DIYbanter - View Single Post - Gas Hot Water Heater and Attic Fan conflict

Bob

wrote:
...A week ago I installed a big attic fan to draw the hot air out and
cool the house and I have observed the fan interfering with the
updraft of the water heater by performing the following test. With the
fan off and water heater running, I tested the draft at the water
heater flue with a smoking match and found it to work fine (drew the
smoke right up). Next, I performed the same test after the attic fan
was running for 2 minutes and it didn't draw the smoke up.

Your big attic fan needs to draw air from somewhere, usually from open
windows. Air will be drawn in from the path(s) of least resistance,
including a downdraft from the water heater vent. This also creates
problems with combustion air. So open enough windows before starting
the fan.