space shall be considered in making this determina- tion. Each opening shall have a minimum free area of 1 in.2 per 1,000 BTUH (2,225 mm2 per kW) of the total input rating of all gas utilization equipment in the con- fined space, but not less than 100 in.2 (645 cm2). One opening shall commence within 12 in. (305 mm) of the top, and one opening shall commence within 12 in. (305 mm) of the bottom of the enclosure. The mini- mum dimension of air openings shall be not less than 3 in. (76 mm) in any direction.
All Air from Outdoors
The confined space shall communicate with the out- doors in accordance with one of the methods below. The minimum dimension of air openings shall not be less than 3 in. (76 mm) in any direction. Where ducts are used, they shall be of the same
1.Two permanent openings, one commencing within 12 in. (305 mm) of the top, and one com- mencing within 12 in. (305 mm) of the bottom of the enclosure, shall be provided. The openings shall communicate directly, or by ducts, with the outdoors or spaces (crawl or attic) that freely com- municate with the outdoors.
a.Where directly communicating with the out- doors or where communicating to the outdoors through vertical ducts, each opening shall have a minimum free area of 1 in.2 per 4,000 BTUH (550 mm2 per kW) of total input rating of all equipment in the enclosure.
b.Where communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of 1 in.2 per 2,000 BTUH (1,100 mm2 per kW) of total input rat- ing of all equipment in the enclosure.
2.One permanent opening, commencing within 12 in. (305 mm) of the top of the enclosure, shall be permitted where the equipment has clearances of at least 1 in. (25 mm) from the sides and back and 6 in. (152 mm) from the front of the appliance. The opening shall directly communicate with the out- doors or shall communicate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with the outdoors, and shall have a minimum free area of:
a.1 in.2 per 3,000 BTUH (740 mm2 per kW) of the total input rating of all equipment located in the enclosure, and
b.Not less than the sum of the areas of all vent connectors in the confined space.
WARNING: Do not use one permanent opening method if the equipment room is under negative pressure conditions or the equipment is common vented with other
Canadian Installations
CAUTION: All combustion air must be drawn from the air outside of the building; the mechanical equip- ment room must communicate directly with the outdoors.
1.Ventilation of the space occupied by the heater shall be provided by an opening(s) for ventilation air at the highest practical point communicating with the outdoors. The total
2.For heaters using a barometric damper in the vent system there shall be a permanent air supply opening(s) having a cross section area of not less than 1 in.2 per 7,000 BTUH (320 mm2 per kW) up to and including 1 million BTUH, plus 1 in.2 per 14,000 BTUH (160 mm2 per kW) in excess of 1 million BTUH. This opening(s) shall be either located at or ducted to a point not more than 18 in. (450 mm) nor less than 6 in. (152 mm) above the floor level. The duct can also “goose neck” through the roof. The duct is preferred to be straight down and terminated 18 in. (450 mm) from the floor, but not near piping. This air supply opening require- ment shall be in addition to the air opening for ventilation air required in 1. (above).
WARNING: Care must be taken to ensure that the equipment room is not under negative pressure conditions or that the equipment is not common- vented with other
3.For heaters not using a barometric damper in the vent system, and when air supply is provided by natural air flow from outdoors for a power burner and there is no draft regulator, drafthood or similar flue gas dilution device installed in the same space, in addition to the opening for ventilation air required in 1., there shall be a permanent air sup- ply opening(s) having a total
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