Blower runs on heat but not a/c

Problems related to residential installations.

Blower runs on heat but not a/c

Postby flo » Tue May 10, 2011 11:28 pm

We have a split system - gas furnace in the attic and a compressor outside. When the thermostat is set to 'heat', the heat and fan both turn on (I can feel hot air blowing from the registers). When the thermostat is set to cool, the compressor will start and continue to run but the circulating fan doesn't turn on and there is no air blowing from the registers. If I put the thermostat in 'test' mode, and run the test to turn the fan on/off, I cannot get the fan to start. In test mode 'heat & fan', it works. In test mode "a/c and fan', the compressor starts but the circ fan does not.

Why would the circ fan only work in heating mode? BTW, the fan worked fine until a few days ago. The thermostat is a Honeywell 8000 series, it has a removable, battery operated module that can be snapped on/off the wall mount. We had a spare thermostat in the house of the same type so I copied the programming exactly and plugged the new module onto the existing wall mount but it didn't make a difference. I'm not sure that it really rules out the thermostat as the problem though, since I didn't replace the wall mount (which is the part with the wires).

Thanks for any advice before we call out the experts...
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- Blower runs on heat but not a/c

Postby paul292 » Wed May 11, 2011 2:40 pm

if it's not a thermostat it must be your circuit board. check for loose connections to the blower and replace board
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- Blower runs on heat but not a/c

Postby cascadehvac » Wed May 11, 2011 3:41 pm

it prolly isnt the stat. it is either the motor, the circuit board or wiring...prolly need to call an expert out.
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