A.O. Smith FPSE50, fps50, FPS40, FPS 75 manual Negative Pressure

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.