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SUF SERVICE HANDBOOK
INSTALLATION
Technical Training Department State Water Heaters
STC-077
10 Ashland City, Tennessee © 2004
MAKE-UP AIR
Direct Communication
A fresh supply of make-up air for combustion can be supplied to the heater through make-up air
openings, 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 dilu-
tion air and acts as a relief opening for flue gases should the vent become obstructed or a down
draft condition occur.
MAKE-UP AIR
Vertical Ducts
Often it is more practical to install vertical make-up air ducts to the outdoors. Again, 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 4000 BTUH of the total input of all appliances within the enclosure.