State SBL85 390NE A - Manual (Page 7)

5 Servicing should only be performed by a Qualified Service Technician
AIR SUPPLY:
Stoichiometric or theoretical complete combustion requires 10 cubic feet of air per 1,000 BTU 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.
INSUFFICIENT MAKE-UP AIR, NEGATIVE AIR PRESSURE AND DOWNDRAFTS:
A lack of combustion and ventilation air can create a negative ambient air pressure in the installed space. The
vent system on one or more gas fired appliances can experience down drafts due to the outdoor air pressure
being greater than the ambient air pressure in the installed space. Where multiple gas fired appliances are
installed, one or more gas fired appliances can “pull air” through the vent system(s) of other appliances
installed nearby.
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.
• Down drafts can cause flue gases to spill into the installed space.
Downdraft Caused by Kitchen Vent Hood