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MINIMUM AIR FOR COMBUSTION
Stoichiometric or theoretical complete combustion requires 10 cubic feet of air per 1000 BTUH input of the gas
input. The National Fuel Gas Code also recommends an additional 2.5 cu.ft. of "excess air". This
12.5 cu.ft. minimum supply air per 100 BTUH input applies to natural and propane gas models.
The National Fuel Code also specifies minimum make-up air opening sizes for various building installations.
(Ref: NFPA 54, ANSI Z223.1, sec 5.3).
This model is approved for direct venting either horizontally or vertically or conventional venting horizontal or
vertical. Direct venting avoids using room air for combustion and eliminates the need for additional air
intake ducts. The direct vent installation may have up to 50 equivalent feet of exhaust and 50 equivalent feet
of air intake piping
A. O. Smith Water Products Company
©
Irving, Texas 2000
10 CUBIC FEET OF AIR PER 1,000 BTU
1,000
BTU
+
MAKE-UP AIR
DIRECT VENT INSTALLATION
10 CU.
FT.
COMB.
AIR
2.5 CU. FT.
EXCESS
AIR
5
Service Handbook
Training Department
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