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FPS/FPSE/FPD TECHNICAL TRAINING AND SERVICE WORKBOOK
AIR FOR COMBUSTION cont.
Negative
Pressure
Diagnosing pilot outage complaints requires an examination of the environment in
which the heater is installed. Mechanical vent fans are the chief cause of
negative pressure. If the house is too tight, more air will be exhausted from the
structure than is being brought in from the outside, resulting in a negative
pressure condition. Fireplaces, furnaces, wind, and thermal stacking within the
house can also playa role in creating negative pressure conditions.
Negative pressure can result from air being backdrafted through the water heater
vent down into the combustion chamber, blowing out the pilot. This may happen
randomly because it may be a particular combination of appliances which cause
the condition.