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-   -   A/C always on with fan switch "on". (

bkvanbek 07-25-2010 10:16 AM

A/C always on with fan switch "on".
Why does the a/c run non-stop when the thermostat fan switch is set to “on”?

We just moved in with friends and to get some air flow we switched the fan control from "auto" to "on", we left for a few hours and when we came back it was 58 degrees in the house.

I verified the Honeywell thermostat wires were coordinated with the wires on the "R","W","Y" and "G" on the circuit board of the furnace. and the "C" and "Y" go to the a/c compressor. So I thought it might be the thermostat, so I switched it with another and it also turns on the a/c compressor when the fan switch is turned on, even when the "heat off cool" switch is "off". What is going on?

Marty S. 07-25-2010 10:32 AM

Likely a short between the G and Y wire in the wall. Cheap meter will let you know for sure.

yuri 07-25-2010 10:35 AM

Where do you live? In Canada we have new homes with a central exhaust system. The system is interlocked to the furnace. You can get a backfeed to the AC from G if it is not wired correctly. Happens to electric furnaces also. Post the make and model # of the furnace.

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