A. O. Smith Water Products Company Service Handbook
Irving, Texas 2000©
Training Department
6
MAKE-UP AIR
DIRECT COMMUNICATION
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 out of doors. 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 shall have a free area of not
less than 1 square inch per 4000 BTUH of the total input of all appliances within the enclosure.
The lower opening is primarily providing combustion air. The upper opening is providing vent dilution air and
acts as a relief opening for flue gases should the vent become obstructed or a down draft condition occur.
Additionally, when the heater is installed in a confined space and communicating with the outdoor air, one
permanent opening, commencing within 12 inches (30 cm.) of the top of the enclosure, shall 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 shall directly communicate with the outdoors
and shall communicate through a vertical or horizontal duct to the outdoors or spaces (crawl or attic) that
freely communicate with the outdoors, and shall have a minimum free area of:
a. 1 sq. in. per 3000 BTU per hr (7 cm2 per kW) of the total input of all equipment located in the
enclosure, and
b. Not less then the sum of the areas of all vent connectors in the confined space.
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