2021 IRC Section G2407
Every fuel-burning appliance needs air from somewhere, and G2407 tells an inspector or construction manager where that air is allowed to come from and how much of it there has to be. The air does three jobs at once: combustion air feeds the flame, ventilation air keeps the enclosure from overheating, and dilution air enters a draft hood to stabilize draft and keep flue gases moving up the vent instead of spilling into the room. Starve any one of the three and the same failure shows up: incomplete combustion, sooting, poor draft, and carbon monoxide. This lesson works through the volume and opening-sizing methods GCHS staff use to judge whether a furnace closet or mechanical space has what it needs, one of the most common findings in older rehab houses.
The simplest test is whether the room itself is big enough. Under the standard, or indoor air, method a space qualifies on its own volume if it provides at least 50 cubic feet of room volume for every 1,000 Btu/h of total input of every fuel-burning appliance in it. Only appliances that actually draw air from the space count; a direct-vent appliance, which takes its air from outdoors through its own sealed piping, is left out entirely. A small interior furnace closet almost never has enough volume to pass this test, which is why the code gives two other paths: openings to additional indoor volume, or a connection to outdoor air.
Where the appliance space is too small by itself, it can be connected to enough additional indoor volume through two permanent openings, one within 12 inches of the top of the enclosure and one within 12 inches of the bottom (G2407.5.3.1). That high-low placement is not decorative: cooler air enters low toward the burner while warmer air leaves high, setting up the circulation the calculation assumes. Each opening on the same story needs a free area of at least 1 square inch per 1,000 Btu/h of total input, with a floor of 100 square inches per opening no matter how small the appliance is. Where the connected spaces are on different stories, the permanent openings in doors or floors need a combined free area of 2 square inches per 1,000 Btu/h of total input (G2407.5.3.2), double the same-story figure, since a floor or ceiling between the spaces makes air exchange far less reliable. Two rooms only count as connected, or communicating, through openings that have no door; a doorway with a door that can close does not qualify, even if it is usually left open.
Outdoor air is an effectively unlimited supply, so openings that reach it can be smaller than the indoor equivalents. Two permanent openings direct to the outdoors, or through vertical ducts, need 1 square inch per 4,000 Btu/h each; through horizontal ducts instead, 1 square inch per 2,000 Btu/h each, twice the vertical figure, since a horizontal run gets no help from buoyancy and adds friction (G2407.6.1). A single permanent-opening method is also allowed: one opening within 12 inches of the top of the enclosure, sized at 1 square inch per 3,000 Btu/h and not less than the sum of the areas of all vent connectors in the space, but only with at least 1 inch of clearance on the appliance's sides and back and 6 inches in front, so air can circulate around the unit to the burner (G2407.6.2). Any opening must have a minimum dimension of not less than 3 inches, since a long narrow slot clogs easily in the field. Under G2407.11, ducts are galvanized steel or equivalent, each duct serves a single enclosure only, ducts serving the upper and lower openings are kept separate all the way to the source of air, and a horizontal upper duct may not slope downward toward the source of combustion air, which would trap warm air instead of relieving it. An outdoor opening's lowest edge sits not less than 12 inches above grade, any screen must have openings not smaller than 1/4 inch mesh, and a duct terminating in an attic is never screened there, since a screen would only collect blown insulation until it blocks.
Every opening size in this section is a free area, the portion actually open to airflow, not the gross size of the hole cut in the wall or door. Where a louver or grille's actual free area is not published, the code assumes a wood louver passes only 25 percent of its gross area and a metal one passes 75 percent, so a wood-louvered door needs roughly four times the gross opening a calculated free-area requirement would suggest. A manufacturer's actual tested free area is used instead where it is published. Non-motorized louvers must be fixed open, since an adjustable one only invites someone to close it; motorized louvers are allowed only when interlocked with the appliance so the burner cannot fire unless the damper is proven open.
A third method credits the air a house naturally leaks through its envelope instead of relying on a fixed volume or dedicated openings. It uses a volume constant, 21 cubic feet for a non-fan-assisted appliance or 15 cubic feet for a fan-assisted one, divided by the space's air changes per hour, times the total input in thousands of Btu/h. The fan-assisted constant is smaller because a power burner meters its own air rather than depending entirely on natural draft, so it needs less volume. The method has one hard limit: an infiltration rate greater than 0.60 air changes per hour may never be used, even where a blower-door test measures higher, since assuming too leaky a house is exactly the mistake that starves an appliance later. The method also becomes mandatory, instead of the standard method, where the infiltration rate is known to be less than 0.40 air changes per hour (G2407.5).
A direct-vent, sealed-combustion appliance draws all combustion air from outdoors through its own intake piping and is provided with air per the manufacturer's instructions (G2407.1), so it needs no room openings, making it the standard fix wherever a combustion air problem cannot practically be solved another way. Location matters as well: an appliance may not be located in a sleeping room, bathroom, toilet room, storage closet, or surgical room, or in a space that opens only into such rooms (G2406.2). The exceptions include a direct-vent appliance installed per its listing and instructions, and an appliance in a room or space that opens only into a bedroom or bathroom, is used for no other purpose, and has a solid weather-stripped door with an approved self-closing device, with combustion air taken directly from outdoors. A combustion air grille may never be cut through the wall separating a dwelling from an attached garage, since openings between the garage and the residence are limited to doors of specified construction (R302.5.1). A garage itself is not a prohibited location, so an appliance there is sized by the ordinary methods like any other space.
In the field
A recurring finding in GCHS rehab work is a furnace closet where a solid door has replaced an original louvered one, or a homeowner has stuffed insulation into a grille "to stop the draft," quietly removing the appliance's air supply years after inspection. Weatherization compounds the problem: air sealing and new windows remove the leakage that used to feed an older natural-draft appliance informally, so an energy-efficiency scope on such a house should be paired with a combustion air and draft check. The fastest permanent fix is often to replace the natural-draft appliance with a direct-vent unit, which removes the room-air question altogether.
Worked example: a rehab utility room holds a 90,000 Btu/h furnace and a 40,000 Btu/h water heater, both natural draft, for a total input of 130,000 Btu/h. Under the standard method the room would need 130 × 50 = 6,500 cubic feet, far more than a typical closet provides. Two openings to the adjacent indoor space instead need 130 square inches of free area each (130,000 ÷ 1,000), which controls over the 100-square-inch floor. The same two openings run directly to the outdoors would only need 130,000 ÷ 4,000 = 32.5 square inches each, which is why an outdoor connection is usually the more practical retrofit in a tight closet.
1. An inspector finds a furnace and water heater installed in a small interior closet with a solid, gasketed door and no grilles. Both appliances are natural draft. Based on the general combustion air rule, this installation is:
2. Under the standard indoor-air method, what minimum room volume is required per 1,000 Btu/h of total appliance input?
3. Two indoor combustion air openings connecting a furnace closet to the house are sized under the indoor method. Each opening's minimum free area is 1 square inch per 1,000 Btu/h, with an absolute floor of:
4. A rehab crew adds a second gas appliance to an existing furnace closet that previously had adequate combustion air openings sized for one appliance. What must happen to the openings?
5. A furnace closet uses two openings directly to the outdoors. Each opening's minimum free area is:
6. Compared to two openings run through vertical ducts, two openings run through horizontal ducts to the outdoors must be:
7. An inspector finds a wood louver of unknown free area covering a combustion air opening, sized exactly to the calculated free-area requirement. This is:
8. Metal louvers or grilles of unknown free area are assumed to pass what percentage of their gross area for sizing purposes?
9. The known air-infiltration-rate method may not be used where the space's air infiltration rate exceeds:
10. Compared with a non-fan-assisted appliance, the known-air-infiltration-rate method's volume constant for a fan-assisted appliance is:
11. During a weatherization retrofit, new windows and extensive air sealing are added to a house with a natural-draft water heater that previously passed inspection. The concern is:
12. A natural-draft gas appliance may not be located in which of these spaces under the general rule?
13. A sealed-combustion, direct-vent furnace is installed in a small interior closet with no grilles. Regarding combustion air, this is:
14. An inspector finds a combustion air grille to a water heater closet stuffed with fiberglass insulation to "stop the draft." The most likely consequence if left in place is: