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Technical Training Department
Ashland City, Tennessee
SBN TANK TYPE COMMERCIAL GAS WATER HEATER
SERVICE HANDBOOK
©2003
Stoichiometric or theoretical complete combustion requires 10 cubic feet of air per
1,000 BTUH of gas supplied. The National Fuel Gas code also recommends an
additional 2.5 cubic feet of “excess” air.
For information on minimum make-up air opening sizes for various building installations,
refer to the National Fuel Gas Code NFPA 54, ANSI Z223.1, Sec. 5.3.
Air Supply
One common example is in a restaurant installation where exhaust vent equipment was
not considered in sizing make-up requirements. This condition may result in air being
backdrafted by the restaurant exhaust equipment through the heater causing the draft
proving switch to open and/or erratic heater shutdown.
Insufficient Make-Up Air - Backdraft
1,000
BTU
10 CU.
FT.
COMB.
AIR
Insufficient make-up air is a major cause of combustion problems.
2.5 CU.
FT.
EXCESS
AIR