Combustion Air Sizing Chart
How much combustion air a fuel-fired appliance needs and how to bring it in, per the IFGC, IRC, and NFPA 54. The standard method sizes by room volume — 50 cubic feet per 1,000 BTU/hr of input — and where the room is too small (a confined space), permanent openings supply the air at the ratios below. A starved appliance produces carbon monoxide, so this sizing is a safety essential.
Combustion air opening ratios
| Opening method | Free-area ratio |
|---|---|
| Outdoors — direct opening or vertical duct | 1 sq in per 4,000 BTU/h (each of two openings) |
| Outdoors — horizontal duct | 1 sq in per 2,000 BTU/h (each of two openings) |
| Outdoors — single opening | 1 sq in per 3,000 BTU/h (one opening) |
| Indoor adjacent space, same story | 1 sq in per 1,000 BTU/h (each; ≥ 100 sq in) |
| Indoor adjacent space, different stories | 2 sq in per 1,000 BTU/h (total input) |
Confined vs. unconfined, and why it matters
The first question is whether the space is big enough to supply combustion air on its own. Divide the room volume by the total appliance input in thousands of BTU/hr: 50 cubic feet per 1,000 BTU/hr or more and it is an unconfined space that may not need any openings; less than that and it is confined, and you must bring in air through permanent openings sized from the ratios above. The ratios reward the more effective air path — a straight shot outdoors or a vertical duct needs half the opening area of a horizontal duct, and drawing from another indoor room needs the most area of all because that room has to get its air from somewhere too.
This is not a comfort calculation — it is a combustion-safety one. An appliance that cannot get enough air burns incompletely, backdrafts, and produces carbon monoxide. Modern tight construction makes it worse, which is why direct-vent and sealed-combustion appliances, which pull their air straight from outdoors through a dedicated pipe, have largely replaced open-combustion units in new work.
Common questions
How much combustion air does a gas appliance need?
By the standard (indoor air) method, the room needs at least 50 cubic feet of volume per 1,000 BTU/hr of total appliance input. A space with less than that per 1,000 BTU/hr is a "confined space" and needs permanent openings to bring in outside or adjacent-space air. A 100,000 BTU/hr furnace, for instance, needs a room of at least 5,000 cubic feet to qualify as unconfined.
How do you size combustion air openings?
It depends on where the air comes from. Two openings direct to outdoors (or through vertical ducts) are sized at 1 square inch of free area per 4,000 BTU/hr each; horizontal ducts need twice that (1 sq in per 2,000 BTU/hr) because they move air less effectively. Openings to an adjacent indoor space are sized at 1 sq in per 1,000 BTU/hr and never smaller than 100 square inches each.
Where do the two combustion air openings go?
One opening within 12 inches of the top of the enclosure and one within 12 inches of the bottom, so air can circulate — cool combustion air low, warm air high. The minimum opening dimension is 3 inches, and you deduct for the louver: a metal louver passes about 75% of its gross area, a wood one only about 25%.
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