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Furnace C wire connected to condenser. Can it be used for thermostat?

Home Improvement Asked by Nid15 on December 15, 2020

I have a 20+ year old Lennox furnace and AC unit. I opened up the back of the furnace since I had a disconnected C wire.
Basically, it looks like the C wire from furnace is connected to to one of the wires going out to condenser outside the house. The other wire being the Y wire.
Question is can I still use this same C wire terminal for thermostat? Currently the C(blue) wire to thermostat is disconnected and I can connect it to the wire going to the condenser.

Update 11/22 – Looks like I messed up something. I realized today that I had connected the thermostat C wire to the Y wire.
There are 2 wire in the Y screw and one of them goes to the thermostat and other goes to the air-handler. The C wire from thermostat also goes out to the air handler with my non-smart thermostat. Accidentally I had connected the thermostat C wire to the Y wire-nut.
Now I connected it back to the C wire-nut and at some point the outside-condenser came on.
Did I screw up something or is this setup not compatible with C wire?

furnace board wiring

One Answer

Yes. I assume you are trying to replace the regular thermostat with a smart one.

Answered by Ted Mittelstaedt on December 15, 2020

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