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Combustion air calculator
A gas appliance that burns room air needs enough of it. Under the IFGC standard method a 100,000 BTU/h furnace needs 5,000 cubic feet of space, and a 12 by 10 ft utility room with an 8 ft ceiling holds 960. Enter the appliances and the room to see whether the indoor air is enough, and if it is not, the size of the openings to other rooms or to the outdoors by 2021 IFGC Section 304. For the gas line to the same appliances, use the gas pipe sizing calculator.
- Use the standard method, or the known infiltration rate method when the air change rate of the house is known.
- Enter the input rating and vent type of each gas appliance in the room. Direct-vent appliances, which take all their air from outdoors, drop out of the sum.
- Enter the room size and any rooms that open to it, then read the required volume and the openings.
Example result for the default inputs
140,000 BTU/h of appliance input in 960 cu ft, 2021 IFGC Section 304
Required volume
7,000 cu ft
Volume of the space
960 cu ft
Indoor air short by
6,040 cu ft
| Input counted | 140,000 BTU/h |
|---|---|
| Method | Standard, 50 cu ft per 1,000 BTU/h of input |
| Each of two openings to more rooms on the same story | 140 sq in free area |
| Openings to a space on another story, total | 280 sq in free area |
| Two outdoor openings, direct or vertical ducts, each | 35 sq in free area |
| Two outdoor openings, horizontal ducts, each | 70 sq in free area |
| One outdoor opening, 1 sq in per 3,000 BTU/h (vent connectors not entered) | 46.7 sq in free area |
| Indoor and outdoor combined (304.7), outdoor size factor | 0.86, so two direct openings of 30.2 sq in each |
| Mechanical supply from outdoors (304.9) | 49.0 CFM |
| Smallest dimension, same-story and outdoor openings | 3 in or more |
For estimation purposes only. Consult a licensed engineer for final designs.
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How the volume and openings are calculated
IFGC Section 304 first asks whether the air inside the building is enough for the appliances in the space. If the space has the volume, the indoor air counts. If it does not, you connect more rooms or bring air in from outdoors through openings sized by the total input.
- Standard method (304.5.1)
V = 50 x I / 1,000V is the required volume in cubic feet and I the total input in BTU/h of the appliances in the space that the method covers.- Known infiltration rate (304.5.2)
V = 21 / ACH x I_other / 1,000 + 15 / ACH x I_fan / 1,000Equation 3-1 covers appliances other than fan-assisted, Equation 3-2 the fan-assisted ones. Below 0.40 ACH this method is required, and no rate above 0.60 ACH may be used.- Openings to other rooms (304.5.3)
same story, each of two = max(1 sq in per 1,000 BTU/h, 100 sq in); other story, total = 2 sq in per 1,000 BTU/hOn the same story, one opening starts within 12 inches of the top and one within 12 inches of the bottom of the enclosure.- Two outdoor openings (304.6.1)
each = I / 4,000 direct or vertical duct, I / 2,000 horizontal ductSquare inches of free area. The openings reach the outdoors directly, by ducts, or through spaces that freely communicate with the outdoors, such as a ventilated attic or crawl space.- One outdoor opening (304.6.2)
A = max(I / 3,000, sum of vent connector areas)A single opening starts within 12 inches of the top of the enclosure, and the appliance needs 1 inch clearance at the sides and back and 6 inches at the front.- Indoor and outdoor combined (304.7.3)
factor = 1 - available volume / required volumeThe full outdoor opening from 304.6 is multiplied by this factor, so a room that has part of the volume needs smaller outdoor openings.- Mechanical supply (304.9)
CFM = 0.35 x I / 1,000Outdoor air delivered by a fan, interlocked so the burners cannot run without it.- Louvers (304.10)
gross = free area / 0.75 metal, / 0.25 woodUse the louver's published free area when you have it. The code assumes 75 percent for metal and 25 percent for wood only when it is not known. Openings sized by 304.5.3.1 and 304.6 are at least 3 inches in their smallest dimension.
Five setups, from a closet to a basement
Each row goes through the same function as the form. The first row is the worked example in the form.
| Setup | Input counted, BTU/h | Required, cu ft | Space, cu ft | Indoor air, or two outdoor openings |
|---|---|---|---|---|
| Furnace 100,000 fan-assisted and water heater 40,000 in a 12 x 10 x 8 ft room | 140,000 | 7,000 | 960 | 35 sq in each, direct |
| Same two appliances in an open 30 x 40 x 8 ft basement | 140,000 | 7,000 | 9,600 | Enough |
| Same basement, tight house measured at 0.30 ACH | 140,000 | 7,800 | 9,600 | Enough |
| Direct-vent furnace and the 40,000 water heater in the 12 x 10 x 8 ft room | 40,000 | 2,000 | 960 | 10 sq in each, direct |
| Water heater 40,000 alone in a 3 x 3 x 8 ft closet | 40,000 | 2,000 | 72 | 10 sq in each, direct |
Swapping the furnace for a direct-vent model takes it out of the sum, and the room then has to supply only 2,000 cu ft instead of 7,000. The same pair of appliances in a tight house measured at 0.30 ACH needs 7,800 cu ft, 800 more than the standard method asks for, because less outdoor air leaks in to replace what the burners use.
Codes and data sources
- 2021 International Fuel Gas Code, Section 304, Combustion, Ventilation and Dilution Air - every factor on this page. The 50 cu ft standard method, Equations 3-1 and 3-2 with the 0.40 and 0.60 ACH limits, opening sizes for indoor, outdoor, combined and mechanical air, the 3 inch minimum dimension and the louver free area assumptions, plus the 304.1 rule that sends direct-vent and non-Category I appliances to the manufacturer's instructions.
- 2021 Seattle Fuel Gas Code, Chapter 3 - a second published copy of the same Section 304 text, used to check the wording.
- 2021 IFGC Chapter 2, Definitions - what counts as a direct-vent appliance. All air for combustion comes directly from the outdoors and all flue gases go directly outdoors.
- Appliance rating plate and installation instructions - the input rating, the vent category and the combustion air the manufacturer requires, which governs any appliance outside the Section 304 methods.
Limitations of this calculator
- A preliminary check. Combustion air problems can produce carbon monoxide. The appliance installation instructions, the adopted code and the local inspector govern, and a licensed contractor does the work.
- Section 304 sizes air for natural draft and fan-assisted Category I appliances. A direct-vent appliance takes all of its air from outdoors and is left out of the sum. A power burner or a Category II, III or IV appliance that burns room air is counted in the sum, and its manufacturer's instructions govern its own air (IFGC 304.1).
- The standard method does not ask how tight the house is. If the infiltration rate is known to be below 0.40 ACH, IFGC 304.5 requires the known rate method instead.
- ACH here is the natural infiltration rate. This page does not convert a blower door ACH50 into it.
- The room is taken as a box, length x width x ceiling height. Add connected rooms only if they open to it with no door or through openings sized by 304.5.3.
- Exhaust fans, dryers and kitchen hoods can pull air away from the appliances. IFGC 304.4 calls for makeup air where they interfere, and this page does not size it.
- Duct material and construction for combustion air ducts (IFGC 304.11), opening placement, screens and motorized louver interlocks are not checked.
- In the combined method the factor is shown for two direct openings. The same factor applies to whichever 304.6 opening you use.
- Written to the 2021 IFGC. States and cities can adopt another edition, amend Section 304, or enforce NFPA 54 or the Uniform Mechanical Code instead, so check the code your inspector uses.
Common combustion air mistakes
- Counting only the furnace. IFGC 304.5 sums the required volume of every appliance in the space. A water heater or a boiler in the same room adds to the total.
- Counting a direct-vent furnace. A direct-vent furnace takes all of its air through its own intake from outdoors. Leave it out of the sum and follow its manual, then size for what is left. A condensing furnace with only an exhaust pipe is not direct vent. It burns room air and belongs in the sum.
- Using the output rating. Every factor in Section 304 is per 1,000 BTU/h of input. Read the input on the rating plate.
- Measuring the louver instead of its free area. The openings are sized by net free area. A metal grille with unknown free area counts at 75 percent and a wood louver at 25 percent, so the frame has to be larger than the number on this page.
- Putting ACH50 into the known rate equations. Equations 3-1 and 3-2 use the natural infiltration rate. ACH50 from a blower door is far higher than the natural rate. Entered here it hits the 0.60 cap and understates the volume a tight house needs.
- Counting a room behind a closed door. A connected room adds volume only through an opening with no door or through openings sized by 304.5.3. A door that closes cuts it off.
Common questions about combustion air
How much combustion air does a gas furnace need?
By the IFGC standard method, 50 cubic feet of space per 1,000 BTU/h of input. A 100,000 BTU/h furnace needs 5,000 cu ft, and a 10 x 10 ft room with an 8 ft ceiling holds 800. That room needs openings to other rooms or to the outdoors.
What size openings does a furnace closet need?
Two openings, one starting within 12 inches of the top and one within 12 inches of the bottom. To rooms on the same story, each is 1 sq in per 1,000 BTU/h and at least 100 sq in. Straight to the outdoors or through vertical ducts, each is 1 sq in per 4,000 BTU/h, and through horizontal ducts 1 per 2,000.
Does a high efficiency furnace need combustion air from the room?
A direct-vent furnace takes all of its combustion air from outdoors through its own intake, so it stays out of the room calculation and its manual sets the air. A condensing furnace piped to take its air from the room is different. It still burns room air, so count it in the room total, since Section 304 adds up every appliance in the space. It is not a Category I appliance, so IFGC 304.1 also sends you to its installation manual, which governs where it asks for more.
When do I have to use the known infiltration rate method?
When the infiltration rate of the building is known to be below 0.40 air changes per hour. The method uses 21 cu ft per 1,000 BTU/h divided by ACH for natural draft appliances and 15 for fan-assisted ones, and the code does not allow a rate above 0.60 ACH in the equations.
Can I use one opening instead of two?
Yes, if it goes to the outdoors and starts within 12 inches of the top of the enclosure. It needs 1 sq in per 3,000 BTU/h and at least the total area of the vent connectors in the room, and the appliances need 1 inch clearance at the sides and back and 6 inches in front.
Does a water heater closet need combustion air?
Yes, the same Section 304 rules apply to a natural draft water heater. A closet is usually far too small for the volume, so it gets two openings to the next room or to the outdoors.
How big should the louver be?
Size the free area first, then the louver. Use the free area the louver maker publishes. If it is not known, the code assumes 75 percent for metal and 25 percent for wood, so a 35 sq in free area needs about 47 sq in of metal grille.
Can I use a fan to bring in combustion air?
Yes. IFGC 304.9 allows a mechanical supply from outdoors of at least 0.35 CFM per 1,000 BTU/h of input, interlocked so the main burners cannot run when the fan is off.
Used on the same job
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The chat does not size combustion air. It runs the free whole-house Manual J heating and cooling load, so you know how much heat the house needs at its design temperature. The input rating of the furnace you install is the number to bring back to this page.
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