Boiler Combustion Air Calculator
Combustion air openings for a gas or oil boiler room.
Enter your values
ⓘ Live results
Results
- Free area per opening
- 100 sq in
- Two openings required
- High and low, each this size
- Approx square opening
- 10 x 10 in
- Louvre derate
- Multiply by ~2x if opening is behind a louvre (50% free area typical)
Quick answer
For a boiler room using outdoor combustion air, code typically requires 1 sq in of free area per 4,000 BTU/hr input for each of two permanent openings (high and low).
Free area (sq in) = input BTU/hr / 4000, per opening (outdoor air, direct)
How to use the Boiler Combustion Air Calculator
- 1Total the BTU/hr input of all fuel-burning appliances in the room.
- 2Choose whether combustion air comes directly from outdoors or through ducts.
- 3Provide two permanent openings (high and low) each sized per the calculated free area, doubling for louvred openings.
Worked example
Using the values the calculator loads with:
Inputs
- Total appliance input400000 BTU/hr
- Air source pathDirect outdoor openings
Results
- Free area per opening100 sq in
- Two openings requiredHigh and low, each this size
- Approx square opening10 x 10 in
- Louvre derateMultiply by ~2x if opening is behind a louvre (50% free area typical)
Frequently asked questions
Why two openings?
One near the ceiling and one near the floor provide both combustion air and a natural chimney effect for the room.
Can I use indoor air instead?
Only if the surrounding space itself has adequate infiltration/volume per code; otherwise outdoor air is required.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.