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BTR TANK TYPE HEATERS
SERVICE HANDBOOK
A.O. Smith Water Products 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 twelve 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 downdraft condition occur. Additionally, when the heater is installed in a
confined space and communicating with the outdoor air, one permanent opening,
commencing within 12 in. (30 cm) of the top of the enclosure, shall be permitted where
the equipment has clearances of at least 1 in. (2.5 cm) from the sides and back and 6 in.
(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 cm
2
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.
1 Square
Inch Per
4,000
BTUH