Conventional Combustion Air Supply
U.S. Installations
All Air from Inside the Building
If all combustion air is drawn from the air inside the building (the mechanical equipment room does not receive air from outside):
1.The mechanical equipment room must be provid- ed with two permanent openings communicating directly with additional room(s) of sufficient volume so that the combined volume of all spaces meets the criteria for an unconfined space. (An uncon- fined space is defined as a space whose volume is more than 50 cubic feet per 1,000 BTUH of the aggregate input rating of all appliances installed in that space.)
2.Each opening must have a minimum free area of one (1) square inch per 1,000 BTUH of the total input rating of all gas utilization equipment in the mechanical equipment room.
3.One opening must commence within twelve (12) inches of the top, and one opening must com- mence within twelve (12) inches of the bottom of the room.
4.Refer to NFGC, Part 5, for additional information.
All Air from Outdoors
If all combustion air is drawn from the air outside the building (the mechanical equipment room directly communicates with the outdoors), either of the follow- ing two methods can be used:
Method 1:
1.The mechanical equipment room must be provid- ed with two permanent openings, one commencing within (twelve) 12 inches of the top, and one commencing within twelve (12) inches of the bottom of the room.
2.The openings must communicate directly, or by ducts, with the outdoors.
3.Each opening must have a minimum free area of one (1) square inch per 4,000 BTUH of total input rating of all equipment in the room when the open- ing is communicating directly with the outdoors or
through vertical ducts. The minimum free area required for horizontal ducts is one (1) square inch per 2,000 BTUH of total input rating to all the equipment in the room.
4.Refer to NFGC, Part 5, for additional information. Refer to Appendix A for additional information about combustion air quality.
Method 2 (normally applied in cold climate regions):
1.The mechanical equipment room must be provid- ed with at least one permanent opening, commencing within 12 inches of the top of the enclosure.
2.The opening must communicate directly or by ducts with outdoors.
3.The opening must have a minimum free area of 1 sq. in per 3000 BTUH of the total input rating of all equipment in the room, or no less than the sum of the areas of all vent connectors in the confined space. Refer to the latest version of NFGC, part 5 for additional information.
WARNING: Do not use one permanent opening method (Method 2) if the equipment room is under negative pressure condition or the equipment is common vented with other
Canadian Installations
CAUTION: All combustion air has to be drawn from the air outside the building (the mechanical equipment room directly communicates with the outdoors).
1.Ventilation of the space occupied by the boiler shall be provided by an opening(s) for ventilation air at the highest practical point communicating with outdoors. The total
2.When air supply is provided by natural air flow from the outdoors for natural draft, partial fan assisted,
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