Measure Guideline: 

Combustion Safety for Natural 

Draft Appliances Through 

Appliance Zone Isolation

J. Fitzgerald and D. Bohac 

NorthernSTAR

April 2014
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iii

Measure Guideline: Combustion Safety for Natural Draft 

Appliances Through Appliance Zone Isolation

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: 

J. Fitzgerald and D. Bohac 

Center for Energy and Environment 

212 Third Avenue North, Suite 560 

Minneapolis, MN 55401 

NREL Technical Monitor: Stacey Rothgeb 

Prepared under Subcontract No. KNDJ-0-40338-03

April 2014 
iv

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v 

Contents

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

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

Definitions ..............................................................................................................................viii

Acknowledgments .................................................................................................................... 1

Abstract ..................................................................................................................................... 2

Foreword ................................................................................................................................... 2

Progression Summary.............................................................................................................. 3

1 1 Introduction ...................................................................................................................... 3

2 Screening Protocol ............................................................................................................. 5

2.1 Combustion Appliance Assessment .....................................................................................6

2.1.1 Identify Natural Draft and/or Category I Combustion Appliances in Isolatable 

Spaces ......................................................................................................................6

2.1.2 Check Entry Conditions and Combustion................................................................7

2.2 Isolated Appliance Zone Work Scope .................................................................................8

2.2.1 Assess and Specify Work To Isolate Appliances From Living Space.....................8

2.2.2 Assess and Specify Work for Proper Combustion Gas Venting..............................9

2.2.3 Determine Outside Combustion Air Requirement ...................................................9

2.3 Job Cost Estimation .............................................................................................................9

3 Isolation Work Protocol ....................................................................................................10

3.1 Creating the Isolated Zone .................................................................................................11

3.1.1 Venting System ......................................................................................................11

3.1.2 Air Sealing the Enclosure ......................................................................................12

3.1.3 Duct and Cabinet Sealing.......................................................................................12

3.1.4 Combustion Air......................................................................................................12

3.1.5 Makeup Air and Exhaust Fans ...............................................................................15

3.2 Field Confirmation .............................................................................................................15

3.2.1 Visual Inspection of Air Handler ...........................................................................15

3.2.2 Smoke Test of Duct and Cabinet Leakage .............................................................15

3.2.3 Isolation Pressure Test ...........................................................................................16

4 Isolation Examples............................................................................................................17

4.1 Furnace in Attic..................................................................................................................17

4.1.1 Furnace in Attic Open to Garage ...........................................................................18

4.2 Combustion Appliances in Attached Garages ...................................................................19

4.2.1 Separation Wall From Garage ...............................................................................20

4.3 Combustion Appliances in Mechanical Closets ................................................................20

4.3.1 Air Handlers on Enclosed Support Platform .........................................................20

4.4 Combustion Appliances in a Closet or Separate Room With a Door That Opens to a 

Bedroom or Bathroom .......................................................................................................22

4.5 Building Cavities Used as Returns, Opening Into Combustion Appliance Space .............22

5 Materials ............................................................................................................................24

5.1 Rigid Air-Impermeable Panel Materials ............................................................................24

5.2 Sealants ..............................................................................................................................24

5.3 Materials for Ductwork and Duct Sealing .........................................................................24

5.4 Materials for Filling Cavities .............................................................................................25
vi 

5.5 Additional Resources .........................................................................................................25

References............................................................................................................................................... 26

Appendix .................................................................................................................................................. 27

Additional Resources ........................................................................................................................... 33
vii 

List of Figures

Figure 1. Two openings method ............................................................................................13

Figure 2. Two openings method with vertical duct...............................................................13

Figure 3. Two openings method with horizontal ducts ........................................................14

Figure 4. One opening method...............................................................................................14

Figure 5. Attic bypasses.........................................................................................................17

Figure 6. Category 1 fan assisted attic furnace with no gaskets or sealing will pull air 

from mechanical room until properly sealed. ................................................................. 18

Figure 7. New return added to attic furnace: joints sealed with mastic and cabinet seams 

taped ..................................................................................................................................18

Figure 8. Metal elbow for new return to inlet side of attic fan compartment ......................19

Figure 9. Not acceptable: As-found view inside platform return: debris, moisture, wall 

cavities potentially open to combustion vent, burner, and attic above ........................21

Figure 10. Not acceptable: Restricted platform return in newer home ...............................21

Figure 11. Acceptable: View after opening inlet to fan cabinet ...........................................21

Unless otherwise noted all figures were created by NorthernSTAR. 

List of Tables

Table 1. Screening Protocol Job Breakdown.......................................................................... 5

Table 2. Isolation Job Breakdown..........................................................................................10

Table 3. Carbon Monoxide Thresholds (Table A.4.1 of NFGC Annex)................................. 32

Unless otherwise noted all tables were created by NorthernSTAR. 
viii

Definitions

ANSI American National Standards Institute

ASHRAE American Society of Heating, Refrigerating and Air-Conditioning 

Engineers 

ASTM American Society for Testing and Materials

BA Building America

Btu British thermal unit

CO Carbon monoxide

ICC International Codes Council, Inc.

ICC ES International Codes Council Evaluation Service

IFGC International Fuel Gas Code

IRC International Residential Code

NFGC National Fuel Gas Code

NFPA National Fire Protection Association

NREL National Renewable Energy Laboratory

UL Underwriters Laboratories
1 

Acknowledgments

This Guideline is the product of a collaborative effort. The authors acknowledge the funding and 

support of the U.S. Department of Energy’s Building America Program. Special thanks to Iain 

Walker, Lawrence Berkeley National Laboratory; Dan Cautley, Wisconsin Energy Conservation 

Corporation; John Tooley, Advanced Energy; Tom Andrews, INCAP; Gary Nelson, Energy 

Conservatory; Jim Cummings and Neil Moyer, Florida Solar Energy Center; Rick Chitwood; 

John Proctor; Joe Lstiburek, Building Science Corporation; Larry Brand, Gas Technology 

Institute; Paul Cabot, America Gas Association; Anthony Cox, New River Center for Energy 

Research and Training; Paul Francisco, Illinois Sustainable Technology Center; and Ted 

Williams, American Gas Association. The authors also wish to thank Rick Chitwood and 

Anthony Cox for giving us permission to use their photos in this Guideline. 
2 

Abstract

This Measure Guideline covers how to assess and carry out the isolation of natural draft 

combustion appliances from the conditioned spaces of low-rise residential buildings. It deals 

with combustion appliances located either within the living space in enclosed closets or side 

rooms or outside the living space in an adjacent area like an attic or garage. 

This subset of houses does not require comprehensive combustion safety tests, and simplified 

prescriptive procedures can be used to address safety concerns. This allows residential 

contractors inexperienced in advanced combustion safety testing to effectively address 

combustion safety issues and allow retrofits, including air sealing and changes to distribution and 

ventilation systems, to proceed. 

Foreword

It is often challenging for residential contractors to conduct appropriate combustion safety test 

procedures in existing homes and use effective actions to address failures. However, there are 

situations where comprehensive combustion safety tests are not required and prescriptive 

procedures can be used to address safety concerns. A method to easily identify those situations 

and conduct the required actions will reduce test costs and provide a higher level of confidence 

that contractors are adequately addressing combustion safety issues. For this reason, this 

Measure Guideline has been created for a subset of these types of houses—houses that have or 

can have an isolated combustion appliance zone. This situation allows for combustion appliances 

with draft diverters to be segregated from the living space. 

This guide uses existing code requirements for Category I appliances installed in isolated spaces. 

Those requirements may not always be sufficient and have been augmented with performance 

tests and additional details to ensure safe operation for houses that are undergoing 

weatherization. The purpose of this guide is to achieve similar protection against 

depressurization as is provided by direct-vent appliances installed in the living space. 

Consequently, successful application of this guide will allow houses to be tightened and 

ventilation added without having to be concerned with the effect on the combustion appliances. 
3 

Progression Summary

ss 

1 

OUTDOOR AIR FOR 

COMBUSTION

Residential Appliance Zone Isolation. The purpose of this Guideline is to assess and 

carry out the isolation of natural draft combustion appliances from the conditioned space of 

low-rise residential buildings where outdoor combustion air is being used.

APPLIANCE 

ASSESSMENT

Combustion Appliance Assessment. Determine that the Category I 

appliances and draft hood-equipped water heaters are located in a zone that 

can be isolated and that they are operating safely.

1

WORK SCOPE

Isolated Appliance Zone Work Scope. Define the air-sealing work needed to 

isolate the zone where the appliances are located. Determine work scope for 

proper venting of combustion gases and providing outside combustion air

2

JOB COST 

ESTIMATE

Estimate Total Job Cost. Compare job cost to other combustion safety 

3 approaches such as replacement with power or direct vent appliances.

ISOLATE ZONE

Create an isolated space for the appliance(s) that is separated from the 

living space. This includes air sealing, ductwork sealing, adding combustion 

and makeup air openings, and inspecting venting.

4

FIELD 

CONFIRMATION

Field Confirmation. This includes performing an isolation pressure test and if 

applicable, inspecting the air handler and performing a smoke test for duct and 

cabinet leakage.

5

Do Not Proceed if:

• Combustion appliances are not operating safely.

• Appliance zone can not be isolated.

• The cost of the isolation exceeds any suitable alternative 

approaches.

! !
4 

1 Introduction

This Measure Guideline covers how to assess and 

carry out the isolation of natural draft combustion 

appliances from the conditioned space of low-rise 

residential buildings. It deals with combustion 

appliances located either within the living space in 

enclosed closets or side rooms or outside the living 

space in an adjacent area like an attic or garage. This 

Measure Guideline concentrates on these house 

characteristics in order to provide general contractors 

a process to proceed with work on existing homes 

with legacy combustion appliances without the need 

for extensive diagnostics or performance-based 

protocols. 

The purpose of this Measure Guideline is to define a 

subset of houses where the combustion appliances 

are or can be effectively isolated from the living 

space, allowing retrofits, including air sealing and 

changes to distribution and ventilation systems, to 

proceed. 

According to the National Fuel Gas Code (NFGC) 

(Section 3.3.6.11), a Category I Vented Appliance is “an appliance that operates with a nonpositive vent static pressure and with a gas temperature that avoids excessive condensate 

production in the vent.” 

The isolated zone approach deals with a subset of houses in which combustion safety issues do 

not require extensive diagnostics or performance-based protocols. Other conditions that allow 

retrofit work to proceed without extensive combustion safety diagnostics or performance-based 

protocols include:

1. No vented combustion appliances are present. 

2. All combustion appliances are depressurization resistant.

3. Extremely leaky houses with little or no potential for depressurization. 

4. Tighter house with high depressurization potential so that only option is replacement of 

depressurization susceptible appliances.

5. All vented combustion appliances are located in a space isolated from living space.

This Guideline:

1. Provides a guide to the conditions 

that can allow natural draft 

appliance locations to be isolated 

from house pressures.

2. Clarifies requirements for the 

separation of Category I natural 

draft combustion appliances or 

draft-hood equipped water 

heaters from living spaces so that 

all flue gases go outdoors and all 

air for combustion and dilution 

comes from outdoors.

3. Defines a screening protocol to 

assess whether the assessment 

and protocols are applicable to 

the home.

4. Describes the work required to 

achieve an isolated appliance 

zone. 

5. Describes some specific cases 

 
5 

2 Screening Protocol 

The goals of the screening protocol are to determine whether the isolation approach is: 

1. Applicable for the types of combustion appliances that are being used in the house. 

2. Compatible within the elements, structure, and conditions of the building.

3. Cost effective compared to alternative methods; for example, replacement with power-vented or 

direct vent appliances.

The screening will determine if the isolation approach is applicable to the home. Table 1 shows 

the job breakdown for the screening protocol.

Table 1. Screening Protocol Job Breakdown

Important Steps Key Points Reasons

1. Combustion 

Appliance 

Assessment

a. Natural draft and/or fan-assisted 

Category I appliances are located in a 

space that can be isolated.

b. Combustion appliances do not pose 

an immediate hazard.

a. Determine if Measure Guideline 

can be applied.

b. Safe working and living 

conditions exist. 

2. Isolated Zone 

Work Scope

c. Define air sealing work to isolate 

appliance from living space.

d. Define venting work needed to vent 

combustion gases to the outside.

e. Define work required to provide 

outside combustion air to the zone.

c. Determine need for air sealing 

and work required.

d. Determine need for mechanical 

contractor and work required.

e. Determine need for mechanical 

contractor and work required.

3. Estimate Cost of 

Total Job

f. Compare cost to other combustion 

safety approaches such as 

replacement with power or direct 

vent appliances.

f. Adopt the most cost-effective 

combustion safety strategy.
6 

2.1 Combustion Appliance Assessment

The initial combustion appliance assessment needs to determine that:

1. The natural draft and fan-assisted Category I appliances are located in areas that can be isolated.

2. These combustion appliances are 

operating safely. 

3. The work needed to isolate those appliances is feasible. 

4. The estimated cost of the work justifies the deployment of this measure with 

respect to other combustion safety measures such as replacement with power or direct vented 

appliances.

The findings from these steps will determine whether isolation work will continue. 

2.1.1 Identify Natural Draft and/or Category I Combustion Appliances in 

Isolatable Spaces

Conduct a house walkthrough to identify any natural draft and/or Category I combustion 

appliances that are located in spaces that can be isolated. The primary concern is furnaces, 

boilers, and water heaters, with special interest in those having venting systems that can allow 

combustion gases to spill through draft hoods. 

The second requirement for this measure is that 

these appliances must be located in spaces that 

are capable of being isolated. This means that a 

pressure boundary separating the location of the 

combustion appliances from the living space can 

be defined and that this boundary can be made 

airtight with respect to the living space. Likely 

locations for these appliances are:

• Mechanical closets open to the outdoors

• A separate room open to the outdoors 

• The garage

• An attic.

Based on the walkthrough, this Measure Guideline should NOT be used 

if no natural draft or Category I combustion appliances are found in the 

house or if the natural draft or Category I combustion appliances are 

located in areas that are not airtight and not amenable to air sealing.

! !

If not installed properly, combustion 

appliances represent a potential source 

of hazardous pollutants that should be 

kept out of the living space and away 

from its occupants. Specifically, this 

Measure Guideline is concerned with 

natural draft and Category I combustion 

appliances (water heaters, furnaces, 

and boilers) that can spill combustion 

gases back into the living space.
7 

Combustion appliance testing should

be performed under two conditions: 

with the equipment off and then with 

the equipment running. Portions of 

the combustion equipment check 

require a qualified technician. The 

entire test requires skilled staff with 

access to and knowledge of specific 

code requirements and test 

equipment. Below are the tests that 

should be performed during the two 

conditions. These tests are to be 

performed with the appliance zone 

open to the house and in standby 

conditions (e.g. all exhaust fans and 

air handlers off).

Many of the tests and 

inspection items can be 

performed by a home 

performance contractor. 

Others will require a qualified 

mechanical technician: 

1. Compare chimney vent sizing 

and installation to code 

requirements. 

2. Inspect appliance equipment 

inspection. 

3. Confirm proper operation of 

appliance safety and protective 

devices.

2.1.2 Check Entry Conditions and Combustion

Appliances, as Found 

Make sure that the screening is taking place under safe 

entry working conditions and that appliance operation 

does not pose an immediate hazard to the tester and 

occupants. Confirm that the indoor air has safe levels of 

fuel gas and carbon monoxide (CO) before conducting the 

inspection. Remedy any immediate hazards before 

continuing with this Measure Guideline. 

2.1.1.1 On Entry and With Equipment Off

The following assessment should be performed with the 

combustion appliance equipment off (see the Appendix 

for procedures):

• House CO level 

• Gas odor 

• Venting system sizing and installation (compliant with 

manufacturers’ instructions and code)

• Check installed combustion air (International Fuel Gas 

Code [IFGC] and National Fire Protection Association 

[NFPA] outdoor combustion air requirements, net free 

area of louvers and grilles)

• Appliance equipment inspection that includes safety 

systems, inside-wiring, burners, heat exchanger, etc.

2.1.1.2 Combustion Appliance Tests With 

Equipment On 

The following assessments should be performed with the 

combustion appliance equipment running: 

• Check house CO level during operation. 

• Check ignition and controls function. 

• Conduct a spill/draft check.

• Check input for over-firing: clock meter or measure the size of the orifice and pressure.

• Oil smoke check and set draft at barometric damper.

• Check flue CO (manufacturer recommendations, American National Standards Institute [ANSI] 

standards) % carbon dioxide, and % oxygen with a gas analyzer.
8 

• Verify that there is no flame distortion when the fan is running (furnaces only). 

• Measure temperature rise to determine if it is within the manufacturer’s recommendation. 

• Check safety and protective devices: pilot shut off, high temperature limit, low water cutout, and 

temperature pressure relief.

A worst-case appliance zone depressurization test is not applicable. Do not open the isolated 

appliance zone to the house to perform a worst-case test.

2.2 Isolated Appliance Zone Work Scope

The next step of the screening process is to determine 

the scope for the isolation work that results in:

1. A code-compliant vent system

2. Adequate combustion air 

3. No combustion gases entering the living space 

through the wall, ceiling, and/or floor of the zone or 

via the air handler.

2.2.1 Assess and Specify Work To Isolate 

Appliances From Living Space

The first step is to confirm that there is a continuous, 

durable boundary separating the appliances from the 

house. If the boundary is not complete, the work scope 

must include work necessary to complete the boundary. 

The next step is to specify air sealing work such that:

1. The combustion appliances are isolated from the 

living space. Adequate isolation is achieved when 

the pressure difference between the appliance zone 

and outside is less than 5 Pa for a blower door 

induced house depressurization of 50 Pa or 1/10 of 

the house depressurization for houses that can’t 

reach 50 Pa. If isolation is not achieved, include 

isolation work as per Section 3.1.2. 

2. Ductwork and cabinet within the isolation 

zone are completely sealed so that while 

operating they will not create a negative 

pressure in the space with the burner and vent 

openings, and during off cycle they will not 

allow house pressures to interfere with other 

combustion appliances in the isolated space or 

reverse furnace startup firing before the fanon temperature is reached. Adequate sealing is 

achieved when the exterior duct leakage is 

less than 6% of the nominal system flow rate 

The screening protocol may uncover 

current hazards that must be 

remedied before screening 

continues. However, even with no 

immediate hazard, alterations to meet 

standards in appliance operation, 

chimney and vent capacity, and 

combustion air are still needed for 

long-term safe use.

When the air handler fan is 

on, return leaks can capture 

combustion air and draw in 

products of combustion.

Depressurization 

Test Protocol

1. Measure and record the pressure in 

the space where the combustion 

appliance is located with respect to 

outside.

2. Turn on any central forced air 

system blowers and measure and 

record the pressure in the space 

where the appliance is located with 

respect to outside. 

3. The depressurization is equal to the 

pressure with the fans on minus the 

pressure with the fans off.
9 

and a smoke test shows that there is no return duct leakage (see Section 3.2.2). Measure the 

amount of duct leakage to the exterior in accordance with ASTM E1554-2007 C. The total duct 

leakage test can be used (method D) as long as the total leakage is less than 6% of the system 

flow rate. If adequate sealing is not achieved, include work as per Section 3.1.3. 

2.2.2 Assess and Specify Work for Proper Combustion Gas Venting

The vent system inspection shall identify any work required to ensure compliance with the 

NFGC (NFPA 54, 31, and 211) or local code requirements. This inspection should be performed 

by a qualified mechanical technician. A qualified mechanical technician should perform the 

venting work and ensure that it complies with code requirements. 

2.2.3 Determine Outside Combustion Air Requirement

The work specified by the auditor or mechanical technician must allow for outside combustion 

air as required by the NFGC (NFPA 54) or local code requirements. It is recommended that 

practitioners either be provided guidance to implement minimum requirements or carry a 

recognized credential such as the International Code Council (ICC) Fuel Gas Inspector 

certificate. 

2.3 Job Cost Estimation

The final step of the screening process is to determine the least cost option. For instance, when 

the appliances can either be isolated or replaced with power/direct vent appliances, select the 

least expensive option. 
10 

3 Isolation Work Protocol

Once it has been established that natural draft and/or Category I appliances will continue to be 

operated in the house and that an isolated zone can be created, the next course of action is to 

successfully perform the work necessary to create an isolated zone or to repair the one that exists 

if the pressure testing in Section 2 fail. This work is defined as: 

• The combustion appliance equipment is documented to be installed and is operating within 

manufacturer’s recommendations and according to code. 

• The venting system meets code.

• All combustion air is supplied from the outside. 

• The appliance space is partitioned off as a separate room or enclosed space from the living space, 

per the 5 Pa at 50 Pa rule. 

• Ductwork, including air handler cabinets, in the isolated combustion zone is sealed so that the 

exterior duct leakage is less than 6% of the system flow and the smoke test indicates no return 

leakage. 

• No return air is drawn from a garage, boiler room, furnace room, unconditioned attic, or areas 

with flammable vapors. 

This section details the necessary steps for the physical creation of the isolated zone and the 

commissioning process to be used to ensure that the system is working properly. Table 2 shows 

the job breakdown for the isolation protocol. 

Table 2. Isolation Job Breakdown

Important Steps Key Points Reasons

1. Create an isolated 

space that is 

separated from the 

living space

a. Inspect venting system.

b. Air seal the enclosure from 

the house.

c. Seal ducts and cabinets.

d. Provide combustion air from 

outside to zone.

e. Provide makeup air for 

clothes dryer and remove all 

other exhaust fans.

a. Identify any work required to 

ensure that venting system is 

within manufacturers’ spec and 

codes.

b. Minimize impact of living space 

depressurization on combustion 

gas venting.

c. Limit the effect that the ductwork 

leakage and air handler have on the 

zone pressure and the movement 

of zone air into the house.

d. Comply with code for outside 

combustion air.

e. Minimize opportunity for 

depressurization in the zone. 
11 

Important Steps Key Points Reasons

2. Field Confirmation

f. Visually inspect air handler

and measure exterior duct 

leakage. 

g. Perform smoke test of duct 

and cabinet leakage. 

h. Perform isolation pressure 

test.

i. Visually inspect venting, 

combustion air, and makeup 

air systems. 

f. Ensure that sealing work on air 

handler has been done.

g. Confirm completion and/or 

compliance of duct and cabinet 

sealing work. 

h. Confirm that zone is air sealed to 

within sufficient isolation. 

i. Confirm that venting and 

combustion and makeup air comply 

with code and/or manufacturer’s 

instructions.

3.1 Creating the Isolated Zone

There are five steps to creating an isolated zone:

1. Ensure that the venting system complies with manufacturer’s specifications and local codes. 

2. Air seal cracks, gaps, or openings in the physical boundary to ensure that it is airtight to the 

living space.

3. Seal all ducts and cabinet leakage located in the zone to ensure that ducts and cabinet meet 

the leakage requirements. 

4. Provide code required outside combustion air openings. 

5. Remove all exhaust devices located in the area that can depressurize the zone or provide 

makeup air if there is a clothes dryer in the isolated zone.

3.1.1 Venting System

The chimney venting system should be installed to manufacturer’s specifications and in 

accordance with specifications detailed in NFPA 54 chapter 12. The chimney vent sizing should 

meet the capacity requirements of the vent tables published in NFPA 54 chapter 13, NFPA 31, 

NFPA 211, and IFGC chapter 5. This is true for both the common vent and vent connector(s) 

after all requirements are evaluated including number of elbows, length of lateral runs, vertical 

height, and actual input/firing rate. 
12 

3.1.2 Air Sealing the Enclosure

Once the structural enclosure has been completed, the boundary needs to be made airtight. A 

blower door and smoke pencil should be employed to find any air leaks. Seal air leakage with 

caulk, gaskets, weather-stripping or other closure methods and materials to limit flow. Any 

louvers between the zone and conditioned space shall be removed and the opening covered with 

sheet metal or gypsum board. Fill in large openings in the wall, ceiling, or floor with 1⁄2-in. 

gypsum board or its equivalent. Seal all joints, seams, and penetrations between the zone and 

living space with joint tape, sealant foam listed for use as a fire stop and approved for uncovered 

use, or sealant caulk (see International Residential Code [IRC] 314.6). 

Weather-strip the door to the inside of the house (if any) and provide a door sweep and make 

sure the door is self-closing and self-latching. Any louvers in doors to the inside of the house 

shall be removed and filled with metal or gypsum board. 

3.1.3 Duct and Cabinet Sealing 

Seal all joints, penetrations and openings in the cabinets of the air handling unit. Use metal tape 

or suitable gaskets on service openings and permanent seals on fixed joints and seams. All air

handling units should be mechanically attached to other air distribution system components. 

Provide access as needed to seal all return and fan cabinet leaks in the space. Provide continuous 

sealed ductwork from the living space directly into the blower housing for the forced air furnace, 

or other air handler. Maintain at least the cross sectional area of the return inlet and size the liner 

or duct to meet the manufacturers’ required airflow without restriction. 

Furnace returns without manufacturers’ information must provide at least 2 in.2

 of total crosssectional area for every 1000 Btu of output (IMC-2012 918.2). 

Mechanically fasten the duct to the furnace cabinet and at the connection to the interior. Tape all 

cabinet service openings and joints. Seal all joints and seams with mastic and mesh tape meeting 

Underwriters Laboratories (UL)181-M, or foil tapes meeting UL 181 A-P and UL 181 B-FX. 

Sealant foam approved for use as a fire stop in combustible construction is acceptable at the 

connection to the interior construction materials. 

No return air can be taken from a garage, boiler room, furnace room, crawlspace, or 

unconditioned attic. Line any enclosed support platform or building cavity used as a return air 

duct with continuous, durable, air impermeable material and seals that show a flame spread of 25 

or less and smoke developed of 50 or less when tested to ASTM E 84 or UL 723 or use a 

standard material listed for use in duct systems, i.e., sheet metal, duct board, flexible duct. 

3.1.4 Combustion Air

Combustion air supplied to the appliances shall meet the requirements set forth by the NFGC or 

applicable local codes. Do not use house indoor air for combustion air when following this 

Measure Guideline. (A separate Measure Guideline for houses using indoor air for combustion is 

available.) All combustion air must be supplied from the outside or from spaces that freely 

communicate with the outside such as a vented attic (minimum vent to attic area = 1:300) or 

vented crawlspace (1:150 minimum) with house to attic bypasses and attic duct leakage sealed. 
13 

The permanent air openings to the outdoors cannot be smaller than 3 in. diameter. Two methods 

based on fuel gas code (IFGC and NFPA 54) can be employed: 

1. A two openings method with one within 12 in. of the top and the other within 12 in. of the bottom 

of the zone. If the opening is directly to the outside or connected through vertical ducts, the 

minimum net free area of each opening should be 1 in.2

/4,000 Btu/h of the total input of all the 

appliances in the zone. If the openings are connected to the outside through horizontal ducts, the 

minimum net free area of each opening should be at least 1 in.2

/ 2,000 Btu/h of the total input of 

all the appliances in the zone.

Figure 1. Two openings method

Figure 2. Two openings method with vertical duct
14 

Figure 3. Two openings method with horizontal ducts

2. A one opening method with the opening within 12 in. of the top of the zone. The minimum net 

free area of the opening should be 1 in.2

/3,000 Btu/h of the total input of all the appliances and 

not smaller than the sum of all the vent connectors in the space. 

Figure 4. One opening method

If a mechanical damper is installed in the combustion air opening, interlock this with the furnace 

or boiler but provide a passive combustion air opening in the same space for a natural draft water 

heater, if present, because there is no electrical control circuit on residential water heaters. 
15 

Consider the presence of louvers, screens, or grilles over the openings when calculating the free 

area of the opening(s). Use manufacturers’ labeled free air where provided. If unlabeled, assume 

free air is 75% of the metal louver area and 25% of the wood louver area. Refer to NFGC 2012 

section 9.3.7.1, IFGC, NFPA 54, and NFPA 31. 

3.1.5 Makeup Air and Exhaust Fans 

Makeup air is required for clothes dryers located in the zone. Each clothes dryer should be 

provided with 100 in.2 of makeup air. If closure is required during the off cycle of the dryer, a 

mechanical damper interlocked with the dryer should be provided. The interlock should be wired 

to prevent dryer operation if the damper fails. An interlock with the furnace, boiler, or water 

heater is not needed if dedicated combustion air is provided with those appliances. Refer to IFGC 

614.5, IRC-2012 G2439.4 and IMC 504.5. 

All other exhaust fans should be removed from these isolated combustion appliance zones. 

If the appliances are installed in an attic, powered attic fans in an attic space should either be 

disconnected or additional air inlets should be provided as per the requirements of the fan 

manufacturer.

3.2 Field Confirmation

Field confirmation of the isolated zone ensures that the system is operating as designed and 

meets code requirements. This section is adapted from California Energy Commission 2008, 

Reference Appendixes for the 2008 Building Efficiency Standards for Residential and NonResidential Buildings RA3.1.4.3.7-8 2010, p. 249. 

3.2.1 Visual Inspection of Air Handler

Visually inspect to verify that the following locations have been sealed. These locations include: 

1. Seams, joints, and openings at the fan cabinet, furnace cabinet, the cabinet section with the air 

conditioner coil, and connections to return and supply plenums 

2. Refrigerant line and other penetrations into the forced air unit 

3. Tape or adjust approved gasket at air handler door and other service panels (no permanent 

sealants)

4. Plenum and duct seams 

5. Return plenum joints at filter grille and to the back side of interior finish materials

6. Gap between any duct and duct chase where it passes out of the combustion space. 

3.2.2 Smoke Test of Duct and Cabinet Leakage

Perform a smoke test to confirm completion or compliance of duct and cabinet sealing work:

1. With the furnace fan and service disconnect off, set up for fan pressurization of the duct system 

for the furnace or air handler located in the combustion space. Close registers to limit smoke 

entry to the interior. Seal the grilles and registers necessary to reach +25 Pa inside the ducts in the 

specific combustion space.
16 

2. With the duct pressurization fan running, inject theatrical fog or equivalent non-toxic smoke into 

the fan inlet while observing smoke coming out of the ducts and cabinet. Mark the spots and stop 

adding smoke. 

3. Seal all joints, penetrations, and seams where smoke came out, use foil tape or approved gasket 

on service openings in the cabinet and permanent seals on all other openings. 

4. Inject additional smoke and inspect the seals. If smoke continues to come out of the return 

plenum or cabinet, turn off the pressurization fan and open the air handler fan cabinet to access 

and seal the remaining return leaks in the combustion space.

5. Ensure that there are no return leaks. Return air from outside the combustion space must be 

brought to the furnace fan through “continuous airtight ducts.” 

6. The furnace duct section is sealed when smoke no longer comes out of any openings in the 

combustion space. 

Note any smoke coming out of the burner area, heat exchanger, or vent connector for follow-up 

service. 

3.2.3 Isolation Pressure Test 

The following conditions must be met, as applicable: 

1. Appliance zone pressure change to outside is not more than 5 Pa. To check, perform a blower 

door test placing the house at 50 Pa house depressurization. 

2. Appliance zone pressure change to outside with dryer and makeup air is not greater than 2.5 Pa 

during dryer operation. Note: 

a. If the mechanical damper does not open, the dryer operation is prevented. 

b. For dryer makeup air, the airflow through 100 in.2

 net opening is approximately 165 cfm 

at –2.5 Pa, 200 cfm at –3 Pa depressurization, with a metal louver reduction factor of 

0.55. 

3. Garage pressure change to outside is not greater than 2.5 Pa during garage exhaust fan operation 

if the appliances are located in the garage.

Windy conditions will cause greater uncertainty in the pressure measurements. The uncertainty 

can be reduced by averaging the pressures over longer periods. When the outdoor reference tube 

is placed outside the house, locate the end of the tube at the intersection of an exterior wall with 

grade on the leeward side of the house. 
17 

4 Isolation Examples

In this section we describe the application of this guide to six specific situations. 

4.1 Furnace in Attic

The attic must be vented with the net vent area meeting combustion air requirements. Connect 

combustion air openings directly to the outside and select louvers to maintain free air. The 

minimum recommended proportion of attic vents to attic area is 1:300 ratio, with 3.3 ft2 of 

venting per 1,000 ft2 of attic area. Seal bypasses and duct leakage to the outside before adding 

any additional attic vents. 

Photo Rick Chitwood 2010 Photo Anthony Cox 2008

Figure 5. Attic bypasses

Attic bypasses should be sealed wherever practical. The minimum bypass sealing should result in 

an attic pressure less than 5 Pa at 50 Pa house depressurization with a blower door. If after 

bypass sealing, the attic pressure is greater than 5 Pa, perform additional sealing or increase the 

amount of attic vents to 1:150 ratio or the amount needed to reduce the pressure to 5 Pa. Refer to 

IRC, NFPA 54, and IFGC for details. 

As described in Section 3, continuous sealed ductwork must be provided from the living space 

directly into the blower housing for the forced air furnace, or other air handler with a return open 

in the combustion space. 
18 

Photo Rick Chitwood 2010

Figure 6. Category 1 fan assisted attic furnace with no gaskets or sealing will pull air from 

mechanical room until properly sealed.

Photo Rick Chitwood 2010

Figure 7. New return added to attic furnace: joints sealed with mastic and

cabinet seams taped

4.1.1 Furnace in Attic Open to Garage

Use sheet metal ductwork to connect the return to the interior, as only sheet metal ductwork can 

be exposed to the garage. Alternatively, provide taped 1⁄2-in. gypsum board on the garage side of 

the wall that is open to the attic with all penetrations sealed.
19 

Photo Rick Chitwood 2010

Figure 8. Metal elbow for new return to inlet side of attic fan compartment

4.2 Combustion Appliances in Attached Garages

In the attached garage, add combustion air openings that comply with the combustion appliance 

manufacturers’ recommendation and according to code, as defined in Section 2. Any garage 

exhaust fans must have the appropriate amount of makeup air required. Garage depressurization 

with respect to the outdoors caused by exhaust fans should not exceed 2.5 Pa. 

Seal openings in the garage walls and ceiling adjacent to the living space with a material 

approved to resist the free passage of flames and products of combustion. Fill in large openings 

to the living space with 1⁄2-in. gypsum board on the garage side of wall to the living space, garage 

side of wall to attic, and any walls or columns supporting a living space above the garage. As 

necessary, reduce cavity leakage by installing dense pack insulation using approved materials. 

Fill in openings in the ceiling to living space with 5⁄8-in. Type X gypsum board. Weather-strip 

garage door to house and seal trim. Confirm that the garage door is solid, has a 20-minute fire 

rating (if new), is equipped with a door closer, and has a self latching mechanism. Tape all joints, 

seal gypsum gap to foundation at sill, and seal penetrations to the house with sealant or foam 

listed for use as a fire stop in wood frame construction. For details refer to IBC 406.4, IRCR302.5.2, and IRC-R302.11. 

Use sheet metal ductwork to connect the return to the interior, as only sheet metal ductwork can 

be exposed to the garage. Seal any exposed sheet metal ducts in the garage with mastic and mesh 

tape and seal any penetrations through walls or ceilings. Enclose nonmetal ducts with taped 1⁄2-in. 

gypsum board or replace with metal. Seal between ducts and duct enclosure. Seal air handler 

cabinet operable doors with tape and other openings with mastic and mesh tape or other 

approved sealant listed below. Attach air handlers to continuous sealed ducts that extend to the 

return grille at interior, close all openings and joints (see IMC 603.7 and 918.5). 

For water heaters made before July 2003 and for any combustion appliances: the ignition source 

must be at least 18 in. above the floor of the garage. Raise the appliance or replace it with a 

current flammable vapor ignition resistant model. For details refer to IRC M1307.3 and ANSI 

Z21.10.1-2001. Follow local codes if they are more restrictive. Spaces with openings to the 

garage are considered as part of the garage.
20 

4.2.1 Separation Wall From Garage

For some appliance(s) that are located in a garage or a space that is open to the garage, it may be 

most efficient to construct walls that separate the appliance(s) from the garage. Provide a 

continuous wall between the combustion appliances and the rest of the garage. Tape joints in a 

1⁄2-in. gypsum board covered separation wall between the garage and combustion appliances, 

with no openings to the garage. Seal top and bottom gaps in wall to garage. The door to the space 

should be from the outside. If the door to the space is to the garage, the appliance ignition source 

must be at least 18 in. above the floor of the garage. Raise the appliance or replace it with a 

current flammable vapor ignition resistant model.

4.3 Combustion Appliances in Mechanical Closets

For mechanical closets located within the living space, seal the walls, floor, ceiling, and door 

from the house. Bring all combustion air from the outside. Fill in large openings at interior finish 

with 1⁄2-in. gypsum board, duct board or equivalent air impermeable sheet material, tape seams. 

Seal all joints, seams and penetrations in the ceiling floor and walls with mastic and tape, foam 

listed for use as a fire stop and approved for uncovered use, sealant meeting the standards set 

forth in ASTM814-09 ASTM C 920, and ASTM C1193-09. 

Remove and infill louvers in walls or door to the house living space with metal or gypsum board. 

Provide a closet door with weather-stripping and a bottom sweep or threshold. Provide a door 

closer or other self-closing mechanism that confirms closing and self-latching.

Provide a sealed duct from the return grille in the room to the inlet of the fan cabinet as above. 

The return cabinet also should be sealed. Zero leakage for returns is allowed in the zone. Refer to 

NFPA 54, IFGC, and IRC. 

4.3.1 Air Handlers on Enclosed Support Platform

Sealing alone is not adequate for air handler platforms. Code and this Guideline do not allow 

adjacent construction materials to be in contact with return air. The minimum requirement is to 

line the support platform with sealed duct board, sheet metal, or other air barrier material rated 

for use in ducts. No adjacent walls, floors, ceilings, or other part of the building can be used as 

part of the return duct or plenum in the combustion space. (cf. NFPA54 10.3.7.4, IFGC, 

618.5.EX2.3, NFPA 90B 4.2.5, 2010 FL mechanical code table 603, and California title 24 parts 

4 and 6. Install, line, or extend a properly sized duct section constructed entirely of rigid metal, 

rigid fibrous glass duct board, or flexible duct between the return air inlet(s) in conditioned space 

and the inlet of the air handling unit or furnace. Mechanically fasten one end to the return fan 

inlet at the cabinet and the opposite end to the return grille inside occupied space. Seal both 

attachments completely without obstructing airflow. Fasten and seal all joints in between to 

allow no return leakage into the mechanical space.
21 

Photo: J Fitzgerald 2005

Figure 9. Not acceptable: As-found view inside platform return: debris, moisture, 

wall cavities potentially open to combustion vent, burner, and attic above

Photo Rick Chitwood 2010 

Figure 10. Not acceptable: Restricted platform return in newer home

Photo Rick Chitwood 2010 

Figure 11. Acceptable: View after opening inlet to fan cabinet
22 

Figure 12. How to seal existing platform returns

Illustration by Ray David, NREL/PIX 19500. Photos from Warren Gretz (NREL/PIX 10929) and 

Iberdola Renewables Inc. (NREL/PIX 15185)

For further information refer to the NREL Standard Work Specifications Tool 

(https://sws.nrel.gov/spec/316025).

4.4 Combustion Appliances in a Closet or Separate Room With a Door That 

Opens to a Bedroom or Bathroom 

Provide combustion safety check and re-vent to code as needed and provide all combustion air 

from outside. Weather-strip the door and provide a door closer. Seal all openings to adjacent 

spaces. Seal all ductwork openings including air handler as above. Cf. IFGC 303.3.5 and NFPA 

54 10.3.11, 10.28.1. 

4.5 Building Cavities Used as Returns, Opening Into Combustion Appliance 

Space

Line building cavity with continuous, durable, non-porous, air-impermeable material and seals 

that show a flame spread of 25 or less and smoke developed of 50 or less when tested to ASTM 
23 

E 84 or UL 723. Or use a standard material listed for use in duct systems, such as sheet metal, 

duct board, or flexible duct. 

Seal joints and penetrations with a suitable long-life mastic material. Mechanically fasten and 

seal to the return inlet, extend and seal to a grille, filter grille or duct section at the interior. Seal 

the annular space between the building cavity and new air impermeable liner to prevent all return 

leakage in the combustion space. 

Maintain at least the cross sectional area of the return inlet and size the liner or duct to meet the 

manufacturers required airflow without restriction. Furnace returns must provide at least 2 in.2 of 

total cross-sectional area for every 1000 Btu of output.
24 

5 Materials

Use materials and sealants that match the listing where they are applied.

5.1 Rigid Air-Impermeable Panel Materials 

Acceptable rigid air-impermeable materials include:

• 1⁄2-in. gypsum board 

• 5⁄8-in. Type X gypsum board 

• 3⁄4-in. and 1-in. thick FSK faced duct board labeled for use in ducts and installed to Sheet Metal 

and Air Conditioning Contractors’ National Association fibrous Glass Duct Construction 

Standards and North American Insulation Manufacturers Association Fibrous Glass Duct 

Construction Standards 

• Attic: rigid foil faced foam board rated for use exposed to interior (Thermax or equivalent) with 

ICC Evaluation Service (ES) report confirming testing to code requirements 

• Rigid extruded polystyrene insulation board for use where 1⁄2-in. gypsum board separation to 

interior is in place.

5.2 Sealants 

Acceptable sealants include:

• Joint compound and tape on gypsum board joints 

• High temperature room temperature vulcanizing silicone for use on gas vent gaps to metal 

barriers 

• Meets or exceeds Federal Specification TT-S-001543a, TT-S-00230c, A-A-272, Type IV, 

C.G.S.B. 19 GP-9B and ASTM C 920-86,2008 

• Furnace cement, ASTM E 136-09 non-combustible sealant for masonry chimney gaps 

• Acrylic and silicone sealant caulks for attics and closets

• 15-minute rated sealants caulks for small gaps, penetrations, and seams in garage wall to house or 

similarly rated areas.

5.3 Materials for Ductwork and Duct Sealing 

Acceptable materials for ductwork and duct sealing include:

• Tapes for metal ductwork marked UL 181A-P 

• Mastic that meets UL 181 A-M 

• Fiberglass duct board that meets UL 181A 

• Non-metallic flexible duct (do not use cloth backed tapes): 
25 

o Tape labeled UL 181B FX 

o Mastic that meets UL 181B-M 

o Mechanical fasteners labeled UL 181B-C.

• Aerosol sealant with 25/50 class 1 flame spread is approved for all joint seals when applied by 

manufacturer certified installers to manufacturer’s standards.

• Duct coverings, linings and adhesives flame/smoke 25/50 on ASTM E 84 or UL 723. 

• Two part spray foam meeting IRC 2012 M1601.4.1, and IRC R316.6 tested for its intended uses 

to: NFPA 286 within acceptance criteria of IRC R 302.9.4, FM 4880, UL 1040, or UL 1715 and 

reported in an ICC ES report as meeting requirements of code, requires no additional sealants. 

• Single component sealant foam: use material accepted as fire stop in wood frame construction 

and approved for limited exposed use in ICC ES reports see ESR 1961-12, ESR 1862-11, ESR 

1926-12. 

5.4 Materials for Filling Cavities 

For performing practical retrofit air flow separation and firestop in cavities to rooms over a 

garage or other cavity airflows: 

• Select cellulose tested with ICC ES reports see ESR 1996, ESR 2217-09, or 

• Select engineered fiberglass wool tested for BPI 104 to ASTM C 522, E 283 or E 2178 for use in 

BPI-102 “Standard for Air Resistance of Thermal Insulation Used in Retrofit Cavity Applications 

– Material Specification.” 

5.5 Additional Resources

For additional information on how to provide specific details see a recognized provider.
26 

References

Bird, R.E.; Hulstrom, R.L. (1981). A Simplified Clear Sky Model for Direct and Diffuse 

Insolation on Horizontal Surfaces. SERI/TR-642-761. Golden, CO: Solar Energy Research 

Institute.

Graumann, A. (2003). Data Documentation for Dataset 3701, GOES Data User’s Guide, ver 1.1. 

Ashville, NC: National Climatic Data Center.

Maxwell, E.L. (1998). “METSTAT – The Solar Radiation Model Used in the Production of the 

National Solar Radiation Database (NSRDB).” Solar Energy; 62(4); pp. 263–279. 

Myers, D., Wilcox, S.; Marion, W.; George, R.; Anderberg, M. (2005). “Broadband Model 

Performance for an Updated National Solar Radiation Database in the United States of 

America.” Proc. Solar World Congress, International Solar Energy Society, 2005. 

Myers, D.R.; Wilcox, S.; Marion, W.; Al-Abbadi, N.; bin Mahfoodh, M.; Al-Otaibi, Z. (2002). 

Final Report for Annex II – Assessment of Solar Radiation Resources in Saudi Arabia 1998–

2000. NREL/TP-560-31546. Golden CO: National Renewable Energy Laboratory, April 2002. 

NCDC (2004). Federal Climate Complex Data Documentation for Integrated Surface Hourly 

Data, June 21, 2004. Asheville, NC: National Climatic Data Center. 

NREL (1993). Users Manual for SERI QC Software – Assessing the Quality of Solar Radiation 

Data. NREL/TP-463-5608. Golden, CO: National Renewable Energy Laboratory. 

NREL (1995). Final Technical Report, National Solar Radiation Data Base (1961-1990). 

NREL/TP-463-5784. Golden, CO: National Renewable Energy Laboratory. 

Perez, R.; Ineichen, P.; Seals, R.; and Zelenka, A. (1990). “Making Full Use of the Clearness 

Index for Parameterizing Hourly Insolation Conditions.” Solar Energy, vol. 45, No. 2, pp. 111–

114. 

Perez, R.; Ineichen, P.; Moore, K.; Kmiecikm, M.; Chain, C.; George, R.; Vignola, F. (2002). “A 

New Operational Satellite-to-Irradiance Model.” Solar Energy, 73 5, pp. 307–317. 

Rymes, M.; Myers, D. (2001). “Mean Preserving Algorithm for Smoothly Interpolating 

Averaged Data.” Solar Energy, 71, 4, pp. 225–231. 
27 

Appendix

Safe Entry

Only after the indoor air environment is found to be safe shall further work be undertaken. In the 

event that the indoor air environment is determined to be unsafe to the auditor and/or 

occupant(s), the auditor shall not conduct further work and shall take action in accordance with 

guidance provided in the subsections that follow. 

Combustible Gas Detectors and Carbon Monoxide Measurement Instruments

Calibrate instrument outside the building away from any combustion outlets or automobile traffic 

areas, or in accordance with manufacturer’s instructions. 

Combustible Fuel Gases 

If at any time the auditor smells combustible fuel gas (characterized by a garlic, rotten egg, or 

“gassy” odor) the auditor and occupants upon notification by the auditor, shall leave the house, 

and the appropriate emergency services (typically fire services) and fuel gas providers shall be 

notified from outside the home. 

If the auditor uses a combustible gas detector to determine hazards from combustible 

concentrations of fuel gas (as in cases where sense of smell may be compromised), indoor air 

will be sampled at least one location per floor of occupied space upon entering the home. If any 

measured concentrations exceed 20% lower explosive limit (1% concentration in air by volume), 

the occupants and Auditor shall leave the house, and the appropriate emergency services and fuel 

gas providers shall be notified from outside the home. 

The audit shall not proceed until emergency services have deemed it safe to re-enter the home 

and proceed with audit activities. 

Where called for by the local authority having jurisdiction, the response procedures documented 

in the NFGC, ANSI Z223.1/NFPA 54, Annex D, “Suggested Emergency Procedures for Gas 

Leaks” shall be carried out. 

Carbon Monoxide 

Carbon monoxide measurements upon entry shall be made and actions taken in accordance with 

the U. S. Consumer Products Safety Commission document, “Responding to Residential Carbon 

Monoxide Incidents: Guidelines for Fire and Other Emergency Response Personnel,” section 

“CO Levels: Advice to Give, Actions to Take,” available 

at: www.cpsc.gov//PageFiles/117067/coguide.pdf. 
28 

Ambient Atmosphere Safety Action Levels

Based on CO detector readings, the auditor should take the following actions: 

If the CO detector indicates an ambient carbon monoxide level of 70 ppm or higher, the auditor 

shall immediately notify the occupant of the need for all building occupants to evacuate the 

building; the auditor shall immediately leave the building.

If the CO detector indicates an ambient reading between 36 ppm and 69 ppm, the auditor shall 

advise the occupant that elevated levels of ambient CO have been detected and recommend that 

all possible sources of CO be turned off immediately and windows and doors opened. Where it 

appears that the source of CO is a permanently installed appliance, the appliance shall be turned 

off and serviced by a qualified servicing agent. 

If the CO detector indicates an ambient reading between 9 ppm and 35 ppm, the auditor shall 

advise the occupant that CO has been detected and recommend that all possible sources of CO be 

checked and windows and doors opened. Where it appears that the source of CO is a 

permanently installed appliance, the appliance shall be inspected and serviced by a qualified 

servicing agent. 

If the CO detector indicates ambient CO below 9 ppm, the inspection can proceed. 

Building Fuel Gas Distribution Systems 

Fuel Gas Distribution System Inspection

Accessible interior piping and tubing shall be visually inspected for general condition of 

materials, connections, components, and supports. Where the auditor suspects deficiencies of any 

kind in materials, connections, components, or supports, he/she should note the deficiencies in 

the audit report and recommend to the owner to contact a qualified plumber or other gas piping 

expert to review the fitness of the system.

Review to Minimum Code Requirements

Systems shall be inspected and tested in accordance with the NFGC, ANSI Z223.1/NFPA 54, 

Chapter 8, “Inspection, Testing, and Purging” and, where called for by local authority having 

jurisdiction, shall be tested in accordance with Annex C of the NFGC, “Suggested Method for 

Checking for Leakage.” 

Inspection of Combustion Appliances and Venting Systems

Combustion appliances and venting systems serving those appliances shall be inspected for 

damage, leaks, disconnections, inadequate vent and vent connector slope and other safety 

hazards. 
29 

Temporarily disable from operating the appliance to be inspected by means of a consumeraccessible switch or control such as switching off power at the electrical panel or lowering the 

thermostat setting.

Gas-Fired Appliance Venting Systems

The auditor shall carry out one of the following procedures to evaluate venting system design 

and installation for gas-fired appliances:

1. Determine whether the existing venting system complies with Chapters 12 and 13 of the 

NFGC, ANSI Z223.1/NFPA 54) and its requirements for terminations, sizing of 

chimneys, vents, and vent connectors, conditions of chimneys, venting system materials, 

draft hoods and other components for Category I (atmospherically vented, including fan 

assisted) appliances and Category II, III, and IV appliances. 

2. Recommend the owner contact a qualified gas appliance installer to review venting 

system conformance to Chapters 12 and 13 of the NFGC. 

Oil-Fired Appliance Venting Systems

The auditor shall carry out one of the following procedures to evaluate venting system design 

and installation for oil-fired appliances:

1. Determine whether the existing venting system complies manufacturer’s installation 

instructions and its requirements for terminations, sizing of chimneys, vents, and vent 

connectors, conditions of chimneys, venting system materials, draft hoods and other 

components for Category I (atmospherically vented, including fan assisted) appliances 

and Category II, III, and IV appliances. If the manufacturer’s installation instructions are 

not available refer to NFPA 31. 

2. Recommend the owner contact a qualified gas appliance installer to review venting 

system conformance to Chapters 12 and 13 of the NFGC. 

Gas-Fired Appliance Inspection 

CO measurements associated with flue gases and normal operating conditions of gas-fired 

appliances shall be conducted in accordance with NFGC, Annex G and evaluated in accordance 

with thresholds shown in Table A.4.1 of that Annex (reproduced in this Guideline as Table 3).

The following appliance-specific inspections are performed either with the appliance in the off or 

standby mode (indicated by “OFF”) or on an appliance that is operating (indicated by “ON”). 

The CO measurements are to be undertaken only after the appliance is determined to be 

properly venting. 
30 

Forced Air Furnaces

a. OFF. Verify that an air filter is installed and that it is not excessively blocked with dust.

b. OFF. Inspect visible portions of the furnace combustion chamber for cracks, ruptures, holes, and 

corrosion. A heat exchanger leakage test should be conducted. 

c. ON. Verify both the limit control and the fan control for proper operation. Limit control operation 

can be checked by blocking the circulating air inlet or temporarily disconnecting the electrical 

supply to the blower motor and determining that the limit control acts to shut off the main burner 

gas.

d. ON. Verify that the blower compartment door is properly installed and can be properly resecured 

if opened. Verify that the blower compartment door safety switch operates properly.

e. ON. Check for flame disturbance before and after blower comes on which may indicate heat 

exchanger leaks.

f. ON. Measure CO in the vent after 5 minutes of main burner operation. The CO should not exceed 

threshold in Table 3. 

Boilers

a. OFF and ON. Inspect for evidence of water leaks around boiler and connected piping. 

b. ON. Verify that the water pumps are in operating condition. Test low water cutoffs, automatic 

feed controls, pressure and temperature limit controls, and relief valves in accordance with the 

manufacturer's recommendations to determine that they are in operating condition. 

c. ON. Measure CO in the vent after 5 minutes of main burner operation. The CO should not exceed 

threshold in Table 3. 

Water Heaters

a. OFF. Verify that the pressure temperature relief valve is in operating condition. Water in the 

heater should be at operating temperature.

b. OFF. Verify that inspection covers, glass, and gaskets are intact and in place on a flammable 

vapor ignition resistant type water heater. 

c. ON. Verify that the thermostat is set in accordance with the manufacturer’s operating instructions 

and measure the water temperature at the closest tub or sink that it is no higher than 120oF. 

d. OFF. Where required by the local building code in earthquake prone locations, inspect that the 

water heater is secured to the wall studs in two locations (high and low) using appropriate metal 

strapping and bolts. 

e. ON. Measure CO in the vent after 5 minutes of main burner operation. The CO should not exceed 

threshold in Table 3. 
31 

Gas Clothes Dryer

a. OFF. Where installed in a closet, verify that a source of make-up air is provided and inspect that 

any make-up air openings, louvers, and ducts are free of blockage. 

b. OFF. Inspect for excess amounts of lint around the dryer and on dryer components. Inspect that 

there is a lint trap properly installed and it does not have holes or tears. Verify that it is clean 

condition. 

c. OFF. Inspect visible portions of the moisture exhaust duct and connections for loose fittings and 

connections, blockage, and signs of corrosion. Verify that the duct termination is not blocked and 

that it terminates in an outdoor location. Verify that only approved metal vent ducting material is 

installed (plastic and vinyl materials are not approved for gas dryers).

d. ON. Verify mechanical components including drum and blower for proper operation. 

e. ON. Operate the clothes dryer and verify that the exhaust system is intact and exhaust is exiting 

the termination. 

f. ON. Measure CO in at the exhaust duct or termination after 5 minutes of main burner operation. 
32 

Table 3. Carbon Monoxide Thresholds (Table A.4.1 of NFGC Annex)

Appliance Threshold Limit

Central Furnace (All Categories) 400 ppm* air free

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

Vented Room Heater 200 ppm air free

Vent-Free Room Heater 200 ppm air free

Water Heater 200 ppm air free

Oven/Boiler 225 ppm as measured

Top Burner 25 ppm as measured (per burner)

Refrigerator 25 ppm as measured

Gas Log (Gas Fireplace) 25 ppm as measured in vent

Gas Log (Installed in Wood-Burning Fireplace) 400 ppm air free in firebox

* Parts per million 

From ANSI/BSR Z223.1/NFPA 54
33 

Additional Resources

Air Conditioning Contractors of America

2800 Shirlington Road, Suite 300 

Arlington, VA 22206 

ASHRAE

1791 Tullie Circle, NE 

Atlanta, GA 30329 

ASTM International

100 Barr Harbor Drive 

West Conshohocken, PA 19428-2959 

ICC 

International Code Council, Inc. 

500 New Jersey Ave, NW

6th Floor 

Washington, DC 20001 

ICC-Evaluation Service

5360 Workman Mill Road 

Whittier, California 90601 

Phone: 800-423-6587 ext. 66546

Fax: 562-695-4694 

www.icc-es.org

North American Insulation 

Manufacturers Association

44 Canal Center Plaza, Suite 310 

Alexandria, VA 22314

NFPA 

National Fire Protection Association

1 Batterymarch Park

Quincy, MA 02169-7471 

Sheet Metal and Air Conditioning 

Contractors National Association, Inc.

4201 Lafayette Center Drive

Chantilly, VA 20151-1209 

UL

Underwriters Laboratories, Inc.

333 Pfingsten Road 

Northbrook, IL 60062-2096 
34 

References

Cummings, J., Withers, C., “Problems Related to Air Handler Leakage,”

American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 

ASHRAE Journal, January 2008.

2010 California Energy Code

California Code of Regulations, Title 24, Part 6 

California Building Standards Commission 

2525 Natomas Park Drive, Suite 130 

Sacramento, CA 95833-2936 

American Gas Association

NFPA (FIRE) 54/ANSI Z223.1 

400 North Capitol Street, NW 

Washington, DC 20001 

California Energy Commission

2008 Title 24 California Building Energy Efficiency Standards for low-rise residential buildings 

2008 Residential Compliance Manual 

California Energy Commission

1516 Ninth Street, MS-13 

Sacramento, CA 95814 

916-654-5200 

California Energy Commission

Reference Appendices to the 2008 Building Energy Efficiency Standards

1516 Ninth Street, MS-13 

Sacramento, CA 95814 

916-654-5200 

Reference Residential Appendix RA3 Field Verification 

Proctor, John, Rick Chitwood, and Bruce A. Wilcox. (Proctor Engineering Group, Ltd., 

Chitwood Energy Management, Inc. Bruce A. Wilcox). 2011.

Efficiency Characteristics and Opportunities for New California Homes. 

California Energy Commission. 

Publication number: CEC‐500‐2012‐062
35 

2010 California Mechanical Code

California Code of Regulations Title 24, Part 4 

California Building Standards Commission 

Based on the 2009 Uniform Mechanical Code 

Copyright to 2009 UMC held by 

International Association of Plumbing and Mechanical Officials 

5001 East Philadelphia Street

Ontario, CA 91761-2816 

Copyright to all California Provisions held by 

California Building Standards Commission 

2525 Natomas Park Drive, Suite 130 

Sacramento, CA 95833-2936 

IMC 2012

918.2 Minimum duct sizes. 

The minimum unobstructed total area of the outdoor and return air ducts or openings to a forced-air 

warm-air furnace shall be not less than 2 square inches per 1,000 Btu/h (4402 mm2/kW) output rating 

capacity of the furnace and not less than that specified in the furnace manufacturer’s installation 

instructions. The minimum unobstructed total area of supply ducts from a forced-air warm-air furnace 

shall not be less than 2 square inches for each 1,000 Btu/h (4402 mm2/kW) output rating capacity of 

the furnace and not less than that specified in the furnace manufacturer’s installation instructions. 

Exception: The total area of the supply air ducts 

and out- door and return air ducts shall not be 

required to be larger than the minimum size 

required by the furnace manufacturer’s 

installation instructions.”
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