Combustion Air Calculator
BTU input to combustion air opening sizes.
What it does
Gas flames breathe room air, and starved flames make carbon monoxide instead of heat. Combustion air openings feed burners steadily: two permanent grilles sized to total BTU input, one high and one low. Math sizes them in seconds.
The tool above divides total BTU/hr by 1000 for each opening with a 100 sq in floor, using the two-opening indoor-air method. A 150,000 BTU room needs two 150 sq in grilles of free area through louvers.
Fuel-gas codes in the NFPA 54 and IFGC families publish this method beside outdoor-air and mechanical alternatives with smaller divisors. Tight modern rooms often fail indoor-air volume tests, which pushes jobs to outdoor openings.
Add every nameplate in the room: furnace, water heater, dryer, range. Then confirm the method fits: volume, tightness, and louvers all change the answer, and inspectors enforce the adopted code's version.
How to use
- Add nameplate BTU/hr in the room.
- Enter the total input.
- Read each opening size in sq in.
- Cut one high and one low opening.
- Use free area through louvers.
- Confirm method with the inspector.
Formula
- BTU
- Combined appliance input in BTU/hr.
Two-opening indoor-air method only: one opening high, one low. Outdoor-air, single-opening, and mechanical methods size smaller with their own divisors. Louvered free area counts, not frame size.
Worked example
Example: 150,000 BTU/hr
150,000 BTU/hr of gas appliances in one room.
- Appliances total 150,000 BTU/hr input.
- Method: two openings at 1 sq in per 1000 BTU.
- Each: 150000 / 1000 = 150 sq in.
- Minimum checks 100 sq in: 150 passes.
- Cut two 150 sq in openings, high and low.
Reference table
| Input | Each opening | Note |
|---|---|---|
| 80,000 BTU/hr | 100 sq in | Minimum binds |
| 100,000 BTU/hr | 100 sq in | At minimum |
| 150,000 BTU/hr | 150 sq in | Typical home |
| 200,000 BTU/hr | 200 sq in | Large home |
| 300,000 BTU/hr | 300 sq in | Multi-appliance |
Limits
- Indoor-air method only. Other methods differ.
- Needs qualifying room volume and infiltration.
- Louver free area, not frame, counts.
- Inspector confirms the adopted method.
Common mistakes
- Counting one opening instead of two.
- Sizing frames instead of free area.
- Ignoring the 100 sq in minimums.
- Blocking vents with storage.
- Missing appliances in the total.
Try it yourself
Quick checks. Answers sit tucked under each question.
Openings for 80,000 BTU/hr?
80000 / 1000 = 80, under the 100 minimum, so 100 sq in each.
Openings for 300,000 BTU/hr?
300000 / 1000 = 300 sq in each, two openings.
Questions
What is the two-opening method?
Two permanent openings, one high and one low, each 1 square inch per 1000 BTU/hr of input with 100 sq in minimums. A 150,000 BTU room needs two 150 sq in grilles.
When does indoor air qualify?
Volume over 50 cubic feet per 1000 BTU/hr generally qualifies as unconsumed indoor air; tighter or smaller rooms need outdoor air by other methods. Remodels that tighten envelopes change the answer.
What other methods exist?
Outdoor air through vertical ducts (1 per 4000), horizontal (1 per 2000), single opening (1 per 3000), or mechanical supply. Each suits different layouts with smaller holes than indoor air.
Why one high and one low?
Yes: one feeds the burner while the other vents fumes and heat. Blocking either starves flames into carbon monoxide. Keep both clear of storage and snow.
Which appliances count?
BTU/hr input from every nameplate in the room: furnace, water heater, dryer, range. Add them all; the room feeds the hungriest combination running together.
What fails inspections most?
Undersized openings, single openings where two are required, screens that clog, and louvered doors counted at face value. Free area means through the louvers, not the frame.