Combustion Air Calculator

It depends on the room, and on every other gas appliance sharing it. The IFGC unconfined-space rule is 50 cubic feet of room volume per 1,000 Btu/h of total input, so an 80,000 Btu/h furnace alone needs a space of at least 4,000 cubic feet to draw its air from the room. If the space is smaller than that, it is confined and needs combustion-air openings sized by the method you choose. And if a water heater shares the space, its input adds to the furnace's before you size anything: the most common and most dangerous mistake here is sizing on the furnace alone and starving the water heater.

50 cubic feet per 1,000 Btu/h of aggregate appliance input, for the standard indoor-volume method (IFGC 304.5.1). That is the confined-versus-unconfined threshold: a room with at least that much volume can supply combustion air from the room itself, and a smaller room is confined. There is also a known-air-infiltration-rate method (IFGC 304.5.2) that can permit a smaller room when the building's tightness is known, using 21 cubic feet per air change for draft-hood appliances and 15 for fan-assisted, but it caps the infiltration rate at 0.60 air changes per hour.

By the method and the appliance total. For outdoor air through two openings, each opening is 1 square inch per 4,000 Btu/h for a direct opening or a vertical duct, or 1 square inch per 2,000 Btu/h for a horizontal duct, which doubles it. A single outdoor opening is 1 square inch per 3,000 Btu/h and must also be at least the sum of the vent-connector areas. All-indoor openings are 1 square inch per 1,000 Btu/h with a 100 square inch minimum. In the 120,000 Btu/h example on this page, two vertical outdoor openings come out to 30 square inches of net free area each.

Net free area is the actual open area air passes through. Gross area is the size of the hole you cut, and it is larger because the louver or screen blocks part of it. A metal louver passes about 75 percent of its gross area and a wood louver only 25 percent, so a required 30 square inch net opening through a metal louver becomes 80 square inches gross. Cutting the hole at the net size undersizes the opening by a quarter to three quarters, which starves combustion. Always cut to the gross size, and use the louver's actual marked free area if it has one.

The appliance cannot get enough oxygen to burn cleanly. Incomplete combustion produces carbon monoxide, and in a tight space the appliance can backdraft, pulling its own flue gases back into the room instead of up the vent. That is the reason this is a code requirement and not a suggestion: it is a carbon-monoxide safety issue, not a fire one. A calculator sizes the code-required opening, but it cannot confirm the space is actually safe. That takes a draft, spillage, or CO test on the running appliances, and the authority having jurisdiction and the appliance instructions have the final say.

A direct-vent, sealed-combustion appliance pulls its combustion air directly from outdoors through its own pipe, so it does not draw from the room and this room-air sizing does not apply to it. But it still matters on the same job: when a high-efficiency furnace replaces an old atmospheric one, an atmospheric water heater often stays behind in the same space, and now that water heater may be the only appliance left drawing combustion air from a room that used to share it. Re-run the space with only the remaining atmospheric appliances, because the answer changes when the furnace leaves.