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Calculation Machine

Boiler Combustion Air Calculator

Combustion air openings for a gas or oil boiler room.

Reviewed by the Calculation Machine team · How we check our maths

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).

The formula

Free area (sq in) = input BTU/hr / 4000, per opening (outdoor air, direct)

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Total appliance input400000 BTU/hr
  • Air source pathDirect outdoor openings

Result

  • 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)

How to use the Boiler Combustion Air Calculator

  1. 1Total the BTU/hr input of all fuel-burning appliances in the room.
  2. 2Choose whether combustion air comes directly from outdoors or through ducts.
  3. 3Provide two permanent openings (high and low) each sized per the calculated free area, doubling for louvred openings.

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.

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