Improving Gas Furnace 

Performance: A Field and 

Laboratory Study at End 

of Life

L. Brand, S. Yee, and J. Baker

Partnership for Advanced Residential Retrofit

February 2015
NOTICE

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

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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 

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government or any agency thereof.

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iii

Improving Gas Furnace Performance:

A Field and Laboratory Study at End of Life

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:

Partnership for Advanced Residential Retrofit

Gas Technology Institute

1700 South Mount Prospect Road

Des Plaines IL 60018

NREL Technical Monitor: Stacey Rothgeb

Prepared under Subcontract No. KNDJ-0-40346-03

February 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

Acknowledgments ................................................................................................................................... viii

Definitions................................................................................................................................................... ix

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

1 Problem Statement............................................................................................................................... 1

1.1 Introduction..........................................................................................................................1

1.2 Background..........................................................................................................................1

1.3 Installation Conditions.........................................................................................................2

1.4 Relevance to Building America’s Goals..............................................................................3

1.5 HVAC SAVE Program........................................................................................................3

1.6 Performance Degradation With Age....................................................................................3

2 Objective................................................................................................................................................ 5

3 Research Methodology........................................................................................................................ 6

3.1 Research Questions..............................................................................................................6

3.2 Technical Approach—Data Collection and Analysis..........................................................6

3.3 Measurement Methods.........................................................................................................8

3.4 Equipment............................................................................................................................9

3.5 Analysis Methodology.........................................................................................................9

3.6 Furnaces Tested .................................................................................................................10

3.6.1 InterCity Products GN100A016AIN .....................................................................11

3.6.2 InterCity Products GUI100A012GIN....................................................................12

3.6.3 Lennox G12QE382-10...........................................................................................13

3.6.4 InterCity Products GN100A016CIN......................................................................14

3.6.5 Fraser Johnston PBKM-L016N080A ....................................................................15

3.6.6 Lennox G23Q4/5-100-4.........................................................................................16

3.6.7 Amana GUC090X50B...........................................................................................17

3.6.8 Rheem RGOA100CER..........................................................................................18

3.6.9 Armstrong GUK075D14-18 ..................................................................................19

3.6.10 Trane TUS060A936AO.........................................................................................20

3.6.11 Lennox G16Q3757.................................................................................................21

3.6.12 Lennox GH6D 100M.............................................................................................22

4 Analysis ............................................................................................................................................... 23

4.1 Steady-State Efficiency......................................................................................................23

4.2 Annual Fuel Utilization Efficiency and Annualized Efficiency........................................27

4.3 Efficiency Change With Time ...........................................................................................30

5 Results and Discussion..................................................................................................................... 32

6 Conclusions and Recommendations ............................................................................................... 34

References................................................................................................................................................. 35
vi

List of Figures

Figure 1. Section 11.2.12 excerpt from ASHRAE Standard 103-2007.................................................. 10

Figure 2. Section 11.4.12 excerpt from ASHRAE Standard 103-2007.................................................. 10

Figure 3. Furnace #1 InterCity Products................................................................................................. 11

Figure 4. Furnace #2 InterCity Products................................................................................................. 12

Figure 5. Furnace #3 Lennox ................................................................................................................... 13

Figure 6. Furnace #4 InterCity Products................................................................................................. 14

Figure 7. Furnace #5 Fraser Johnston.................................................................................................... 15

Figure 8. Furnace #6 Lennox ................................................................................................................... 16

Figure 9. Furnace #7 Amana .................................................................................................................... 17

Figure 10. Furnace #8 Rheem .................................................................................................................. 18

Figure 11. Furnace #9 Armstrong............................................................................................................ 19

Figure 12. Furnace #10 Trane .................................................................................................................. 20

Figure 13. Furnace #11 Lennox ............................................................................................................... 21

Figure 14. Furnace #12 Lennox ............................................................................................................... 22

Figure 15. Steady-state efficiency field and Standard 103 lab conditions.......................................... 24

Figure 16. Steady-state efficiency field and Standard 103 lab conditions.......................................... 27

Figure 17. AE field and Standard 103 lab conditions ............................................................................ 28

Figure 18. Rated AFUE compared to AE Standard 103 lab conditions ............................................... 29

Figure 19. Efficiency change versus age................................................................................................ 30

Figure 20. Rated AFUE versus efficiency change per year.................................................................. 31

Unless otherwise noted, all figures were created by PARR.
vii

List of Tables

Table 1. Data Collected in the Field........................................................................................................... 7

Table 2. Data Collected in the Laboratory ................................................................................................ 8

Table 3. Measurement Tolerances ............................................................................................................ 8

Table 4. Test Procedure and Measurements............................................................................................ 9

Table 5. Equipment Table........................................................................................................................... 9

Table 6. Furnace #1 InterCity Products .................................................................................................. 11

Table 7. Furnace #2 InterCity Products .................................................................................................. 12

Table 8. Furnace #3 Lennox..................................................................................................................... 13

Table 9. Furnace #4 InterCity Products .................................................................................................. 14

Table 10. Furnace #5 Fraser-Johnston ................................................................................................... 15

Table 11. Furnace #6 Lennox................................................................................................................... 16

Table 12. Furnace #7 Amana.................................................................................................................... 17

Table 13. Furnace #8 Rheem.................................................................................................................... 18

Table 14. Furnace #9 Armstrong ............................................................................................................. 19

Table 15. Furnace #10 Trane.................................................................................................................... 20

Table 16. Furnace #11 Lennox................................................................................................................. 21

Table 17. Furnace #12 Lennox................................................................................................................. 22

Table 18. Efficiency Measurements ........................................................................................................ 23

Table 19. Furnace Field Data and Steady-State Efficiency................................................................... 23

Table 20. Steady-State Efficiency Comparison...................................................................................... 24

Table 21. Airflow in the Field and Standard 103 Conditions ................................................................ 25

Table 22. Furnace Rise Range ................................................................................................................. 25

Table 23. Airflow Rate Differences in the Field and Lab at the Same Static Pressure ...................... 26

Table 24. Steady-State Efficiency Measured in the Lab........................................................................ 27

Table 25. AE Under Field and Standard 103 Conditions....................................................................... 28

Table 26. Rated AFUE and AE Under Standard 103 Conditions .......................................................... 29

Unless otherwise noted, all tables were created by PARR.
viii

Acknowledgments

The Partnership for Advanced Residential Retrofit would like to thank Energy Stewards 

International for collecting the furnaces and field data used in this project.
ix

Definitions

AE Annualized efficiency

AFUE Annual fuel utilization efficiency

ANSI American National Standards Institute

ASHRAE American Society of Heating, Refrigerating and AirConditioning Engineers

BEoptTM Building Energy Optimization software

Btu British thermal unit

DOE U.S. Department of Energy

ESP External static pressure

HVAC Heating, ventilation, and air conditioning

HVAC SAVE HVAC System Adjustment & Verified Efficiency 

kWh Kilowatt hour

MEEA Midwest Energy Efficiency Alliance

w.c. Water column
x

Executive Summary

In 2010, natural gas provided 54% of total residential space heating energy in the United States

on a source basis, or 3.5 quadrillion Btu, according to the U.S. Department of Energy (DOE)

Building Energy Data Book. Natural gas burned in furnaces accounted for 92% of that total, and 

boilers and other equipment made up the remainder. A better understanding of installed furnace 

performance is key to energy savings for this significant energy use. 

Natural gas furnace performance can be measured in many ways. The DOE annual fuel 

utilization efficiency (AFUE) rating provides a fixed value under specified conditions, analogous

to the U.S. Environmental Protection Agency miles per gallon rating for new vehicles. The 

AFUE rating is provided by the manufacturer to the consumer and is a way to choose between 

models tested on the same basis. This value is commonly used in energy modeling calculations.

ASHRAE 103 is a consensus furnace testing standard developed by the engineering community.

The procedure provided in the standard covers heat-up, cool-down, condensate heat loss, and 

steady-state conditions and an imposed oversize factor. The procedure can be used to evaluate 

furnace performance with specified conditions or with some variation chosen by the tester. In 

this report the ASHRAE 103 test result will be referred to as annualized efficiency (AE) to avoid 

confusion with the DOE rating, and any nonstandard test conditions will be noted. Aside from 

these two laboratory tests, steady-state or flue loss efficiency can be measured in the field under

many conditions, typically as found or tuned to the manufacturers’ recommended settings. In this 

report, AE and steady-state efficiency are used as measures of furnace performance.

Modeled pre-retrofit energy consumption and retrofit energy savings commonly exceed those of 

actual energy consumption and savings. This is especially true in homes that are leaky, poorly 

insulated, and that have older mechanical systems (Polly et al. 2011). The discrepancy may be 

related to poor load modeling that increases runtimes; to the performance of equipment tested 

under laboratory conditions, which differs from field performance; or to equipment performance 

that degrades with time. The latter two theories are tested in this study.

The objective of this project is to examine the impact that common installation practices and ageinduced equipment degradation may have on the installed performance of natural gas furnaces

over the life of the product, as measured by steady-state efficiency and AE. The Partnership for 

Advanced Residential Retrofit identified 12 furnaces of various ages and efficiencies that were 

operating in residential homes in the Des Moines, Iowa, metropolitan area and worked with a 

local heating, ventilation, and air conditioning contractor to retrieve furnaces and test them for 

steady-state efficiency and AE in the lab. Prior to removal, system airflow, static pressure, 

equipment temperature rise, and flue loss measurements were recorded for each furnace as 

installed in the house. After removal from the field, the furnaces were transported to the Gas 

Technology Institute laboratory, where the Partnership for Advanced Residential Retrofit

conducted steady-state efficiency and AE testing. Nine of the 12 furnaces could be tested in the 

lab with minor component repair that did not impact efficiency. Steady-state efficiency was 

calculated for each furnace from the field data. Each furnace was then tested for steady-state

efficiency and AE under field conditions and AE under test conditions specified in the ASHRAE 

103-2007 (consensus) test standard, based on furnace type. In this report, rated AFUE is used as 

the “before” condition and AE under ASHRAE Standard 103 conditions is used as the “after” 
xi

condition in the absence of individual furnace efficiency lab test results when the furnaces were 

new.

The test results show that steady-state efficiency in the field was 6.4% lower than that measured 

for the same furnaces under Standard 103 conditions in the lab, which included tuning the 

furnace input and airflow rate to the conditions recommended by the manufacturer. This value is 

based on the average of nine of the 12 furnaces that could be tested after minor repair.

Comparing AE measured under Standard 103 conditions with the rated AFUE shows similar 

values, implying that equipment performance did not significantly decrease over 15–24 years of 

operation. The conclusion is that rated AFUE values can be used as a good indicator of the 

performance of natural gas furnaces throughout their useful lives if they are installed according 

to the manufacturers’ installation instructions. Installing a furnace correctly in the lab or in the 

field is a key driver in this finding: increase the blower speed to provide the correct airflow to 

match the manufacturer’s recommended temperature rise without exceeding the manufacturer’s 

design static pressure. If the fan speed cannot be adjusted properly, changes to the duct system 

will need to be made. 
1

1 Problem Statement

1.1 Introduction

Natural gas furnaces are rated for efficiency using the U.S. Department of Energy (DOE) annual 

fuel utilization efficiency (AFUE) test standard under controlled laboratory test conditions. In the 

home, these furnaces are then installed under conditions that can vary significantly from the 

standard, require adjustment by the installing contractor to adapt to field conditions, may or may 

not be inspected over their useful lifetimes, and can operate with little maintenance over a 30-

year period or longer. At issue is whether the installation practices, field conditions, and wear 

over the life of the furnace reduce the efficiency significantly from the rated efficiency. In this 

project, nine furnaces, with 15–24 years of field service, were removed from Iowa homes and 

tested in the lab under four conditions to determine the effects of installation practices, field 

operating conditions, and age on efficiency. 

1.2 Background

As required by the Energy Policy and Conservation Act of 1975, the National Bureau of 

Standards developed test procedures to establish methods of energy consumption of certain 

appliances. On May 10, 1978, DOE promulgated the test procedures for furnaces and boilers. 

Those procedures and the amendments of August 10, 1980 provided the basis for ASHRAE 

Standard 103, the gas furnace and boiler efficiency test standard. ASHRAE developed the 

standard using the American National Standards Institute (ANSI) consensus process. It was 

approved by the ASHRAE Standards Committee on June 27, 1982. The original standard was 

ANSI/ASHRAE 103-1982 (ASHRAE 1982). ASHRAE 103-2007 will be referred to as 

“Standard 103” in this report.

Minimum appliance efficiency levels came into effect in the mid-1980s. When the Energy Policy 

and Conservation Act was amended, DOE was required to promulgate energy conservation 

standards for certain major appliances, including residential furnaces. In 1987, the National 

Appliance Energy Conservation Act established initial minimum efficiency standards for 

residential gas furnaces at 78% AFUE for most gas furnaces effective January 1, 1992. The 

minimum efficiency for furnaces was revised upward in 2007—gas furnaces produced after 2015

are required to have an AFUE of 80% or higher.

The DOE AFUE rating procedure and ASHRAE Standard 103 are not developed in tandem. In 

1997, DOE published a final rule that amended the original test procedure for furnaces

incorporating provisions contained in test procedure waivers granted to several manufacturers 

from 1985 to 1996 and adding test procedures for new product designs. In 2010, DOE published 

a final rule that amended the original test procedure for residential furnaces to include standby 

and off-mode energy use. ASHRAE re-evaluates Standard 103 on a 5-year interval and published 

the most recent version in 2007.

ASHRAE SPC 103 is currently reviewing Standard 103 for the next publication cycle, and DOE 

has issued a Request for Information seeking input on the DOE AFUE test procedure. Because

the timeline for the ASHRAE and DOE standards activities are not coincident, the furnaces 

tested in this report under ASHRAE 103 2007 may have originally been listed according to an 

older DOE test procedure. The authors believe the differences in ASHRAE Standard 103 and 
2

DOE AFUE test procedure over time are not significant and are within the 0.1% experimental 

error of the laboratory test setup in this report (Brand and Rose 2012).

Previous Building America research has shown that correct sizing and proper installation of 

heating, ventilation, and air conditioning (HVAC) systems are crucial to achieving the desired 

occupant comfort and efficiency levels (Burdick 2011; Brand 2012). However, while there are 

several directly applicable and well-known installation, sizing, and distribution design 

guidelines, the DOE Building American team, Partnership for Advanced Residential Retrofit 

(PARR), has observed that installing contractors often do not follow these guidelines, especially 

during an upgrade of existing equipment. It is common practice within the HVAC industry to 

replace existing equipment without determining if the existing duct system is properly designed 

and sized to match the needs of the new equipment, or to fail to adjust the new equipment to 

match the existing distribution system. The result is equipment installed and operating at 

conditions outside the manufacturer’s recommended range.

This project explored the effects these adverse operating conditions have on equipment 

performance ratings by conducting several measurements:

• Calculated steady-state efficiency based on field reported conditions.

• Measured steady-state efficiency in the lab at the same field conditions without adjusting 

the furnaces.

• Measured steady-state efficiency in the lab at ASHRAE Standard 103 conditions.

• Measured, annualized efficiency (AE) in the lab under conditions reported from the field 

and without adjusting the furnaces.

• Measured AE in the lab under Standard 103 conditions.

AE measurements are as described in ANSI/ASHRAE 103-2007. Although Standard 103 uses 

AFUE as the test value, AE is used in this report to avoid confusion with the DOE rating. PARR 

has evaluated the performance of each furnace according to ASHRAE Standard 103 and has 

modified the test to simulate conditions found in the field for some furnace test cases.

1.3 Installation Conditions

In retrofit situations HVAC installers regularly fail to address the existing distribution system 

when installing new equipment, resulting in equipment operating outside the manufacturer’s 

specifications. Ductwork that was installed many years ago to meet the previous load and match 

the original equipment (generally higher delivered air temperature at lower flow rates) often 

cannot handle the airflow requirements of newer high efficiency units. Manufacturers address 

this situation in their installation instructions by providing guidelines on setting the fan speed so 

the furnace operates in its rise range (air temperature rise across the furnace). A second common 

field adjustment is to set the input rate on the furnace to match the value on the label. This is 

done by adjusting the gas valve pressure setting and measuring the gas flow rate by observing the 

meter. Some installers fail to perform these adjustments and the performance is impacted.
3

1.4 Relevance to Building America’s Goals

Overall, the goal of DOE’s Building America program is to reduce home energy use by 30%–

50% compared to the 2009 International Energy Conservation Code for new homes and preretrofit energy use for existing homes. To this end, Building America conducts research to 

“develop market-ready energy solutions that improve efficiency of new and existing homes in 

each U.S. climate zone, while increasing comfort, safety, and durability.”1

This project directly aligns with Building America’s objectives, as it addresses commonly 

observed deficiencies in forced-air natural gas furnace installations, which likely increase energy 

use in many American homes. Presumably, following the furnace manufacturer’s installation 

instructions carefully and employing industry-approved methods for equipment sizing, duct 

design, and filter selection should result in the unit achieving its rated performance, while failure 

to do so may reduce performance, thus increasing energy use. Further, degradation of 

performance over time caused by drift of component settings or other factors is assumed; this 

project provides data to test those assumptions.

1.5 HVAC SAVE Program

The furnaces analyzed in this study were taken from homes participating in Iowa’s HVAC 

System Adjustment & Verified Efficiency (SAVE) program, a statewide HVAC training and 

certification program (Yee et al. 2013). The HVAC SAVE program was developed by the 

Midwest Energy Efficiency Alliance (MEEA) to train contractors in the skills necessary to 

determine in-place performance of functioning systems, including equipment and distribution 

systems. Energy Stewards International has been training HVAC professionals for many years 

on how contractors can use static pressures, system temperatures, and airflows to identify 

existing system deficiencies, allowing them to make targeted repairs or adjustments. The 

furnaces being analyzed in this study, which were removed from the field, help to test the 

hypothesis that previously installed equipment is performing in less than optimal installation 

conditions.

1.6 Performance Degradation With Age

In addition to studying how installed conditions affect energy use, this project examined

potential degradation in equipment performance over time by conducting a second round of AE

testing on each unit under conditions specified in Standard 103 and comparing these results with 

the original rating. Building America Simulation protocols (Hendron and Engebrecht 2010) once 

used a degradation factor for furnace efficiency in Building Energy Optimization software 

(BEoptTM), although that has been discontinued. The equation below provides the calculation 

evaluated in this project:

 1 www1.eere.energy.gov/buildings/residential/ba_research.html
4

AFUE = (Base AFUE) × (1-M)age

where 

Base AFUE = Typical efficiency of pre-retrofit equipment when new

M = Maintenance factor

0.005 annual professional maintenance

0.015 seldom or never maintained 

age = Age of equipment in years, up to a maximum of 20 years
5

2 Objective

The objective of this project was to examine the impact that common installation practices and 

age-induced equipment degradation may have on the installed performance of natural gas 

furnaces, as measured by steady-state efficiency and AE. Recommendations are made on 

methods to improve field performance.
6

3 Research Methodology

3.1 Research Questions

The research conducted in this project addresses the following questions:

1. What is the degradation in furnace efficiency for typical field installation compared to the 

rated performance?

2. How should performance ratings be modified in models (such as BEopt) to account for 

degradation, especially for retrofit situations?

3. How do rated and measured efficiency compare for vintage furnaces?

3.2 Technical Approach—Data Collection and Analysis

PARR identified 12 working furnaces of various production years installed in the field, gathered

operating condition information, transported each furnace to a laboratory, and tested each 

furnace for steady-state efficiency and for AE as described in ASHRAE Standard 103-2007. 

Standard 103 provides test methods, instrumentation tolerances, and calculation techniques to 

determine AE. The following basic tests were conducted:

1. Steady-state test (used for both the steady-state and AE results)

2. Cool-down test

3. Heat-up test

4. Condensate heat loss under cyclic conditions test (condensing furnaces).

In addition to the test conditions recommended in Standard 103, PARR also ran the above tests 

mimicking the operating conditions of each furnace, as found in the field. These conditions differ 

from furnace to furnace as each field installation does vary. Based on MEEA’s recent experience 

with the HVAC SAVE, a regional contractor training and certification initiative based on 

National Comfort Institute principles, some common conditions seen in the field are as follows: 

• High external static pressures (ESPs) caused by either undersized or restrictive 

distribution systems: For testing, Standard 103 recommends a minimum external static of 

0.2 in. w.c. (inches of water column) depending on the furnace size; 0.7–0.9 in. w.c. is 

commonly observed in the field. External static was be measured in the field for each 

furnace and used for testing.

• Oversized equipment: Standard 103 accounts for oversizing by simulating burner on and 

off time per cycle; testing is conducted under conditions that represent a 70% oversized 

unit. In this project it was not possible to test oversizing because house characteristics 

were not logged and utility bills are not available. In previous work done by PARR 

(Brand and Rose 2012), it was determined that oversizing is not a significant factor for 

high-efficiency furnaces.

• Equipment that is off rate: The Standard 103 recommends that burner input rate be 

adjusted to within 2% of the hourly Btu input rating specified by the manufacturer. The 

gas input to the equipment was measured in the field and then tested under the same 
7

conditions in the laboratory during the field conditions round of testing. For testing under 

Standard 103 conditions, input rate was corrected.

The original test plan for this project included collecting furnaces with a range of ages and 

designs: 1960s–1990s, ribbon burners and in-shot burners, atmospheric and fan-assisted 

combustion, lower efficiency and higher efficiency. The furnaces in this study covered all the 

cases except the very early years. The year of manufacture of the nine furnaces tested ranges 

from 1987–1998, three are ribbon burners, and the rest are in-shot burners, two are atmospheric 

combustion systems, and two are high efficiency (condensing) furnaces. This range of ages and 

types is fairly representative of the installed base and is sufficient for this research.

Furnaces were sourced from the Des Moines, Iowa, metropolitan area, leveraging MEEA’s work 

on HVAC SAVE. Twelve furnaces were collected; only nine could be tested without significant 

repair. PARR chose not to test the furnaces requiring significant repair because the performance 

could be altered.

Before each furnace was removed from the field, several measurements were taken to determine 

actual operating conditions. PARR is aware that measurements taken in the field are difficult to 

obtain within the accuracy of that recommended by Standard 103. These measurements were not

used to determine AE, but rather as a general guide for determining field steady-state conditions 

and field conditions for laboratory testing. Table 1 provides information that was collected in the 

field.

Table 1. Data Collected in the Field

Data Notes

Nameplate Data Units, coils, fans, etc.

Temperature Rise Across the Equipment °F, radiation shielded

Static Pressure Using a manometer or digital gage

O2% in Flue Gases Concentration, %

Airflow Rate Flow plate replacing the filter

Once the initial evaluation of the equipment was completed in the field, each furnace was

shipped to the Gas Technology Institute laboratory.

Table 2 shows the major data points that were collected as part of the efficiency testing, in 

accordance with ASHRAE 103:
8

Table 2. Data Collected in the Laboratory

Data Notes

Room Temperature Average of 4 points, radiation shielded

Jacket Loss Thermocouples in 6 × 6 in. square on furnace jacket except 

for blower housing (contact temperature sensor may be used)

Power to Burner and Blower kWh

Dry CO2 in Flue Gases Concentration, ppm

Room Humidity Level Must be less than 80% RH

Time for Several Events Seconds

Flue Gas Temperature In a grid pattern using beaded thermocouples

Supply and Return 

Temperatures Thermocouples

Gas Consumption Cubic feet

Weight of Condensate Using a clean non-scaling container

Gas Heating Value Btu/standard cubic feet

Tracer Gas Concentration Within 2% of value

Table 3 provides measurement tolerances from Standard 103. Instruments were selected and 

power conditioning was applied to meet these requirements.

Table 3. Measurement Tolerances

Measurement Tolerance

Temperature 2°F

Gas Pressure 0.2 in. w.c.

Air Pressure 0.01 in. w.c.

CO2 0.1%

Weight 0.5%

Volume 0.5%

Time 0.5 s/h

Tracer Gas 2%

Electricity 1%

Voltage 1%

Gas 1%

Gas Heating Value 1%

3.3 Measurement Methods

Laboratory steady-state and AE testing for this project has been conducted according to Standard

103. Table 4 provides a summary of the measurements required for each step.
9

Table 4. Test Procedure and Measurements

Section

in

Standard

103

Test Name Description Measurements

9.1 Steady-state test

Measure energy into the 

furnace from natural gas and 

energy lost from the vent 

system and condensate system

Measure CO2, flue gas 

temperature, condensate 

production, gas consumption, and 

jacket losses under steady-state 

conditions.

9.5 Cool-down test

Measure energy lost to the 

flue gases during the postpurge or cool-down period

Flue gas temperature and time 

under cool-down conditions

9.6 Heat-up test

Measure energy lost to the 

flue gasses during heat up 

with delayed blower start

Flue gas temperature and time 

during heat-up

9.8

Condensate heat 

loss under 

cyclic 

conditions

Measure thermal energy lost 

to condensate

Fuel input, heating value, 

temperature, and pressure of the 

gas, and weight of condensate for 

each cycle

3.4 Equipment

Table 5 provides the basic set of equipment used to perform these tests.

Table 5. Equipment Table

Measurement Equipment Needed

Gas Flow Rate Gas flow meter calibrated to be accurate within 1% of flow volume

Electricity

Consumption

Watt meters with an accuracy of 1% of measured energy

(not included in energy balance)

Temperatures Bead thermocouples with an accuracy of ± 2°F

Air Pressure Manometer or pressure gage with an accuracy of ± 0.01 in. w.c.

Power Conditioning Voltage to be controlled within 1% of nameplate requirement on 

furnace

CO2 Concentration Within ± 0.1%

Time Stopwatch or timer accurate to ± 0.5 s/h

Tracer Gas Gas chromatograph accurate to ± 2% of measured concentration

Gas Heating Value Gas calorimeter ± 1% in Btu/ft3

3.5 Analysis Methodology

The full analysis procedure is described in detail in ASHRAE Standard 103-2007. Section 11 

was followed for calculating the AE based on the type of furnace under test. For noncondensing 

and nonmodulating furnaces, Section 11.2 describes the procedures used to calculate steady-state

efficiency, heating seasonal efficiency, maximum fuel input rate, and oversizing factor, all of 

which will be used in step 11.2.12 to calculate the AE. The same process applies for condensing 

furnaces as described in Section 11.3, noncondensing modulating furnaces as described in 
10

Section 11.4, and condensing modulating furnaces as described in Section 11.5. The two 

equations used to calculate AE are shown in Figure 1 for nonmodulating furnaces and Figure 2 

for modulating furnaces. (AFUE in Standard 103 is referred to as AE in this report to avoid 

confusion.)

Figure 1. Section 11.2.12 excerpt from ASHRAE Standard 103-2007

Figure 2. Section 11.4.12 excerpt from ASHRAE Standard 103-2007

3.6 Furnaces Tested

Twelve furnaces were collected from the field. Only nine could be tested in the lab without 

significant repair that would impact performance. A short description of each furnace, a 

photograph, nameplate information, and field measurements are included in Figure 3 through 

Figure 14 and Table 6 through Table 17.
11

3.6.1 InterCity Products GN100A016AIN

The GN100A016AIN model is a mid-efficiency gas furnace with an induced draft combustion 

system. The unit was built in April 1992 and removed from service in September 2012.

Figure 3. Furnace #1 InterCity Products

Table 6. Furnace #1 InterCity Products 

Specifications

Manufacturer InterCity Products Height, in. 40.125

Model GNI100A016AIN Width, in. 22.75

Serial Number R921600151 Depth, in. 28.25

Year April 1992 Duct Depth, in. 18.5

Input Capacity 100,000 Duct Width, in. 21.25

Output Capacity 80,400

DOE-Rated AFUE 80.5

Field Data Reported

Airflow (CFM) 920 Temperature Rise, °F 84

O2% 9.6 ESP, in. w.c. 0.9

Flue Temperature, °F 467
12

3.6.2 InterCity Products GUI100A012GIN

The GUI100A012GIN model is a mid-efficiency gas furnace with an induced draft combustion 

system. This unit was built in March 1991 and removed from service in September 2012.

Figure 4. Furnace #2 InterCity Products

Table 7. Furnace #2 InterCity Products

Specifications

Manufacturer InterCity Products Height, in. 39.25

Model GUI100A012GIN Width, in. 19.25

Serial Number L931034889 Depth, in. 28.25

Year March 1991 Duct Depth, in. 18.5

Input Capacity 100,000 Duct Width, in. 17.75

Output Capacity 80,000

DOE-Rated AFUE 80.1

Field Data Reported

Airflow (CFM) 878 Temperature Rise, °F 73

O2% 11.3 ESP, in. w.c. 0.7

Flue Temperature, °F 504
13

3.6.3 Lennox G12QE382-10

The G12QE382-10 model is an atmospheric gas furnace using indoor combustion air, a draft 

diverter, and an atmospheric burner. The unit was built in June 1989 and removed from service 

in September 2012.

Figure 5. Furnace #3 Lennox

Table 8. Furnace #3 Lennox

Specifications

Manufacturer Lennox Height, in. 49

Model G12QE382-10 Width, in. 16.5

Serial Number 5889607663 Depth, in. 26.25

Year June 1989 Duct Depth, in. 17.75

Input Capacity 82,000 Duct Width, in. 14.25

Output Capacity None Found

DOE-Rated AFUE 72.5

Field Data Reported

Airflow (CFM) 947 Temperature Rise, °F 88

O2% 13.2 ESP, in. w.c. 0.6

Flue Temperature, °F 548
14

3.6.4 InterCity Products GN100A016CIN

The GN100A016CIN model is a mid-efficiency gas furnace using an induced-draft combustion 

system. The unit was manufactured in October 1993 and removed from service in September 

2012. The front panels of this furnace were missing. New panels were fabricated for the test.

Figure 6. Furnace #4 InterCity Products

Table 9. Furnace #4 InterCity Products

Specifications

Manufacturer InterCity Products Height, in. 40.625

Model GN100A016CIN Width, in. 23

Serial Number L934418652 Depth, in. 28.25

Year October 1993 Duct Depth, in. 18.25

Input Capacity 100,000 Duct Width, in. 21.25

Output Capacity 80,400

DOE-Rated AFUE 80.5

Field Data Reported

Airflow (CFM) 792 Temperature Rise, °F 84

O2% 9.6 ESP, in. w.c. 0.9

Flue Temperature, °F 467
15

3.6.5 Fraser Johnston PBKM-L016N080A

The PBKM-L016N080A model is a mid-efficiency furnace using an induced draft combustion 

system. This furnace was received without a front panel, so one was fabricated. The unit was 

built in February 1994 and removed from service in September 2012. 

Figure 7. Furnace #5 Fraser Johnston

Table 10. Furnace #5 Fraser-Johnston

Specifications

Manufacturer Fraser-Johnston Height, in. 40.25

Model PBKM-L016N080A Width, in. 22.25

Serial Number EBCM101513 Depth, in. 28.25

Year February 1994 Duct Depth, in. 19.5

Input Capacity 80,000 Duct Width, in. 21.125

Output Capacity 64,000

DOE-Rated AFUE 80

Field Data Reported

Airflow (CFM) 619 Temperature Rise, °F 86

O2% 10.8 ESP, in. w.c. 0.9

Flue Temperature, °F 511
16

3.6.6 Lennox G23Q4/5-100-4

The G23Q4/5-100-4 model is a mid-efficiency furnace using an induced draft combustion 

system. The unit was built in January 1998 and removed from service in September 2012.

Figure 8. Furnace #6 Lennox

Table 11. Furnace #6 Lennox

Specifications

Manufacturer Lennox Height, in. 46.25

Model G23Q4/5-100-4 Width, in. 22.25

Serial Number 5898A 35920 Depth, in. 28.5

Year January 1998 Duct Depth, in. 18.75

Input Capacity 100,000 Duct Width, in. 19

Output Capacity 80,000

DOE-Rated AFUE 80

Field Data Reported

Airflow (CFM) 841 Temperature Rise, °F 89

O2% 13.3 ESP, in. w.c. 0.8

Flue Temperature, °F 562
17

3.6.7 Amana GUC090X50B

The GUC090X50B model is a high-efficiency direct vent condensing gas furnace. The front 

panels for this furnace were missing and new panels were fabricated. The unit was built in 

September 1996 and removed from service in September 2012.

Figure 9. Furnace #7 Amana

Table 12. Furnace #7 Amana

Specifications

Manufacturer Amana Height, in. 48

Model GUC090X50B Width, in. 24.625

Serial Number 9609152460 Depth, in. 28

Year September 1996 Duct Depth, in. 19.75

Input Capacity 90,000 Duct Width, in. 23.125

Output Capacity 85,000

DOE-Rated AFUE 94.4

Field Data Reported

Airflow (CFM) 1071 Temperature Rise, °F 53

O2% 12.9 ESP, in. w.c. 0.8

Flue Temperature, °F 144
18

3.6.8 Rheem RGOA100CER

The RGOA100CER model is an atmospheric gas furnace with a draft diverter. This furnace 

could not be tested because it was damaged beyond repair during shipping. The unit was built in 

August 1991 and removed from service in September 2012.

Figure 10. Furnace #8 Rheem

Table 13. Furnace #8 Rheem

Specifications

Manufacturer Rheem Height, in. 46.25

Model RGOA100CER Width, in. 17.5

Serial Number MN3D104 F3491 0205 Depth, in. 28.125

Year August 1991 Duct Depth, in. 20

Input Capacity 100,000 Duct Width, in. 16

Output Capacity 65,000

DOE-Rated AFUE 65

Field Data Reported

Airflow (CFM) 997 Temperature Rise, °F 84

O2% 10 ESP, in. w.c. 0.8

Flue Temperature, °F 497
19

3.6.9 Armstrong GUK075D14-18

The GUK075D14-18 model is a direct-vent high-efficiency condensing gas furnace. It was 

manufactured in September 1994 and removed from service in September 2012.

Figure 11. Furnace #9 Armstrong

Table 14. Furnace #9 Armstrong

Specifications

Manufacturer Armstrong Height, in. 46

Model GUK075D14-18 Width, in. 22

Serial Number 8494J33762 Depth, in. 28

Year September 1994 Duct Depth, in. 19.5

Input Capacity 75,000 Duct Width, in. 20.75

Output Capacity 67,000

DOE-Rated AFUE 90

Field Data Reported

Airflow (CFM) 693 Temperature Rise, °F 86

O2% 12.2 ESP, in. w.c. 0.9

Flue Temperature, °F 555*

*Reported value
20

3.6.10 Trane TUS060A936AO

The TUS060A936AO model is an atmospheric gas furnace with a draft diverter. Front panels 

were fabricated to replace those lost in shipment. The center heat exchanger mounting screw 

was replaced. The unit was manufactured in September 1987 and removed from service in 

September 2012.

Figure 12. Furnace #10 Trane

Table 15. Furnace #10 Trane

Specifications

Manufacturer Trane Height, in. 49

Model TUS060A936AO Width, in. 18.125

Serial Number B36599305 Depth, in. 28.125

Year September 1987 Duct Depth, in. 20

Input Capacity 60,000 Duct Width, in. 17

Output Capacity None Found

DOE-Rated AFUE 68.4

Field Data Reported

Airflow (CFM) 604 Temperature Rise, °F 87

O2% 11.3 ESP, in. w.c. 0.8

Flue Temperature, °F 497
21

3.6.11 Lennox G16Q3757

The G16Q3757 model is a mid-efficiency gas furnace using an induced draft combustion system. 

This was damaged beyond repair during shipping and could not be tested. It was manufactured in 

March 1989 and removed from service in September 2012.

Figure 13. Furnace #11 Lennox

Table 16. Furnace #11 Lennox

Specifications

Manufacturer Lennox Height, in. 49

Model G16Q3757 Width, in. 16.25

Serial Number 5889C07272 Depth, in. 26.25

Year March 1989 Duct Depth, in. 18

Input Capacity 75,000 Duct Width, in. 14

Output Capacity None Found

DOE-Rated AFUE 78.5

Field Data Reported

Airflow (CFM) 720 Temperature Rise, °F 76

O2% 13.2 ESP, in. w.c. 0.8

Flue Temperature, °F 531
22

3.6.12 Lennox GH6D 100M

The GH6D 100M model is an atmospheric gas furnace that uses a draft diverter. This furnace 

could not be tested because the height exceeded the test chamber capability. The unit was 

manufactured in May 1963 and removed from service in September 2012.

Figure 14. Furnace #12 Lennox

Table 17. Furnace #12 Lennox

Specifications

Manufacturer Lennox Height, in. 60.75

Model GH6D 100M Width, in. 21

Serial Number None Found Depth, in. 27.5

Year May 1963 Duct Depth, in. 17

Input Capacity 100,000 Duct Width, in. 19

Output Capacity 80,000

DOE-Rated AFUE N/A

Field Data Reported

Airflow (CFM) 861 Temperature Rise, °F 88

O2% 11.5 ESP, in. w.c. 0.8

Flue Temperature, °F 485
23

4 Analysis

Gas furnace efficiency was determined using five different metrics to compare laboratory and 

field performance. Table 18 below shows the efficiency metrics.

Table 18. Efficiency Measurements

Efficiency

Metric Description

SSE FC Calc Steady-state efficiency under field conditions calculated from field data

SSE FC LM Steady-state efficiency under field conditions as measured in the lab

SSE SC LM Steady-state efficiency under ASHRAE Standard 103 test conditions 

measured in the lab; airflow and input rate adjusted

AE FC LM AE using ASHRAE Standard 103 under field conditions measured in the lab

AE SC LM AE using ASHRAE Standard 103 test conditions measured in the lab; 

airflow and input rate adjusted to Standard 103 conditions

4.1 Steady-State Efficiency

Steady-state efficiency was measured under constant load conditions in the field and in the lab

(thermostat calling for heat). Field data were reported by the HVAC contractor. Lab testing 

included the Gas Technology Institute rack-mounted combustion analyzer system that is 

calibrated daily and the instruments used in the Standard 103 test procedure as discussed earlier 

in this report. Field conditions are based on handheld combustion analyzers, digital static 

pressure transducers for ESP, and a flow plate for airflow rate. 

Table 19 shows the data collected in the field and the manufacturer-recommended temperature 

rise. Note that in almost every case, the furnaces were operating in the field above the 

manufacturer’s temperature rise recommendation. 

Table 19. Furnace Field Data and Steady-State Efficiency

Number Manufacturer O2 

(%)

ESP 

(in. 

w.c.)

Airflow 

Rate 

(CFM)

Flue 

Temperature

(°F)

Temperature 

Rise 

(°F)

Manufacturer 

Temperature

Rise Range

(°F)

1 ICP 9.6 0.9 920 467 84 30–60

2 ICP 11.3 0.7 878 504 73 40–70

3 Lennox 13.2 0.6 947 548 88 70–100

4 ICP 12 0.9 792 433 90 30–60

5 Fraser-Johnston 10.8 0.9 619 511 86 20–50

6 Lennox 13.3 0.8 841 562 89 30–60

7 Amana 12.9 0.8 1071 144 53 35–65

9 Armstrong 12.2 0.9 693 555 86 50–80

10 Trane 11.3 0.8 604 497 87 40–70
24

Because AFUE and AE cannot be measured in the field, steady state efficiency is used as a field

efficiency indicator. A comparison was made with the calculated steady-state efficiency value 

from field data and steady-state efficiency under the Standard 103 conditions as measured in the 

lab, SSE SC LM. Table 20 shows the steady-state efficiency calculated from the field data and 

SSE SC LM measured in the lab under Standard 103 test conditions of 0.2 in. w.c. ESP. In all 

cases, the field value is less than the laboratory-measured value.

Table 20. Steady-State Efficiency Comparison

Number Manufacturer SSE FC Calc SSE SC LM

1 ICP 76.8 80.4

2 ICP 73.3 81.4

3 Lennox 67.5 77.2

4 ICP 75.1 80.5

5 Fraser Johnston 73.7 79.7

6 Lennox 66.5 81.2

7 Amana 87.0 93.7

9 Armstrong 88.1 88.6

10 Trane 73.5 76.3

Figure 15 compares the steady-state efficiency between the field and Standard 103 conditions. 

The 45-degree line represents perfect agreement, above the line means the lab measured value is 

greater, or conversely that the calculated value from field measurement is lower. The difference 

between the means is 6.4%. A paired t-test of the null hypothesis that the sample means are not 

different from each other at p < 0.05 is rejected based on a statistical analysis. The means are 

significantly different for these two data sets, indicating poor performance in the field caused by

poor installation practices or misadjustment.

Figure 15. Steady-state efficiency field and Standard 103 lab conditions
25

Field installation parameters were evaluated to determine the root cause for this difference. The 

airflow rate measured in the field and in Standard 103 steady-state efficiency measurement (not 

the same static pressure) is provided in Table 21. Note that the Standard 103 numbers are 

significantly higher than in the field because of improper air distribution system design.

Table 21. Airflow in the Field and Standard 103 Conditions

Number Manufacturer Field Airflow

(CFM)

Standard 103

Conditions Airflow

(CFM)

1 ICP 920 1925

2 ICP 878 1635

3 Lennox 947 943

4 ICP 792 1754

5 Fraser Johnston 619 1488

6 Lennox 841 1664

7 Amana 1071 1516

9 Armstrong 693 1389

10 Trane 604 945

An indirect measure of how well the field airflow is adjusted is to measure the temperature rise 

across the furnace and identify where it lies in the manufacturer’s recommended “rise range” as 

follows: The midpoint value is the target; higher values result in decreased efficiency and lower 

values increase the risk of corrosion in the heat exchanger. Table 22 shows the rise values from 

the field and Standard 103 test along with the manufacturers’ recommendations. Note that fan 

speed adjustments are very coarse and meeting the requirement is not always possible.

Table 22. Furnace Rise Range

Number Manufacturer

Manufacturer 

Temperature Rise 

Range 

(°F)

Field 

Temperature 

Rise 

(°F)

Standard 103

Temperature 

Rise 

(°F)

1 ICP 30–60 84 43

2 ICP 40–70 73 49

3 Lennox 70–100 88 69

4 ICP 30–60 90 47

5 Fraser-Johnston 20–50 86 45

6 Lennox 30–60 89 52

7 Amana 35–65 53 66

9 Armstrong 50–80 86 58

10 Trane 40–70 87 51
26

Airflow was not set within the rise range for many of these furnaces in the field, or the ductwork 

was too small to achieve the required airflow. Note that most field furnaces were operating at the 

top or above the rise range, resulting in lower steady-state efficiency.

As a final note on field adjustments, input rate adjustments were not required for laboratory 

testing of the furnaces except for furnace #10. This furnace was received set at 76,000 Btu/h 

compared to the 60,000 Btu/h on the label. There was no individual test to determine the 

efficiency change associated with adjusting the input in isolation, but adjusting both the input 

rate and airflow improved the steady-state efficiency by 2.8% for this furnace. 

PARR attempted to measure the steady-state efficiency of the furnaces under field airflow and 

static pressure conditions (SSE FC LM) to determine if the field conditions could be replicated in 

the lab. Table 23 shows the field-measured airflow rate and the lab-measured airflow rate when 

the furnaces were installed to the same reported ESP conditions needed to replicate the ductwork

pressure drop from the field. There is a significant discrepancy between these two values, for 

reasons unknown to the investigator. It is likely that either the reported field ESP or field airflow

measurements are not reliable. Potential causes are that the flow plate replaced a dirty filter or 

that the return duct geometry was not suitable for use with the flow plate.

Table 23. Airflow Rate Differences in the Field and

Lab at the Same Static Pressure

Furnace

Field-Measured 

Airflow Rate 

(CFM)

Lab-Measured 

Airflow Rate

(CFM)

1 920 1,288

2 878 1,206

3 947 661

4 792 1,048

5 619 938

6 841 1,407

7 1,071 1,516

9 693 955

10 604 665

PARR elected to set up the furnaces in the lab according to the static pressure reading rather than 

the airflow reading, because it was considered to be the more accurate of the two field 

measurements. SSE FC LM was compared to the steady-state efficiency under the conditions in 

Standard 103 (SSE SC LM). Recall that that the Standard 103 condition test is done after airflow

adjustments are made and the input rate is adjusted. 

Table 24 shows the differences between the two values. 
27

Table 24. Steady-State Efficiency Measured in the Lab

Furnace

Field Conditions LabMeasured Steady-State 

Efficiency

(SSE FC LM), %

Standard 103 Conditions 

Lab-Measured SteadyState Efficiency 

(SSE SC LM), %

1 79.3 80.4

2 80.6 81.4

3 74.8 77.2

4 77.2 80.5

5 79.1 79.7

6 80.9 81.2

7 90.9 93.7

9 88.3 88.6

10 76.7 76.3

Figure 16 shows the two efficiency values on a chart; the 45-degree line represents perfect 

agreement.

Figure 16. Steady-state efficiency field and Standard 103 lab conditions

The test results show that there is still a statistically significant difference between the two 

means, but it is only on the order of 1.2% because replicating the field environment in the lab is 

difficult. 

4.2 Annual Fuel Utilization Efficiency and Annualized Efficiency

AFUE is the DOE efficiency rating point as measured under specific conditions in the lab. In this 

report, AE is used for the Standard 103 rating point. When tested according to Standard 103 
28

without change, Annualized Efficiency Standard Conditions Lab Measured (AE SC LM) is used.

When Standard 103 is modified to reflect field conditions, Annualized Efficiency Field 

Conditions Lab Measured (AE FC LM) is used. Table 25 provides the test results.

Table 25. AE Under Field and Standard 103 Conditions

Furnace AE Field Conditions

(AE FC LM), %

AE Standard 103 Conditions

(AE SC LM), %

1 80.47 81.04

2 82.7 82.9

3 70.6 73.0

4 79.1 81.3

5 80.5 81.1

6 81.5 82.5

7 92.7 94.9

9 91.3 91.8

10 71.7 69.6

Figure 17 shows the data in graphical form where the 45-degree line indicates perfect agreement. 

Figure 17. AE field and Standard 103 lab conditions

Using the same paired t-test for significance with this data set, the mean difference of 0.8% is not

significant when testing AE under field and AE under Standard 103 conditions in the lab. The 

difficulty associated with setting up field conditions in the lab, as mentioned in the steady-state

efficiency analysis, coupled with the four heat-up, cool-down, and condensate collection parts of 

the test masks the differences.
29

Table 26 shows the difference between the rated AFUE on the furnace and the lab-measured 

value under Standard 103 conditions (AFE SC LM). This test does not rely on replicating the 

field conditions in the lab. A comparison is made between these values, though the individual 

furnaces were not rated for AFUE when they were new and the test procedures are not identical.

Table 26. Rated AFUE and AE Under Standard 103 Conditions

Furnace Rated AFUE 

(%)

AE Standard Conditions 

(AE SC LM), %

1 80.5 81.0

2 80.1 82.9

3 72.5 73.0

4 80.5 81.3

5 80.0 81.1

6 80.0 82.5

7 94.4 94.9

9 90.0 91.8

10 68.4 69.6

Figure 18 shows the data in graphical form.

Figure 18. Rated AFUE compared to AE Standard 103 lab conditions

Using the same hypothesis testing, there is a significant difference between the rated AFUE and 

the lab-measured AE from Standard 103 for this data set, at 1.3%. There are several potential 

reasons for the slight difference, including differences in the test procedures, changes to the 

standards over time, sampling differences, bias in the test, or emissivity changes from oxidation 

of the heat exchangers. Standard 103 is revised every 4 years and the DOE standard also 
30

undergoes periodic revision, so it is possible that the DOE rating test procedure was significantly 

different than the 2007 Standard.

The hypothesis that there is a time dependence to furnace efficiency is tested in Section 4.3. 

4.3 Efficiency Change With Time

The efficiency measurements for the furnaces collected from the field provide a unique 

opportunity to evaluate changes to the efficiency of furnaces with time. 

Figure 19 plots the age of the furnace versus percent change per year of furnace efficiency as 

measured by the difference from rated AFUE and AE measured in the lab (AE SC LM), which is 

independent of field measurement.

Figure 19. Efficiency change versus age

A linear fit of the data did not result in a slope or intercept value that was statistically significant.

The implications are that there is no time dependency in this metric. It is also possible that there 

is some bias in the AE test compared to the rated DOE value, as mentioned earlier. At minimum, 

the authors contend that these data can be used to conclude that there is no time dependency to 

the AE measurement when using rated AFUE as a baseline; thus, the degradation predicted by 

Equation 1 is not supported in this research.

A final comparison was made between efficiency change, as calculated by rated AFUE-AE SC 

LM, versus age to determine if there was a relationship between the two. The graph is provided 

in Figure 20.
31

Figure 20. Rated AFUE versus efficiency change per year

This analysis shows that there is no relationship between rated AFUE and a linear efficiency 

change per year. 
32

5 Results and Discussion

The primary objective of this project was to determine how age and adverse installation 

conditions affect natural gas field performance as measured by AE. The results from this study 

are summarized below:

1. There is a significant difference between the steady-state efficiency calculated from field 

data and the steady-state efficiency measured in the lab under Standard 103 conditions. 

The difference in the mean is 6.4%. An analysis of the nine data points in Figure 16 using 

a paired student t-test and p < 0.05 for significance indicates that the two data sets are 

significantly different. The implication is that field performance is 6.4% lower than rated 

conditions for this sample because of poor installation practices or misadjustment. 

2. There is a significant difference between steady-state efficiency measured in the lab 

under field conditions and Standard 103 conditions, at 1.2%. Replicating the field 

environment proved to be difficult in this test. Because the furnaces were removed from 

their installation sites, additional field data could not be collected. 

3. There is no significant difference between AE under field conditions and AE under 

Standard 103 conditions measured in the lab. The difficulty associated with setting up 

field conditions in the lab, as mentioned above, coupled with the four heat-up, cooldown, and condensate collection parts of the test may mask the difference.

4. The mean lab-measured AE from the Standard 103 is 1.3% higher than rated AFUE, a 

statistically significant difference. This test does not rely on replicating field conditions in 

the lab. There are several potential reasons for this finding, including differences in the 

test procedures, changes to the standards over time, sampling differences, bias in the test, 

or emissivity changes from oxidation of the heat exchangers. Standard 103 is revised 

every 4 years and the DOE standard also undergoes periodic revision, so it is possible 

that the older equipment was measured under a procedure no longer included in the 

Standard 103.

5. Testing a linear fit of efficiency with time was not significant for either the slope or 

intercept values. The implications are that there is a nonzero increase in rated AFUE, but 

it is not time dependent, indicating that the change happens in the first 15 years of field 

use. It is also possible that there is some bias in the lab test compared to the DOE 

standard, as mentioned earlier. The authors conclude that the degradation predicted by 

Equation 1 is not supported in this research.

The research questions are addressed below:

Q1: What is the degradation in furnace efficiency for typical field installation compared to the 

rated performance?

A1: Furnace steady-state efficiency calculated from field data is 6.4% below that measured in the 

laboratory under Standard 103 conditions, a statistically significant difference.
33

Q2: How should efficiency values be modified in models (such as BEopt) to account for 

degradation, especially for retrofit situations?

A2: BEopt and other models do not need to adjust for differences between rating conditions and 

the field environment for furnaces that are installed properly. 

Q3: How do rated and measured efficiency compare for vintage furnaces? 

A3: Older furnaces show no degradation of performance over an average lifetime of 19 years.

From the data, time is not a significant variable in the regression, so there may be an initial 

increase or a slight bias caused by differences in test standards. 
34

6 Conclusions and Recommendations

The conclusions of this research are that the rated DOE AFUE is a good measure of the 

performance of natural gas furnaces in the field where the installation instructions are followed. 

Field conditions are difficult to replicate in the laboratory, so lab testing that replicates the field 

environment requires more study. Efficiency comparisons between the rated value and the 

Standard 103 test in the lab show no degradation after an average lifetime of 19 years, in fact a 

slight increase was measured. Using this comparison, the efficiency levels do not show a 

significant time dependency. 

PARR recommends that BEopt and other models continue to use the DOE-rated AFUE without 

adjustment for installation practices or degradation with time. This data set supports those 

recommendations. Field measurements of efficiency, static pressure, and airflow (using a flow 

plate) were inconsistent in this study when compared to laboratory measurements. It is 

recommended that field measurements be used only as a general indicator of performance when 

tuning a furnace or adjusting airflow, not as an absolute indication of performance for modeling 

or energy savings analysis. 

PARR conducted this investigation to determine which deficiencies in the field, if any, hold the 

largest energy saving potential. Setting the furnace on rate, and tuning the fan to the 

manufacturer’s recommended rise range (temperature rise across the furnace) are the two most 

significant factors in achieving good performance, as recommended by the manufacturer. If the 

fan speed cannot be adjusted properly, changes to the distribution system need to be made. A

qualified installer should follow the manufacturer’s installation instructions regarding these and 

other settings to get the best performance. 

Future natural gas furnace testing is recommended to add data to the cases where a relationship 

could not be determined in this project: replicating field conditions in the lab and establishing 

efficiency change with time. For example, a test where furnaces are rated, installed in the field 

for normal operation, and then re-rated by the rating agency using the same DOE test procedure 

to determine if there is a significant degradation in the rating point. It may also be valuable to 

determine if a poorly installed furnace would operate significantly below the rating point. 

Examples would be furnaces working outside the rise range (very high or very low), and 

furnaces cycling off the high limit controller for a significant part of the time. Finally, simulation 

models have improved significantly since rated AFUE was first used as an indicator of 

performance. Research is recommended to identify several performance metrics that can be used 

by simulations to improve the agreement between calculated and actual field performance. 
35

References

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Furnaces and Boilers.” ANSI/ASHRAE Standard 103-1982, Approved by the ASHRAE Board 

of Directors on October 1, 1982.

ASHRAE (2007). Standard “Method of Testing for Annual Fuel Utilization Efficiency of 

Residential Central Furnaces and Boilers.” ANSI/ASHRAE Standard 103-2007, Approved by 

the ASHRAE Board of Directors on June 27, 2007.

Brand, L.; Rose, W. (2012). Measure Guideline: High Efficiency Natural Gas Furnaces. Gas 

Technology Institute, Des Plaines IL, DOE/GO-102012-3684.

Brand, L. (2012). Expert Meeting Report: Achieving the Best Installed Performance from HighEfficiency Residential Gas Furnaces. Gas Technology Institute, Des Plaines IL, 2012 

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CO: National Renewable Energy Laboratory. NREL/TP-550-49426. 

Polly, B.; Kruis, N.; Roberts, D. (2011). Assessing and Improving the Accuracy of Energy 

Analysis for Residential Buildings. Golden, CO: National Renewable Energy Laboratory.

NREL/TP-5500-50865.

Yee, S.; Baker, L.; Brand, L.; Wells, J. (2013). “Energy Savings from System Efficiency 

Improvements in Iowa’s HVAC SAVE Program.” Chicago, IL: Midwest Energy Efficiency 

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DOE/GO-102015-4624 ▪ February 2015