furnace fan blows even when unit is off

Old 06-28-09, 05:13 AM
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furnace fan blows even when unit is off

We have a forced air gas furnace & A/C at our house. We have had an hvac company come four times to our house because our furnace fan blows even when the tstat is set to off. When we have the furnace or A/C on, it will also blow when the fan is set to auto. The compressor on the a/c is not on during these times. They have replaced the control board, the tstat, and lately the fan capacitor (correct spelling?). We turned our air off yesterday and the fan will blow for about 5 min. every so often. Doesn't seem like a regular cycle and seems like way too often to be just "normal operation". Any thoughts? We are moving and don't want to leave this problem for the people that are buying our home...
Old 06-28-09, 08:59 AM
Join Date: Mar 2006
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Older furnaces used a plenum switch to control the blower and if your furnace has one then it could be adjusted too low but the technician(s) who replaced the control board should have checked for this before doing anything else.

It seems to me that having replaced the control board and the thermostat the only possibility remaining would the plenum switch or an intermittent fault in the thermostat cable.

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