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Options for adding “C” wire to thermostat

I am upgrading to a newer thermostat that requires a constant 24 volt feed, I have an extra (unsused) wire in the cable at the thermostat. Do I simply connect it to the transformer at the furnace?

  • There should be a wiring diagram on the furnace (usually on the service access panel), use that to figure out where to connect the wire.
    – Tester101
    Jul 23, 2012 at 12:11

1 Answer 1


Once you confirm the power supply in the furnace matches your thermostat's requirements, then yes, use the extra wire. Not only confirm a constant 24v is supplied, but whether it should be AC or DC. (DC would be really weird, but worth confirming)

Since the wire you use may not match the usual color coding, label both ends of the wire to eliminate any confusion in the future.

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