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Step 5: If Indoor Combustion Air is Used, Provide Air as Follows:
1) Buildings of other than unusually tight construction:
Unconfi ned Space– Natural infi ltration into the boiler room will normally provide adequate air for combustion and
ventilation without additional louvers or openings into boiler room.
Confi ned Space – Provide two openings into the boiler room, one near the fl oor and one near the ceiling. The top edge of
the upper opening must be within 12” of the ceiling and the bottom edge of the lower opening must be within 12” of the
fl oor (Fig 3).
• Each opening must have a free area of 1 square inch per 1000 BTU/hr input of all gas burning appliances in the boiler
room. The minimum opening dimension is 3 inches. Minimum opening free area is 100 square inches per opening.
• If the total volume of both the boiler room and the room to which the openings connect is less than 50 cubic feet per 1000
BTU/hr of total appliance input, install a pair of identical openings into a third room. Connect additional rooms with
openings until the total volume of all rooms is at least 50 cubic feet per 1000 BTU/hr of input.
• The “free area” of an opening takes into account the blocking effect of mesh, grills, and louvers. Where screens are used,
they must be no fi ner than ¼” (4 x 4) mesh.
2) Buildings of unusually tight construction:
If at all possible, direct vent the boiler. Where the boiler must be installed in unusually tight construction and cannot be
direct vented, openings must be installed between the boiler room and the outdoors or a ventilated space, such as an attic
or crawl space, which communicates directly with the outdoors. Two openings are required. The top edge of the upper
opening must be within 12 inches of the ceiling. The bottom edge of the lower opening must be within 12 inches of the
fl oor. Size openings and ducts as follows:
• Vertical ducts or openings directly outdoors (Fig 4, Fig 5, Fig 6) – Each opening must have a free cross sectional area of
1 square inch per 4000 BTU/hr of the total input of all gas-fi red appliances in the boiler room but not less than 100 square
inches. Minimum opening size is 3 inches.
• Openings to outdoors via horizontal ducts (Fig 7) - Each opening must have a free cross sectional area of 1 square inch
per 2000 BTU/hr of the total input of all gas fi red appliances in the boiler room but not less than 100 square inches.
Minimum opening size is 3 inches.
• The “free area” of an opening takes into account the blocking effect of mesh, grills, and louvers. Where screens are used,
they must be no fi ner than ¼” (4 x 4) mesh.
FIGURE 3: BOILER INSTALLED IN A CONFINED SPACE - VENTILATION OR
COMBUSTION / VENTILATION AIR FROM INSIDE