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SUF SERVICE HANDBOOK
INSTALLATION
State Water Heaters Technical Training Department
Ashland City, Tennessee © 2004
11 STC-077
MAKE-UP AIR
Horizontal Ducts
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
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 SUF 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.