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Combustion air defects & hazards:
This article explains how to recognize and fix combustion air defects on heating appliances such as boilers, furnaces, and water heaters.
Lack of adequate combustion air causes improper heater operation, increased maintenance cost, and risks dangerous production of carbon monoxide gas. This article series explains how to recognize & diagnose problems with residential heating boilers, including loss of heat, heating boiler noises, leaks, odors, or smoke, and high heating costs.
This article series answers most questions about central hot water heating system troubleshooting, inspection, diagnosis, and repairs. Our photo at page top shows an oil fired furnace installed in a closet with an airtight door; there was no outside combustion air supply. The heating system could not work properly nor safely in this home.
Visible Signs of Inadequate Combustion Air in Buildings
How to recognize, diagnose & fix inadequate combustion air, sooting, or burn marks at oil-fired or gas-fired heating systems.
Watch out: inadequate combustion air not only causes improper and wasteful operation of heating equipment it can also produce fatal carbon monoxide gas hazards indoors.
The quantity of combustion air needed depends on the fuel type, the input BTUh rating of the heating equipment, and additional air needed to assure effective exhaust draft to carry combustion products safely out of the building.
Topics discussed: Diagnose signs of inadequate combustion air for an oil or gas burner. Dangers of carbon monoxide poisoning if there is soot production at gas fired heating appliances. Inadequate combustion air can cause dangerous carbon monoxide gas in buildings.
Combustion air safety check procedure for gas fired heating equipment. How to test for safe combustion air for gas fired heating appliances, water heaters, etc. Combustion Air Defects & Safety Hazards at Mobile Home Heating System. How is outside combustion air provided to a woodstove?
These photographs show a heating furnace with soot blow-back around the oil burner probably means there is backpressure in the combustion chamber - an improper operating condition that may be unsafe.
Watch out: inadequate combustion air supply to a gas burner (and less often to an oil burner) is very dangerous and can produce potentially fatal carbon monoxide. If you suspect unsafe heating system operation or a carbon monoxide problem be sure everyone leaves the building immediately and then call your local fire department for assistance.
Clues Indicating Possible Lack of Combustion Air & Related Safety Hazards
Lack of adequate combustion air can be indicated by or can result in these heating system operating and safety worries:
Watch out: Gas burner sooting or odors: SAFETY WARNING: Small amounts of soot or flame marks right at the gas burner also indicate an operating problem but may not be producing carbon monoxide.
But soot produced at a gas fired appliance such as chunks of soot found around a gas flue vent or draft hood is a RED DANGER FLAG as dangerous carbon monoxide may be produced and a chimney may be blocked.
Oil burner sooting or dirty operation (the photo at above-left)
Burn marks on the boiler (photo at above right), furnace, or water heater, (these conditions may be caused by a blocked exhaust flue and inadequate venting).
Watch out: burn marks on a heating boiler or furnace (shown in our page top photo) can also be due to collapse or damage to the combustion chamber liner - a serious fire hazard needing immediate attention. Notice that in our photo at right, the brown burn marks around the oil boiler combustion chamber inspection port (that rusty round door above the oil burner) have been cleaned-off.
The presence of these burn marks does not necessarily mean that the oil burner backpressure problem remains - in this case the system has been cleaned and adjusted, but no one has re-painted the front of the boiler. But if you see fresh peeling paint or soot in such an area further investigation is needed.
That's why we recommend that after repairing a back-pressure problem at heating equipment the service technician should clean the boiler or furnace exterior - to remove confusing debris and to make it easier to see if the problem recurs.
Heating equipment located in a small utility room with no provision for combustion air intake. When the service technician adjusts the system she probably worked with the utility room door open, but when the service tech left the job he may have closed the door - completely changing the availability of combustion air for the equipment.
We need about one square inch of un-louvered (unobstructed) combustion air intake per 1000 btuh of the oil fired heating boiler, furnace, or water heater.
Increased heating system operating cost, spending more on heating oil than necessary
Damage to oil burner components (backpressure heat can destroy an ignition transformer),
Higher and more frequent heating service calls & costs
Loss of heat, the heating system goes off on safety reset
Noises in the heating equipment or chimney when the boiler, furnace, or water heater starts, is operating, or is shutting down
Smoke or soot indoors,coming out of the heating equipment or its draft regulator
Carbon monoxide or CO alarms: Potentially, the production of carbon monoxide or other flue or combustion gases which escape into the building - potentially dangerous or even fatal. It is harder to produce with oil heat than with gas heat, but not impossible.
Square Inches of Combustion Air Intake: 1 sq. in. per 1000 BTUH
For heating equipment installed in confined spaces, an old-timer's rule of thumb is to add up the total INPUT BTUH numbers from all of the data tags on all of the heating equipment installed. You want to see at least one sq. in. of open fresh air intake per 1000 btuh.
Total Sq .In. of Combustion Air Intake Opening = Total Input BTUH / 1000
Watch out: this combustion air rule of thumb needs to be adjusted to account for the air flow restriction caused by louvers and screening over the combustion air intake opening. As a rule of thumb we
Reduce the effective total square inches of combustion air intake opening by at least 1/3 for louvers & screens
Reduce the effective total square inches of combustion air opening further if the screen is or can easily become clogged by lint, leaves, trash & debris
Watch out: The combustion air estimate provided by outside combustion air openings or openings into other, presumably larger building areas (see below) also needs to account for the effects of building exhaust fans, tight buildings, and similar interferences.
Watch out: this rule of thumb falls apart if the fresh air is not being vented directly into the heating equipment area through an outside wall.
That is, if air has to move through vent pipes or ductwork into the area where it is needed, the equivalent square inches of fresh air intake venting may need to be increased depending on the length, number of bends, angles of bends, and diameter of the fresh air or combustion air intake venting system.
Standard engineering approaches to calculating air flow through round or rectangular ductwork can solve the question of impact on combustion air of routing it through ducts.
Cubic Feet of Room Space as a Measure of Adequacy of Combustion Air: Total Input BTUH / 1000 x 50
For heating equipment installed in larger spaces, a common rule of thumb for computing the required total cubic feet of free space to assure adequate combustion air is to provide 50 cubic feet of free space per 1000 Input BTUH for the total of all of the heating appliances installed in the area. The assumption behind this old rule is that buildings leak air and that larger rooms or spaces have more air intake leaks than smaller ones.
Total Cubic Feet of Free Area = Total Input BTUH / 1000 x 50
Watch out: this formula may not adequately consider the reduction in volume of the room or open space attributed to contents, storage, etc. and it certainly does not adjust for modern tight building construction.
Some writers simplify the formula to express this rule of thumb as
Total Cubic Feet of Free Area = Total Input BTUH x .05
Example: if we have a 180,000 Input BTUH boiler and a 40,000 input BTUH water heater installed in an enclosed utility room, how many cubic feet of space in that room would make us think we had adequate combustion air?
(180,000 + 40,000) / 1000 x 50
220,000 BTUH / 1000 = 220
220 x 50 = 11,000 cubic feet.
If our room is smaller than 11,000 cubic feet we probably don't have adequate combustion air (unless an outside combustion air source is also provided).
Calculate the total cubic feet of space in a room by multiplying the room width x room length x room height
Example: if the heating equipment is installed in an open basement that is 40 ft. x 20 ft. x 8 ft. high, we have
Cubic Feet = 40 x 20 x 80
Cubic Feet = 6400 - this basement will probably not provide adequate combustion air for the example input BTUH total given above.
Special thanks to reader Joe who corrected a math error in the above, 14 Jan 2016
Table of Combustion Air Supply Duct Sizes vs BTUh
Watch out: square duct area is not equal to round duct area in air flow capacity. That's because air flowing through a square or rectangular duct (or chimney) does not flow uniformly - the area of the corners of the rectangle moves less air. For you who left your calculator at home and left Pi in the refrigerator, here is the square inches of cross section opening size for common round duct diameters:
Table of Combustion Air Supply Duct Sizes vs. Input BTUh for Natural Draft Heating Appliances
Approximate Total Input BTUH Supported at This Combustion Air Duct-Vent Opening Diameter2
1. This data is for round ducts and smooth metal duct sides; lengths of flex duct with ribbed or corrugated sides restricts air flow and will not provide as much equivalent air flow. Reference: "Evaluating Duct Work, How to Evaluate Furnace Duct Work &
Cure Short Cycling or Inadequate Ductwork Problems" Vermont Department for Children and Families, Office of Economic Opportunity, - dcf.vermont.gov/sites/dcf/files/pdf/oeo/WAPManual/AppendixI.pdf retrieved 12/5/2013
2. Really? We need further research on these figures. they significantly exceed the 1000 BTUH per square inch of area. Citation: David Clark, Home Inspection Newsletter, retrieved 12/5/2013 http://turnkeyhomeinspectors.com/turnkeyhomeinspectors_004.htm
3. These numbers are for round opening sizes used to provide combustion air to heating equipment.
See SUPPLY DUCTS & REGISTERS for details about HVAC heating or cooling supply & return air duct sizing, air flow, and for matching HVAC air duct sizes to equipment BTUH rate or heating capacity.
How to Convert Round Opening Diameters (say an air duct cross section) to Opening Size Equivalent
If we are using smooth-walled round ducts to bring combustion air into the space where it is needed, and before considering the restrictions on air flow caused by duct bends and length (friction losses) we start by simply calculating the cross-sectional area of the duct:
Pi r2 = the area of a circle or the cross-sectional area of a round combustion air supply duct
Pi (also written as Π) = 3.1416 - a constant
r = the radius = half of the diameter of the circle
We can use any unit (cm, inches, feet) as long as we stick to the same unit through.
Example: a 6-inch diameter round air duct has a cross section (or area) of
Area = 3.1416 x (6 / 2)2 inches
Area = 3.1416 x (3)2 inches
Area = 3.1416 x (3 x 3)
Area = 28 sq. in. of space - which, if unrestricted by duct length, bends, or screens, and if we use our first rule of thumb (one inch per 1000 BTUH) would support about 28,000 Input BTUH
Combine Combustion Air Sources to Check the Combustion Air Requirements
When room volume in cubic feet is inadequate to provide safe combustion air we can add combustion air by providing an outside air source.
Combustion Air Requirements Specifications for Power Burners
Questions & answers or comments about combustion air requirements and safety hazards for oil and gas fired heating appliances
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Handbook - Fundamentals, 1993, Chapter 15, page 15.9 Air For Combustion.
ASME CSD-1- Controls and Safety Devices for Automatically Fired Boilers, 1992 with addendum 1a 1993. section CG-260 Combustion Air.
BOCA - National Mechanical Code, 1990, article 10, Combustion Air.
NFPA 31 - Installation of Oil Burning Equipment, 1992, section 1-5 Air for Combustion and Ventilation.
NFPA 54: National Fuel Gas Code (2015), addresses heating appliance combstion air ventilation specifications. NFPA 54, ANSI Z223.1 provides minimum safety requirements for the design and installation of fuel gas piping systems in homes and other buildings.
NFPA 85: Boiler and Combustion Systems Hazards Code: NFPA 85 contributes to operating safety and prevents explosions and implosions in boilers with greater than 12.5 MMBTUH, pulverized fuel systems, and heat recovery steam generators.
NFPA 87: Recommended Practice for Fluid Heaters, This recommended practice provides safety guidance for fluid heaters and related equipment to minimize fire and explosion hazards that can endanger the fluid heater, the building, or personnel
SBCCI- Standard Mechanical Code, 1991, section 305 Combustion and Ventilation Air.
Axtman, William H., "Combustion Air Requirements: The Forgotten Element in Boiler Rooms", Grayh Gull Associates, retired executive director of the American Boiler Manufacturers Association, National Board Technical Series, Winter 1995 National Board Bulletin. Retrieved 26 January 2015, original source: http://www.nationalboard.org/index.aspx?pageID=164&ID=191
Excerpt: Several safety codes such as the National Fire Protection Association's standards, NFPA 54 - National Fuel Gas code, NFPA 31 - Installation of Oil Burning Equipment, and the American Society of Mechanical Engineers (ASME) CSD-l Controls and Safety Devices for Automatically Fired Boilers have sections covering the requirements for combustion air intakes. In addition, building codes such as the Building Officials and Code Administrators International (BOCA) National Mechanical Code and the Standard Mechanical Code published by the Southern Building Code Congress International (SBCCI) have air requirements for combustion. Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.
Nussbaumer, Thomas. "Combustion and co-combustion of biomass: fundamentals, technologies, and primary measures for emission reduction." Energy & fuels 17, no. 6 (2003): 1510-1521.
Utiskul, Yunyong P., Wu, Neil P., Biteau, Hubert, "Combstion Air Requirements for Power Burner Appliances, Final Report", The Fire Protection Research Foundation, The Fire Protection Research Foundation
One Batterymarch Park
Quincy, MA, USA 02169-7471
http://www.nfpa.org/foundation, retrieved 25 Jan 2015, original source: http://www.nfpa.org/~/media/files/research/research%20foundation/rfcombustionairrequirements.ashx, Executive Summary: Gas-fired appliances require combustion air to properly function. Adequate air is necessary for
supporting combustion of the appliance burner, dilution of flue gas, and proper ventilation of the
space where the appliance is installed. Current standards and model codes outline requirements
and methods to supply the combustion air. One method is to provide outdoor combustion air
through openings or air ducts communicating with the outdoors through natural ventilation.
Most standards require the outdoor opening(s) be prescriptively sized based on the total energy
input rating of the appliance. However, in the United States, the current standards contain no
separate provisions to address the opening size supplying the combustion air for
commercial/industrial sized appliances, which typically have a high energy input rating of
greater than 300 kBtu/hr and are equipped with a power burner unit. As a result, the opening(s)
can be excessively sized when determined based on the current standards.
This research project establishes minimum outdoor combustion air requirements specific to gasfired
appliances utilizing power burners with input ratings no greater than 12.5 MBtu/hr. A
review of the available literature, engineering guidelines, and current standards and model codes
related to combustion air requirements was performed. This report provides an understanding
of the technical basis for the existing provisions for combustion air and their applicability to
power burner appliances. This report also identifies the range of energy input ratings for gasfired
appliances equipped with power burners, and compares the combustion air requirements
specified by a range of appliance manufacturers. A theoretical model for air flow through
openings was developed and the modeling results, together with the data gathered through the
literature review, were used to provide a baseline to establish the theorized combustion air
requirements suitable for power burner appliances.
 Jeff Wilcox, "Evaluating Duct Work, How to Evaluate Furnace Duct Work & Cure Short Cycling or Inadequate Ductwork Problems" Vermont Department for Children and Families, Office of Economic Opportunity, - dcf.vermont.gov/sites/dcf/files/pdf/oeo/WAPManual/AppendixI.pdf retrieved 12/5/2013. Geoff Wilcox
Vermont Office of Economic Opportunity
Weatherization Assistance Program
Reference Material for the original article came from:
1. Saturn Mechanical Systems Field Guide
2. Bacharach (Rudy Leatherman)
Field Controls provides instructions for the installation of LP and Natural Gas spill sensor switches, for example for their Gas Spillage Sensing Kit Model GSK-3, GSK-4, GSK-250M switches. Contact your heating service technician directly, or contact Field controls at fieldcontrols.com for more information. These switch models include a manual reset switch. Field Controls, Kingston NC 28504 - Tel 252-522-3031.
Tjernlund Products provides instructions for the installation and use of their controls, including the WHKE Millivolt Interlock Kit for use with their UC1 Universal Control, MAC1E or MAC4E auxiliary controls for gas fired equipment. This document also describes Tjernlund's recommended combustion air safety check which we recommended in this article. Contact Tjernlund Products at tjernlund.com or at 800-255-4208.
Bachrach Corporation, www.bachrach-training.com provides education for HVAC technicians. We found their web pages hanging during loading -01/2009. Readers may want to contact the company directly at: bacharach-inc.com or at 800-736-4666.
R.E. Prescott Company, a producer of other REPCO™ residential and industrial products as well as a provider of design-build engineering services is at 10 Railroad Avenue, Exeter, NH 03833. Tel: 603-722-04321 or 888-786-7482. Trish O'Keefe from Prescott informed us (10/7/2009) that their company had nothing to do with the failed Repco heating boilers discussed at InspectAPedia.com. She wrote:
Our company manufactures & distributes residential water treatment equipment, including our Repco line of conditioners. We have a plumbing & heating supply dept as well. We are mistaken for the Repco Boiler company on a regular basis, most frequently in Oct. and Nov.
Any information you could give me on the other Repco company would be appreciated. We've assumed it is no longer in business, and we'd like to know that for sure. Many callers want to know where to get replacement parts and I would be glad to direct them if I knew...plus there's always a chance they might be interested in a Crown Boiler instead
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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