rise, length, and number of elbows. 

6. Visually inspect any combustion air duct(s). Ducts that have been fully or partially 

blocked by the occupant should be unblocked. Ducts that are fully or partially blocked 

due to deterioration or debris should be left as-is. Record information on the combustion 

air and include number of openings, location of openings, type of duct, duct area, duct 

length, and elbows. Check the combustion air configuration against NFGC and 

International Fuel Gas Code requirements to determine whether the system is properly 

sized.

7. Ensure that the termination of the indoor pressure reference is in the space where the 

vented combustion appliances are located. Use an appropriate application to record the 

house pressure with respect to the outside at 1-second intervals. Continue the 

measurements while the house is being placed in the first test condition. Wait a minimum 

of 15 seconds after each change in the house configuration and document the average 

house pressure difference for each configuration. 

8. Install a static pressure (or draft) sensor in the appliance vent approximately 1–2 ft

downstream of the appliance draft diverter. The sensor will be used to reference the area 

in which the appliance is located. Use an appropriate application to record the house 

pressure and draft pressure at 1-second intervals 

9. Conduct short-term testing under the must-fail condition and the must-pass condition. 

10. Place the appliance being inspected in operation. Follow lighting instructions for each 

appliance. Adjust the thermostat so the appliance will operate continuously. 

11. Measure CO levels in the appliance flue. Measured air-free CO levels should not exceed

the ANSI certification requirement for the type of appliance. 

12. Test for spillage at the draft hood relief opening. Use chemical smoke as an indicator of 

spillage. Note the duration of time required after the burner is ignited until there is no 

visible spillage up to 5 minutes. Evaluate the spillage after 1, 2, and 5 minutes of burner 

operation. 

13. Record the draft pressure after 1, 2, and 5 minutes of burner operation. 

14. Turn on all other fuel-gas-burning appliances within the same room so they operate at 

their full inputs. Follow lighting instructions for each appliance. 

15. Repeat the spillage test for any additional appliances being inspected.

The CEVP is the highest level of CAZ depressurization for which the appliance can overcome a 

cold stack and establish a draft up the flue. The CVEP is measured using the blower door to vary 

the CAZ depressurization. Use an appropriate application (e.g. The Energy Conservatory’s 

Teclog3) to record the house pressure, draft pressure, and blower door fan flow at 1-second 

intervals. Install the termination of the indoor pressure reference to the space where the vented 

combustion appliances are located. Open all interior doors. The outdoor pressure reference will 

be an average of measurements at each side of the house. Test atmospherically vented appliances 

only. Do not test appliances having an interlocked spill sensing switch. 
84 

1. Install a static pressure (or draft) sensor in the appliance vent approximately 1–2 ft 

downstream of the appliance draft diverter. The sensor will be used to reference the area 

in which the appliance is located. 

2. Reference the blower door fan pressure to house pressure. Install the smallest fan ring to 

attain the needed CAZ depressurization.

3. Place all appliances to be tested in standby mode. If testing the water heater, run hot 

water to ensure the appliance will continue to fire during the duration of test. Test only 

one appliance at a time and keep the additional appliances on standby. 

4. Begin TECLOG3.

5. Record a 2-minute period of record baseline.

6. Referencing the previous must-fail test pressure, use the blower door fan to increase the 

CAZ depressurization by an additional 5 Pa.

7. Fire the appliance to be tested.

8. Measure and record the air-free CO at the appliance flue and continue to monitor the CO 

for the duration of test. 

9. Reduce CAZ to outside depressurization by 0.5 Pa, holding the CAZ at that pressure for 

30 seconds. 

10. Repeat step 9 until the vent establishes negative pressure. Record the CAZ pressure and 

fan flow when the vent negative pressure is achieved.

11. Record a 2-minute period of record baseline

12. Repeat for additional commonly vented appliances allowing the blower door fan to run at 

the start of additional tests to adequately cool the flue.

13. When possible, use the gas utility meter to “clock” or measure the maximum burner input 

rate for each appliance tested.

14. Return doors, windows, exhaust fans, fireplace dampers, and all fuel-gas-burning 

appliances to their “as found” conditions of use. 
85 

Appendix B. Monitoring System Parts

Sensor Number 

Used Manufacturer and Model

K Type Thermocouples in Flue 

and Vent 2 

Fabricated by project team using 24 American 

wire gauge fiberglass-insulated K type solid 

thermocouple grade wire

K Type Thermocouples at 

Draft Hood, in CAZ, and 

Outdoors

4 

Fabricated by project team using 24 American 

wire gauge perfluoroalkoxy alkane insulated K 

type solid thermocouple grade wire

K Type Thermocouples in 

CAZ, and Outdoors 3 Same as above

Differential Pressure 

Transducer, Digital Output 1 

All Sensors DLVR I2C digital output 

differential pressure sensor, ±1 in. water column 

range

Nondispersive Infrared CO2

Sensor, Analog Output 1 Digital Control Systems 305E 

Electrochemical CO Sensor 1 Figaro TGS5042 with amplifier conditioned 

output current

20A Current Transducer, 

Analog Output 1-2 Onset Computer CTV-A 20A current transducer 

Custom Wireless Sensor 

Module, Transceiver with A to 

D Converter

Up to 3 Digi Xbee Pro S2B module, Zigbee protocol 

Custom Wireless Sensor 

Module, Pressure Sensor

1 per 

module 

All Sensors BLV analog (bridge) output 

differential pressure sensor, ±1 in. water column 

range
86 

Appendix C. House Data Sample Photos

MN 01

MN 02

MN 03
87 

MN 04

MN 05

MN 06
88 

MN 07

MN 08

MN09
89 

MN 10

WI 01
90 

Appendix D. Combustion Safety Survey

NATIONAL WEATHERIZATION SURVEY ABOUT HOMES 

THAT EXCEED WORST-CASE DEPRESSURIZATION 

LIMITS AND FAIL SPILLAGE

The Partnership for Advanced Residential Retrofit research team is conducting a Building America 

research project designed to increase understanding of combustion appliance spillage test results 

among the housing stock weatherized by DOE Weatherization Assistance Programs nationally. 

The primary purpose of this survey is to understand states’ methods for conducting the worst-case 

depressurization test and identify the number of homes per state that received measures due to 

concerns of spillage from depressurization (i.e., not spillage from a plugged flue). 

This survey should be completed by weatherization team leads or program managers.

Please submit your responses by mail or by e-mail by 7/15/2015 to:

Stacy Gloss, research specialist

2111 S. Oak St. Suite 106

Champaign, IL 61820

sgloss@illinois.edu 

SURVEY QUESTIONS: 

Please provide information about the respondent:

Name:

Title: 

Street Address:

State:

Phone number:

E-mail address:

May we contact you to follow up on your responses? 

Answer the following questions to the best of your knowledge about your state’s program. Be as specific 

as possible. 

1. Please specify the last program year that you have data for (e.g., 7.2013 – 6.2014 or 1.2014 – 

12.2014)

2. How many homes were weatherized in your state in the last program year? 

3. What percentage of homes weatherized use either natural gas or fuel oil as the primary heating 

fuel? 
91 

4. What percentage of homes have natural draft appliances (e.g., a natural draft water heater) 

within the pressure boundary of the home? These appliances would have a draft hood or draft 

diverter. Inside the pressure boundary means in the living space or a room that doesn’t have a 

duct or wall penetration opening to an area outside the living space.

5. What percentage of homes have natural draft appliances outside the pressure boundary of the 

home? For example, vented closets, garages, crawl spaces, attics.

6. What is the test procedure your state uses to address combustion safety? If your state uses a 

BPI procedure please simply state which version. If your state uses another procedure and you 

can share a document or link, please do.

7. Does spillage testing occur when a house exceeds worst-case depressurization limits? Y/N

8. At how many minutes does the Wx assessor check for spillage under WCD conditions? 

9. In homes where the combustion appliance is in a small closet and the assessor cannot be in the 

CAZ with the door closed, how is spillage assessed?

This next section pertains to homes in your program that fail spillage tests. Read each question carefully. 

If you have any questions about this section, or if you need clarification on a question please 

contact sgloss@illinois.edu. 

10. How many homes in the last program year received any remediation measures to alleviate 

expected post-Wx spillage due to approaching or exceeding worst-case depressurization limits 

during pre-Wx assessments? 

11. How many homes in the last program year received any remediation measures to alleviate 

observed spillage due to depressurization (i.e. not from a blocked flue, etc.)?

12. When homes failed a spillage test, how many of those homes failed due to operation of the air 

handler (i.e. air handler return leaks)? 

13. When homes failed a spillage test, how many of those homes failed due to operation of 

appliances that exhaust air (i.e. kitchen or bath ventilation, dryers)?

14. How many homes receiving remediation for expected or observed spillage post-Wx had a new 

appliance installed? 

15. How many homes receiving remediation for expected or observed spillage post-Wx had a power 

vent kit installed? 

16. How many homes did not receive weatherization services (or were deferred) due to combustion 

safety failures? 
92 

Appendix E Logistic Model Definition and Detailed 

Output

Logistic Model 1

0 1 2 3

Where:

spill_500 binary indicator for spillage (CO2 elevation > 500 ppm)

min1 binary indicator for first minute of WH operation

t_out hourly outdoor temperature, F (KMSP or KMSN airport)

p_zone outdoor air pressure with respect to combustion zone, Pa

Logistic Model 2

0 1 2 3 4 5 6*bathfan1 + 

7 8

Where:

spill_500 binary indicator for spillage (CO2 elevation > 500 ppm)

min1 binary indicator for first minute of WH operation

t_out hourly outdoor temperature, F (KMSP or KMSN airport)

airhandler binary indicator for air handler operation

dryer binary indicator for dryer operation

kitfan binary indicator for kitchen exhaust fan operation

bathfan1 binary indicator for bath fan #1 operation

bathfan2 binary indicator for bath fan #2 operation

door binary indicator for door to combustion zone closed

MODEL 1 OUTPUT

MN_01

Logistic regression Number of obs = 9840

Wald chi2(3) = 219.15

Prob > chi2 = 0.0000

Log pseudolikelihood = -380.79947 Pseudo R2 = 0.6603

(Std. Err. adjusted for 594 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 1005.012 539.3742 12.88 0.000 351.0302 2877.384

t_out | 1.104322 .0099767 10.98 0.000 1.08494 1.12405

p_zone | 1.46179 .1368805 4.05 0.000 1.216688 1.756267

_cons | 8.24e-07 7.86e-07 -14.69 0.000 1.27e-07 5.34e-06

------------------------------------------------------------------------------

MN_02

Logistic regression Number of obs = 7169

Wald chi2(3) = 85.42

Prob > chi2 = 0.0000

Log pseudolikelihood = -116.69931 Pseudo R2 = 0.5182

(Std. Err. adjusted for 383 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 213.7774 168.5456 6.80 0.000 45.58902 1002.451

t_out | 1.228326 .0488502 5.17 0.000 1.136218 1.327901

p_zone | 4.288653 1.299554 4.80 0.000 2.368037 7.766999

_cons | 6.39e-11 2.40e-10 -6.25 0.000 4.05e-14 1.01e-07

------------------------------------------------------------------------------

Note: 2 failures and 0 successes completely determined.
93 

MN_03

Logistic regression Number of obs = 9668

Wald chi2(3) = 1084.72

Prob > chi2 = 0.0000

Log pseudolikelihood = -1116.0108 Pseudo R2 = 0.4920

(Std. Err. adjusted for 640 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 171.3938 29.49573 29.89 0.000 122.3232 240.1492

t_out | 1.066799 .0111585 6.18 0.000 1.045152 1.088895

p_zone | 1.647054 .197689 4.16 0.000 1.301792 2.083886

_cons | .0001471 .0001196 -10.86 0.000 .0000299 .0007236

------------------------------------------------------------------------------

MN_04

Logistic regression Number of obs = 4513

Wald chi2(3) = 63.83

Prob > chi2 = 0.0000

Log pseudolikelihood = -898.38429 Pseudo R2 = 0.2309

(Std. Err. adjusted for 314 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | .6472523 .145278 -1.94 0.053 .4168871 1.004914

t_out | 1.095928 .0147926 6.79 0.000 1.067316 1.125308

p_zone | 1.210336 .1375079 1.68 0.093 .968723 1.51221

_cons | .1706596 .1408104 -2.14 0.032 .0338695 .8599103

------------------------------------------------------------------------------

MN_05

Logistic regression Number of obs = 8786

Wald chi2(3) = 145.94

Prob > chi2 = 0.0000

Log pseudolikelihood = -349.86876 Pseudo R2 = 0.1622

(Std. Err. adjusted for 422 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 15.60763 3.978042 10.78 0.000 9.470752 25.7211

t_out | 1.059844 .0177097 3.48 0.001 1.025696 1.095129

p_zone | 2.364723 .3678234 5.53 0.000 1.743328 3.207608

_cons | .0000524 .0000658 -7.84 0.000 4.47e-06 .000615

------------------------------------------------------------------------------
94 

MN_06

Logistic regression Number of obs = 8756

Wald chi2(3) = 20.19

Prob > chi2 = 0.0002

Log pseudolikelihood = -314.91132 Pseudo R2 = 0.0840

(Std. Err. adjusted for 500 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 3.693012 1.425395 3.38 0.001 1.733173 7.869003

t_out | 1.101421 .0358679 2.97 0.003 1.033318 1.174013

p_zone | 1.385881 .1675929 2.70 0.007 1.09343 1.756551

_cons | 4.27e-06 .0000111 -4.77 0.000 2.66e-08 .0006851

------------------------------------------------------------------------------

MN_07

Logistic regression Number of obs = 5758

Wald chi2(3) = 290.19

Prob > chi2 = 0.0000

Log pseudolikelihood = -117.28965 Pseudo R2 = 0.3170

(Std. Err. adjusted for 352 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 31.47917 19.42631 5.59 0.000 9.39146 105.5148

t_out | 1.033595 .0528788 0.65 0.518 .9349816 1.14261

p_zone | 2.316229 .1572334 12.37 0.000 2.027679 2.645842

_cons | .0000596 .0002446 -2.37 0.018 1.92e-08 .1851354

------------------------------------------------------------------------------

MN_08

 

[p_zone removed as a predictor due to missing data]

Logistic regression Number of obs = 1734

Wald chi2(2) = 46.76

Prob > chi2 = 0.0000

Log pseudolikelihood = -22.655858 Pseudo R2 = 0.5559

(Std. Err. adjusted for 82 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 244.1621 227.4903 5.90 0.000 39.31861 1516.207

t_out | 1.274945 .0859171 3.60 0.000 1.117197 1.454966

_cons | 1.02e-11 5.21e-11 -4.95 0.000 4.50e-16 2.31e-07

------------------------------------------------------------------------------
95 

MN_09

Logistic regression Number of obs = 5809

Wald chi2(3) = 284.96

Prob > chi2 = 0.0000

Log pseudolikelihood = -53.367066 Pseudo R2 = 0.5624

(Std. Err. adjusted for 464 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 13.80867 10.01352 3.62 0.000 3.333446 57.2019

t_out | 1.091956 .0522809 1.84 0.066 .9941487 1.199387

p_zone | 2.790795 .2563266 11.17 0.000 2.331028 3.341246

_cons | 3.52e-07 1.50e-06 -3.48 0.000 8.26e-11 .0015009

------------------------------------------------------------------------------

MN_10

Logistic regression Number of obs = 2153

Wald chi2(3) = 31.77

Prob > chi2 = 0.0000

Log pseudolikelihood = -38.108284 Pseudo R2 = 0.4867

(Std. Err. adjusted for 115 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 396.9932 492.3415 4.83 0.000 34.92554 4512.56

t_out | 1.125801 .0567343 2.35 0.019 1.019919 1.242676

p_zone | 2.736392 .8794349 3.13 0.002 1.457521 5.137379

_cons | 1.87e-08 8.92e-08 -3.73 0.000 1.61e-12 .0002163

------------------------------------------------------------------------------

WI_01

 

[min1 removed as a predictor due to near coincidence with observed spillage]

Logistic regression Number of obs = 7016

Wald chi2(2) = 47.65

Prob > chi2 = 0.0000

Log pseudolikelihood = -1317.7445 Pseudo R2 = 0.3192

(Std. Err. adjusted for 502 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

t_out | 1.127809 .0206553 6.57 0.000 1.088043 1.169028

p_zone | 1.072101 .0397713 1.88 0.061 .9969171 1.152955

_cons | .0161489 .0159334 -4.18 0.000 .0023351 .1116827

------------------------------------------------------------------------------
96 

MODEL 2 OUTPUT

MN_01

[airhandler removed as a predictor due to no data]

[bathfan1 removed as a predictor due to no data]

[bathfan2 removed as a predictor due to no data]

[door removed as a predictor due to no variation (always 1)]

Logistic regression Number of obs = 8124

Wald chi2(4) = 286.62

Prob > chi2 = 0.0000

Log pseudolikelihood = -300.16763 Pseudo R2 = 0.6478

(Std. Err. adjusted for 594 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 688.7202 285.3679 15.77 0.000 305.7396 1551.437

t_out | 1.081199 .007868 10.73 0.000 1.065887 1.09673

dryer | 3.17248 1.093024 3.35 0.001 1.614849 6.232551

kitfan | 2.913266 .84958 3.67 0.000 1.644936 5.159546

_cons | 4.33e-06 3.09e-06 -17.28 0.000 1.07e-06 .0000176

------------------------------------------------------------------------------

MN_02

[kitfan removed as a predictor due to no data]

[door removed as a predictor due to insufficient (0) operation observations]

Logistic regression Number of obs = 4901

Wald chi2(6) = 102.33

Prob > chi2 = 0.0000

Log pseudolikelihood = -104.00317 Pseudo R2 = 0.5326

(Std. Err. adjusted for 266 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 232.8499 164.2364 7.73 0.000 58.43819 927.8019

t_out | 1.193579 .0518684 4.07 0.000 1.096128 1.299694

airhandler | 1.928683 .8883368 1.43 0.154 .7819951 4.756832

dryer | 15.02703 10.45056 3.90 0.000 3.845071 58.72753

bathfan1 | 1.777711 .8611749 1.19 0.235 .6878823 4.594179

bathfan2 | 19.16768 20.65279 2.74 0.006 2.31962 158.3879

_cons | 2.78e-10 1.08e-09 -5.65 0.000 1.35e-13 5.73e-07

------------------------------------------------------------------------------
97 

MN_03

[door removed as a predictor due to insufficient (0) operation observations]

Logistic regression Number of obs = 4178

Wald chi2(7) = 388.85

Prob > chi2 = 0.0000

Log pseudolikelihood = -328.51734 Pseudo R2 = 0.6926

(Std. Err. adjusted for 276 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 2030.775 1033.026 14.97 0.000 749.3212 5503.713

t_out | 1.131787 .0240202 5.83 0.000 1.085674 1.179859

airhandler | 2.368736 .9848471 2.07 0.038 1.048609 5.350815

dryer | 3.284451 1.858414 2.10 0.036 1.083514 9.956143

kitfan | 27.06641 11.2824 7.91 0.000 11.9568 61.26982

bathfan1 | 2.399453 .7685997 2.73 0.006 1.280716 4.495433

bathfan2 | .7578319 .5171363 -0.41 0.684 .19894 2.886846

_cons | 3.29e-07 5.71e-07 -8.62 0.000 1.11e-08 9.82e-06

------------------------------------------------------------------------------

MN_04

 

[airhandler removed as a predictor due to no data]

[kitfan removed as a predictor due to spillage always occurring during operation]

[bathfan1 removed as a predictor due to no data]

[bathfan2 removed as a predictor due to no data]

[door removed as a predictor due to insufficient (0) operation observations]

Logistic regression Number of obs = 4586

Wald chi2(3) = 70.64

Prob > chi2 = 0.0000

Log pseudolikelihood = -913.36726 Pseudo R2 = 0.2267

(Std. Err. adjusted for 314 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | .6515985 .1386257 -2.01 0.044 .4294271 .9887143

t_out | 1.080723 .0103077 8.14 0.000 1.060708 1.101116

dryer | 1.901636 1.916203 0.64 0.524 .2638793 13.70406

_cons | .5006158 .2147002 -1.61 0.107 .2159971 1.160276

------------------------------------------------------------------------------
98 

MN_05

[kitfan removed as a predictor due to spillage never occurring during operation]

[bathfan1 removed as a predictor due to insufficient (0) operation observations]

[bathfan2 removed as a predictor due to no data]

Logistic regression Number of obs = 8589

Wald chi2(5) = 128.65

Prob > chi2 = 0.0000

Log pseudolikelihood = -410.4762 Pseudo R2 = 0.0859

(Std. Err. adjusted for 421 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 10.68197 2.786208 9.08 0.000 6.406636 17.81036

t_out | 1.003908 .0149723 0.26 0.794 .9749871 1.033686

airhandler | 1.04318 .3289829 0.13 0.893 .5622377 1.935525

dryer | 2.178424 .7953753 2.13 0.033 1.065016 4.455833

door | 1.951835 .850156 1.54 0.125 .8311652 4.583518

_cons | .0041646 .0041561 -5.49 0.000 .000589 .029447

------------------------------------------------------------------------------

MN_06

[dryer removed as a predictor due to spillage never occurring during operation]

[kitfan removed as a predictor due to insufficient (0) operation observations]

[bathfan2 removed as a predictor due to no data]

[door removed as a predictor due to suspect data]

Logistic regression Number of obs = 3919

Wald chi2(4) = 50.64

Prob > chi2 = 0.0000

Log pseudolikelihood = -219.48276 Pseudo R2 = 0.1228

(Std. Err. adjusted for 224 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 4.71485 2.01405 3.63 0.000 2.041074 10.89123

t_out | 1.128124 .0514146 2.65 0.008 1.031723 1.233533

airhandler | 1.788027 1.184611 0.88 0.380 .4880142 6.551121

bathfan1 | 2.752574 1.334963 2.09 0.037 1.06394 7.121326

_cons | 5.73e-07 1.99e-06 -4.15 0.000 6.41e-10 .0005117

------------------------------------------------------------------------------
99 

MN_07

[dryer removed as a predictor due to spillage never occurring during operation]

[bathfan1 removed as a predictor due to spillage never occurring during operation]

[bathfan2 removed as a predictor due to no data]

[door removed as a predictor due to insufficient (0) operation observations]

Logistic regression Number of obs = 2609

Wald chi2(4) = 127.34

Prob > chi2 = 0.0000

Log pseudolikelihood = -47.881658 Pseudo R2 = 0.4506

(Std. Err. adjusted for 172 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 45.79054 28.72246 6.10 0.000 13.39224 156.5663

t_out | 1.107268 .0788912 1.43 0.153 .9629545 1.273208

airhandler | 16.09379 18.7085 2.39 0.017 1.648786 157.0913

kitfan | 13.09584 8.682303 3.88 0.000 3.571104 48.02467

_cons | 1.31e-07 7.09e-07 -2.94 0.003 3.35e-12 .0051531

------------------------------------------------------------------------------

MN_08

[airhandler removed as a predictor due to insufficient (0) operation observations]

[dryer removed as a predictor due to insufficient (0) operation observations]

[kitfan removed as a predictor due to insufficient (0) operation observations]

[bathfan1 removed as a predictor due to no data]

[bathfan2 removed as a predictor due to no data]

[door removed as a predictor due to insufficient (0) operation observations]

Logistic regression Number of obs = 1734

Wald chi2(2) = 46.76

Prob > chi2 = 0.0000

Log pseudolikelihood = -22.655858 Pseudo R2 = 0.5559

(Std. Err. adjusted for 82 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 244.1621 227.4903 5.90 0.000 39.31861 1516.207

t_out | 1.274945 .0859171 3.60 0.000 1.117197 1.454966

_cons | 1.02e-11 5.21e-11 -4.95 0.000 4.50e-16 2.31e-07

------------------------------------------------------------------------------ 
100 

MN_09

[kitfan removed as a predictor due to spillage never occurring during operation]

[bathfan1 removed as a predictor due to spillage never occurring during operation]

[bathfan2 removed as a predictor due to no data]

Logistic regression Number of obs = 5826

Wald chi2(5) = 229.00

Prob > chi2 = 0.0000

Log pseudolikelihood = -57.106929 Pseudo R2 = 0.5530

(Std. Err. adjusted for 464 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 10.70014 6.672228 3.80 0.000 3.152172 36.32191

t_out | 1.205639 .1365171 1.65 0.099 .9656814 1.505222

airhandler | 93794.19 289391.5 3.71 0.000 221.7628 3.97e+07

dryer | 4.205509 5.350559 1.13 0.259 .347422 50.90729

door | .0635157 .0691925 -2.53 0.011 .0075094 .5372289

_cons | 5.68e-10 5.50e-09 -2.20 0.028 3.27e-18 .0985806

------------------------------------------------------------------------------

MN_10

[airhandler removed as a predictor due to spillage never occurring during operation]

[dryer removed as a predictor due to no data]

[kitfan removed as a predictor due to no data]

[bathfan2 removed as a predictor due to no data]

[door removed as a predictor due to insufficient (0) operation observations]

Logistic regression Number of obs = 2157

Wald chi2(3) = 48.40

Prob > chi2 = 0.0000

Log pseudolikelihood = -49.817469 Pseudo R2 = 0.3727

(Std. Err. adjusted for 115 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

min1 | 110.9796 81.60065 6.40 0.000 26.26486 468.9336

t_out | 1.086181 .0491264 1.83 0.068 .9940397 1.186864

bathfan1 | .5542877 .4476359 -0.73 0.465 .1138454 2.698703

_cons | 4.04e-06 .000014 -3.58 0.000 4.54e-09 .0035951
101 

WI_01

[min1 removed as a predictor due to spillage always occurring in Minute 1] 

[kitfan removed as a predictor due to no data]

[bathfan1 removed as a predictor due to insufficient (3) operation observations]

[bathfan2 removed as a predictor due to no data]

Logistic regression Number of obs = 6147

Wald chi2(4) = 357.10

Prob > chi2 = 0.0000

Log pseudolikelihood = -890.71423 Pseudo R2 = 0.4953

(Std. Err. adjusted for 446 clusters in wh_cycle)

------------------------------------------------------------------------------

 | Robust

spill_500 | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]

-------------+----------------------------------------------------------------

t_out | 1.258112 .0156502 18.46 0.000 1.227809 1.289163

airhandler | .908079 .7186538 -0.12 0.903 .192523 4.283164

dryer | 1.731584 1.135378 0.84 0.402 .4789841 6.259878

door | 1.04558 .236613 0.20 0.844 .671014 1.629231

_cons | .0000428 .0000273 -15.75 0.000 .0000122 .0001497

------------------------------------------------------------------------------
DOE/GO-102015-4805 November 2015

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