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SBN71-120 thru 85-390, SBD30-150/199 TANK TYPE COMMERCIAL
GAS WATER HEATER SERVICE HANDBOOK
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A fresh supply of make-up air for combustion can be supplied to the heater through
make-up air ducts, which directly communicate with the outdoors.(Not Direct Vent)
1 SQUARE INCH PER
4,000 Btu FOR EACH
OPENING
Two openings are required: one within 12 inches of the top of the enclosure and one within 12
inches of the bottom of the enclosure. Each opening must have a free area of not less than 1 square
inch per 4,000 BTUH of the total input of all appliances within the enclosure. The lower opening
primarily provides combustion air. The upper opening provides vent dilution air and acts as a relief
opening for flue gases should the vent become obstructed or a downdraft condition occur.
Additionally, when the heater is installed in a co
nfined space and communicating with the outdoor air,
one permanent opening, beginning within 12 inches (30 cm) of the top of the enclosure, must be
permitted where the equipment has clearances of at least 1 inch (2.5 cm) from the sides and back,
and 6 inches (16 cm) from the front of the appliance. The opening must directly communicate with the
outdoors and must communicate through a vertical or horizontal duct to the outdoors or spaces (crawl
or attic) that freely communicate with the outdoors, and must have a minimum free area of a) 1
square
inch per 3,000 BTUH (7cm2 per kW) of the total input 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.
MAKE-UP AIR – DIRECT COMMUNICATION WITH OUTDOORS