A.O. Smith 960 through 967 - Manual (Page 12)

BTH S E R V I C E H A N D B O O K
N S T A L L A T I O N
MAKE-UP AIR
Horizontal Ducts
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12
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Air Intake Ducts
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When the heater is installed in an interior room with no roof access for vertical ducts, horizontal
make-up ducts should be installed. When using horizontal ducts, two openings are required - within
12 inches (30 cm) of the top of the enclosure and one within 12 inches (30 cm) of the bottom of the
enclosure. Each opening shall have a free area of not less than 1 square inch per 2000 BTUH of
the total input of all appliances within the enclosure.
INSUFFICIENT MAKE-UP AIR
Backdraft
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Insufficient make-up air is a major cause of combustion problems. One common example is in a
restaurant installation where exhaust vent equipment was not considered in sizing make-up air
requirements. This may result in air being backdrafted by the restaurant exhaust equipment
through the heater causing flue gas spillage, flame roll out, improper combustion, inconsistent pilot
operation, and/or erratic heater shutdown.
A possible solution to this situation would be to use a BTH with direct venting.
A less common service issue associated with a backdraft or negative pressure room would be the
opening or closing of air pressure switches. This may result in erratic or no heater operation.
A. O. Smith Water Products Co.
Ashland City, Tennessee © 2004
11
Technical Training Department
TC-044 Revision 6