2021 IFGC Chapter 3
A flame is a slow, steady chemical reaction that needs a constant supply of fresh air and a place to send the exhaust. Starve it and the appliance makes carbon monoxide; pull the wrong air through the wrong room and it can spill flue gas into the occupied space. This lesson covers the two questions that come first on any gas-fired installation: where may the appliance sit (2021 IFGC Sections 303 and 305), and how does its air get there (Section 304). IMC Chapter 7 is only a short pointer back to the fuel gas code. Lesson 1 explains why you must confirm which mechanical and fuel gas codes your city has adopted, so check the local edition before you cite any number below.
Section 303.3 bans fuel-burning appliances from sleeping rooms, bathrooms, toilet rooms, storage closets and surgical rooms, and from any space that opens only into one of them. A small storage closet with a natural-draft water heater is the classic rehab violation. The section then lists narrow ways around the ban. A direct-vent appliance installed to its listing is allowed. A room that opens only into a bedroom or bathroom can hold an appliance if it has no other use, has a solid weather-stripped door with a self-closing device, and takes its combustion air straight from outdoors. Vented room heaters and wall furnaces are allowed in rooms that pass the indoor volume test of Section 304.5, and small wall-mounted unvented heaters are allowed only up to stated inputs (6,000 Btu/h in a bathroom, 10,000 Btu/h in a bedroom).
Appliances in a hazardous location must be listed and approved for it (303.2), and an appliance a vehicle could hit needs approved protection (303.4). Furnaces and boilers in closets and alcoves must be listed for that installation (303.5), and outdoor appliances must be listed for outdoors or protected from weather (303.6). In a pit, the appliance stays 12 inches from the walls and out of contact with soil, and pits deeper than 12 inches get lined walls (303.7).
Gasoline vapor is heavier than air and pools near the floor, so the source of ignition must sit at least 18 inches above the floor in hazardous locations and in garages and fuel-dispensing areas (305.3), unless the appliance is listed as flammable vapor ignition resistant. In public and repair garages an appliance also goes at least 8 feet above the floor, and 1 foot above the tallest garage door opening if vehicles can pass beneath it (305.4). In a private garage the height is 6 feet (305.5). Both can be relaxed when the appliance is protected from impact and its ignition source is elevated. An appliance on grade sits on a level slab that extends at least 3 inches above adjoining grade, or is suspended at least 6 inches above it (305.7).
Section 304 sizes air for ordinary natural-draft appliances. Direct-vent appliances, appliances of other than natural-draft design, vented appliances that are not Category I, and appliances with power burners take their air as the manufacturer directs. A modern condensing boiler with a sealed intake pipe is in that group, but every other appliance in the room is still judged on its own. A draft hood or barometric regulator goes in the same room as its appliance (304.3), and makeup air is required where exhaust fans, dryers or kitchen ventilation interfere with an appliance (304.4).
The standard method wants 50 cubic feet of room volume for every 1,000 Btu/h of input (304.5.1), added up over all appliances in the space. Rooms joined to the appliance room through openings without doors, and openings sized as below, count toward that volume. A known-infiltration method (304.5.2) is required where the building is tighter than 0.40 air changes per hour, but its equations belong to the design professional. Openings between spaces on the same story come in a pair: each has at least 1 square inch of free area per 1,000 Btu/h of total input and never less than 100 square inches, one starting within 12 inches of the top of the enclosure and one within 12 inches of the bottom, with a least dimension of 3 inches (304.5.3.1). Spaces on different stories count as communicating when permanent openings in doors or floors total at least 2 square inches per 1,000 Btu/h (304.5.3.2).
Openings to outdoors also need a least dimension of 3 inches. The two-permanent-openings method uses one opening starting within 12 inches of the top and one within 12 inches of the bottom. Each opening is 1 square inch per 4,000 Btu/h when it opens straight outdoors or through vertical ducts, and 1 square inch per 2,000 Btu/h when it reaches outdoors through horizontal ducts, because a long horizontal run resists flow (304.6.1). The one-permanent-opening method uses a single opening starting within 12 inches of the top, sized at 1 square inch per 3,000 Btu/h and not less than the total area of all vent connectors in the space. The appliance must also have at least 1 inch of clearance at the sides and back and 6 inches at the front (304.6.2).
When the room is partly big enough, the outdoor openings can shrink. Divide the available volume of all communicating spaces by the required volume to get the ratio of interior spaces. Subtract that ratio from one to get the reduction factor, then multiply the full outdoor opening size from 304.6 by the factor (304.7.3).
Where a fan supplies all the combustion air, it must deliver at least 0.35 cubic feet per minute per 1,000 Btu/h of total input from outdoors, and each appliance is interlocked so the burner cannot run when the fan is off (304.9). Louvers and grilles are sized on net free area. If the maker does not say, assume 25 percent free area for wood louvers and 75 percent for metal louvers and grilles, and use screen mesh no finer than 1/4 inch (304.10). Nonmotorized louvers are fixed open. Motorized louvers must be proven fully open before the burner lights and must shut the burner down if they close (304.10). Ducts are galvanized steel or equal, serve a single enclosure, keep upper and lower ducts apart, and a horizontal upper duct may not slope down toward the source. An exterior intake stays at least 12 inches above finished ground (304.11). Under IMC Section 701.2, dampers in combustion air openings are interlocked with the firing cycle, and manual dampers are not allowed in combustion air ducts.
Worked example. A community center mechanical room holds a 200,000 Btu/h boiler and a 100,000 Btu/h water heater, both natural draft: 300,000 Btu/h in all. Volume test: 300 x 50 = 15,000 cubic feet are required. The room is 12 by 15 feet with a 9 foot ceiling, only 1,620 cubic feet, so it fails on its own. Outdoor, two openings straight to the outside: 300,000 / 4,000 = 75 square inches of net free area each. A metal louver with unknown free area counts as 75 percent, so each louver is 75 / 0.75 = 100 square inches gross, about 10 by 10 inches. With horizontal ducts it is 300,000 / 2,000 = 150 net square inches each, and one top opening needs 300,000 / 3,000 = 100. Combined: the equipment room plus an adjoining storage room total 6,000 cubic feet. The ratio is 6,000 / 15,000 = 0.4, the reduction factor is 1 - 0.4 = 0.6, and each outdoor opening becomes 75 x 0.6 = 45 net square inches. The indoor openings to the storage room are then 300,000 / 1,000 = 300 square inches each, and no smaller than 100.
In the field
A crew rehabbing a community center replaces an old atmospheric boiler with a sealed-combustion condensing boiler, then closes the wall louvers because "the new one pipes its own air." The same room still holds a natural-draft gas water heater that depended on those louvers. Starved of air, the heater can backdraft and spill carbon monoxide. Count every appliance in the room, and ask what else is in it before you sign off a louver removal.
1. A Group B office building has a mechanical room with a 90,000 Btu/h natural-draft boiler and a 60,000 Btu/h natural-draft water heater. Using the standard indoor method in IFGC 304.5.1, what room volume is required?
2. An equipment room in a Group A-3 community hall holds 240,000 Btu/h of natural-draft appliances. Two permanent openings connect directly to outdoors. What net free area does each opening need under IFGC 304.6.1?
3. The same 240,000 Btu/h equipment room gets its outdoor air through horizontal ducts. What net free area does each of the two openings need?
4. A designer proposes a single outdoor combustion air opening near the top of a boiler room in a Group B clinic. The total input is 240,000 Btu/h and the vent connector areas are small. What minimum free area does IFGC 304.6.2 set?
5. Two small natural-draft appliances totaling 60,000 Btu/h share a room with a Group R-2 laundry on the same story, and the room will get its air from indoors through two openings. What is the minimum free area of each opening?
6. Each outdoor opening must supply 75 square inches of net free area. The designer picks wood louvers and the maker does not publish the free area. What gross opening area is needed under IFGC 304.10?
7. An inspector in a Group F-1 shop finds combustion air openings to outdoors in a boiler room. The upper opening starts 30 inches below the ceiling. Under IFGC 304.6.1, this installation:
8. A contractor sets a natural-draft gas water heater in the toilet room of a Group B clinic. The heater is not direct-vent. Under IFGC 303.3, this location is:
9. A gas-fired unit heater in a Group S-1 repair garage has an ignition source, but it is not listed as flammable vapor ignition resistant. Vehicles can pass under it. At a minimum, how high above the floor must an appliance be under IFGC 305.4 unless the impact-protection exception is used?
10. A gas-fired boiler is being placed on an outdoor pad beside a Group A-3 community building. The pad rests on grade. Under IFGC 305.7, the slab must extend at least how far above adjoining grade?
11. A boiler room needs a full-size outdoor opening of 60 square inches each for two openings. The equipment room and an adjoining space total 5,000 cubic feet available, and the required volume is 10,000 cubic feet. Under IFGC 304.7.3, what is each outdoor opening?
12. A Group B building supplies all the combustion air for a 400,000 Btu/h boiler room by fan from outdoors. Under IFGC 304.9, what is the minimum supply rate?
13. On a rehab of a Group A-3 community hall, the crew replaces a natural-draft boiler with a sealed-combustion condensing boiler and closes the room's louvers. A natural-draft water heater remains in the room. Under IFGC 304.1, the louvers:
14. A combustion air supply to a boiler room in a Group I-1 facility uses a motorized louver. Under IFGC 304.10, what must be true before the main burner ignites?