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Common Wire Connection at Furnace


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We have an old Wesco furnace circa 1979ish. Trying to install a Sensi touch screen thermostat. I had no problem taking the mercury thermostat off the wall and finding extra wires behind. When crawling under the house, I cannot figure out where I should be connecting the wire of the same color for the common wire. The cream color and red wires are the same as the two on the thermostat upstairs.


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This is the standard I've followed for decades:

White is heat

Yellow is cool (A/C)

Green is blower

Red is 24 volt supply

Black is common

Orange or sometimes purple is the reversing valve on heat pumps.

DIYers get them mixed up all the time.  If they are, you might have to trace them and figure it out.  Spent many a service call doing that.

Edited by Marc
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