Fan runs all the time

Problems related to residential installations.

Fan runs all the time

Postby Longhorn314 » Sun Feb 03, 2013 1:57 pm

My friend has a new "energy efficient" home. When I went to change the filter for her I found the fan was running and the only way I could stop it was to pull the breaker. The thermostat said the fan was off. I've since discovered the fan runs 24-7 no matter what the OA temp is. It runs at a lower speed than it does when it's heating or cooling but still makes for a drafty cold house. The builder says it's for fresh air. I just don't believe him. If it's 30 outside or 100 I don't think fresh air should be taken in. I have a degree in Mechanical Engineering and have designed commercial HVAC in the past. Is this the new normal for residential? Thanks in advance.
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- Fan runs all the time

Postby Freon » Sun Feb 03, 2013 3:10 pm

Much depends on the make and model of the furnace. Some new systems have some very odd features.

But as a simple test, turn off the power to the furnace and go to the furnace control board. At the control board, disconnect the Fan wire coming from the thermostat to the control board, usually connected to the G terminal. Also check the control board to see what motor connections are available. Usually there will be spade connections for Heat and Cool. All other blower motor leads will be "parked" at unused space connections for safety.

Possibly there is a Fan spade connection at the control board and it is connected to the slower speed tap of the motor. Without knowing the furnace these are just guesses. You may not have a multispeed PSC blower motor. Check the schematic on the back of the blower compartment door for more information.
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