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SBN71-120 thru 85-390, SBD30-150/199 TANK TYPE COMMERCIAL
GAS WATER HEATER SERVICE HANDBOOK
REQUIRED EXTERIOR CLEARANCES
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INSUFFICIENT MAKEUP AIR ........ NEGATIVE AIR PRESSURE .....DOWNDRAFTS
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 back drafted by
the restaurant exhaust equipment
through the heater causing the draft
proving switch to open and/or
erratic heater shutdown.
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