Boiler Combustion Air Calculator
Combustion air openings for a gas or oil boiler room.
Enter your values
Enter the total appliance input in BTU/hr.
Choose one of: Direct outdoor openings, Ducted from outdoors (1 sq in / 2000 BTU).
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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)
At a glance
| What it does | Combustion air openings for a gas or oil boiler room. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Total appliance input, Air source path |
| Main output | Free area per opening |
| Formula | Free area (sq in) = input BTU/hr / 4000, per opening (outdoor air, direct) |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Total appliance input(BTU/hr)
- Enter the total appliance input in BTU/hr.
- Air source path
- Choose one of: Direct outdoor openings, Ducted from outdoors (1 sq in / 2000 BTU).
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.
What do I need to enter into the Boiler Combustion Air Calculator?
Just 2 values: total appliance input, air source path. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Boiler Combustion Air Calculator work out the 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). It applies the formula Free area (sq in) = input BTU/hr / 4000, per opening (outdoor air, direct) and shows the working so you can check each step by hand.
Is the Boiler Combustion Air Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.