Mobile home furnace (C) Daniel FriedmanCombustion Air Requirements
How to Recognize, Test, & Diagnose Heating Appliance Combustion Air Problems

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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.

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Visible Signs of Inadequate Combustion Air in Buildings

Backpressure Burn at an oil burner (C) Daniel FriedmanHow 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?

Article Series Contents

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.

Backpressure sooting at an oil fired furnace (C) Daniel Friedman

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

Mobile home furnace (C) Daniel Friedman

Lack of adequate combustion air can be indicated by or can result in these heating system operating and safety worries:

Also see UNSAFE HVAC DUCT OPENINGS which describes the risks of reduced combustion air on hot air heating systems when certain return air duct defects are present,

Heating Equipment Combustion Air Rules of Thumb

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.

COMBUSTION AIR for TIGHT BUILDINGS explains how to provide outside combustion air for tight buildings.

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

Round Duct Diameter
in Inches
Duct Cross Section Square Inches
(Round Opening Size)
Approximate Total Input BTUH Supported at This Combustion Air Duct-Vent Opening Diameter2
84,800 2
115,000 2
151,000 2
191,000 2
236,000 2
339,000 2
462,000 2


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, -‎ 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

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

For combustion air requirements for power burner fired heating appliances see COMBUSTION AIR for POWER BURNERS

Combustion Air Inlet by Automatically Operated Louvers or Dampers

This topic has moved to a new article at COMBUSTION AIR INLET AUTOMATIC LOUVERS

Combustion Air Inlets vs Cold Drafts


Guide to a Simple Combustion Air Safety Check for Gas Fired Heating Appliances


Combustion Air Defects & Safety Hazards at Mobile Home Heating System

We moved this article to COMBUSTION AIR SAFETY in MOBILE HOMES

Combustion Air for Woodstoves - How is It Provided?

Please see the new home for this article topic now found at COMBUSTION AIR for WOODSTOVES

A Guide to Combustion-Air-Related Heating Equipment Malfunctions & Their Implications



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