State Water Heaters Utra-Force STC-077 Service Handbook page 12

S U F S E R V I C E H A N D B O O K
I N S T A L L A T I O N
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.
State Water Heaters Technical Training Department
11
Ashland City, Tennessee © 2004 STC-077