Make-Up Air: Direct Communication With Outdoors - A.O. Smith SERIES 120 Service Handbook

INSTALLATION CONSIDERATIONS

Make-up Air: Direct Communication with Outdoors

A fresh supply of make-up air for combustion can be supplied to the water heater through
make-up air ducts, which directly communicate with the outdoors. (Not Direct Vent.)
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 must have a free area of
not less than 1 square inch per 4,000 BTUH of the total input of all appliances within
the enclosure. The lower opening primarily provides combustion air. The upper opening
provides vent dilution air and acts as a relief opening for flue gases should the vent become
obstructed or a downdraft condition occur.
Figure 3
Additionally, when the water heater is installed in a confined space and communicating with
the outdoor air, one permanent opening, beginning within 12 inches (30 cm) of the top of
the enclosure, must be permitted where the equipment has clearances of at least 1 inch
(2.5 cm) from the sides and back, and 6 inches (16 cm) from the front of the appliance. The
opening must directly communicate with the outdoors and must communicate through a
vertical or horizontal duct to the outdoors or spaces (crawl or attic) that freely communicate
with the outdoors, and must have a minimum free area of a) 1 square inch per 3,000 BTUH
(7cm per kW) of the total input of all equipment located in the enclosure and b) not less
2
than the sum of the areas of all vent connectors in the confined space.
Servicing should only be performed by a Qualifi ed Service Agent
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