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Combustion Air Room Volume Check

Fuel-burning appliances need air to burn safely. Check whether the room alone has enough volume under the standard 50 cubic feet per 1,000 BTU/h rule.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Add the input (not output) ratings from each appliance’s rating plate: furnace or boiler, water heater, dryer and so on.

Result

Not enough room volume

Minimum volume by the 50 ft³ per 1,000 BTU/h rule
5,000ft³
Room volume
2,400ft³

2,400 ft³ available vs 5,000 ft³ needed — not enough room volume.

  • The 50 ft³ per 1,000 BTU/h rule applies only where the building’s air infiltration is not known to be below 0.40 air changes per hour; tighter houses need a different calculation. The fuel gas code adopted where you live, and the appliance manufacturer’s instructions, govern — and where they differ, the more restrictive applies.
  • The room alone is too small for indoor combustion air. A qualified heating contractor or gas fitter must provide combustion air another way (from adjoining spaces or outdoors, as the code allows). Do not block or close off combustion-air openings.
Room volume as share of the minimum
48%
Largest total input this volume supports
48,000BTU/h
Show the working
  1. Volume = length × width × height
    20 × 15 × 8 = 2,400 ft³
  2. Minimum volume = 50 ft³ × input ÷ 1,000 BTU/h
    50 × 100,000 ÷ 1,000 = 5,000 ft³

The volume rule

The DOE-sponsored Standard Work Specifications for home energy upgrades state a minimum volume of 50 cubic feet per 1,000 BTU/h of appliance input for combustion air drawn from indoors. Add the input ratings — not the outputs — of every fuel-burning appliance in the space: a 100,000 BTU/h furnace and a 40,000 BTU/h water heater together need 7,000 ft³.

The rule does not apply where the building’s air infiltration is known to be below 0.40 air changes per hour; tighter houses need a different calculation from the fuel gas code.

If the room is too small

A room that fails the volume check needs combustion air from adjoining spaces or from outdoors, through openings or ducts designed to the fuel gas code adopted where you live and to the appliance manufacturer’s instructions. Where the two differ, the more restrictive applies.

That design and any changes to a gas appliance installation are work for a licensed professional. Never block combustion-air openings, and install carbon monoxide alarms.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Total input rating of all fuel-burning appliances in the space
100,000 BTU/h
Room size known as
Length, width and height
Room length
20 ft
Room width
15 ft
Ceiling height
8 ft

Result

Minimum volume by the 50 ft³ per 1,000 BTU/h rule
5,000 ft³
Room volume
2,400 ft³
Room volume as share of the minimum
48 %
Largest total input this volume supports
48,000 BTU/h

2,400 ft³ available vs 5,000 ft³ needed — not enough room volume.

Assumptions

  • Standard method for buildings of ordinary tightness: at least 50 ft³ of room volume per 1,000 BTU/h of total appliance input.

Limitations

  • Does not size combustion-air openings or ducts, decide which adjoining rooms may count, or handle houses tighter than 0.40 ACH — those follow the adopted fuel gas code.
  • This is a screening check, not an inspection. Gas appliance work must be done by a licensed professional; if you smell gas or suspect carbon monoxide, leave the building and call for help.

Frequently asked questions

How big a room does a 100,000 BTU/h furnace need?

At least 5,000 ft³ under the 50 ft³ per 1,000 BTU/h rule — for example 25 × 25 ft with an 8 ft ceiling.

Which appliances should I add up?

Every fuel-burning appliance that draws its combustion air from the space. Appliances that bring their combustion air in through their own outdoor intake work differently; the appliance’s installation instructions and the adopted code say how to treat them.

Can I count the rest of the basement?

Only where the code allows the spaces to be counted together, which depends on the openings between them. Ask a qualified professional.

Sources

  1. Standard Work Specifications 5.0502.1a — Combustion Air, Fuel-Fired Appliances (design) — National Laboratory of the Rockies (formerly NREL), U.S. DOE. Minimum required volume is 50 cubic feet per 1,000 BTU/h, except where infiltration is known to be below 0.40 ACH (use the IFGC alternate calculation); where code and manufacturer differ, the more restrictive applies; conform to NFPA 54, IFGC or NFPA 31.

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