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

  • I accidentally connected the C wire on thermostat to the Y wire on board (also goes out to AC unit). Attached it to the wrong wire nut :( I realized this after a couple weeks when my battery started going down and thermostat won't show display without battery. Could I have screwed something up badly?
    – Nid15
    Nov 22, 2020 at 22:54

1 Answer 1


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

  • Yes. Old school one with the Sensi. Thank you.
    – Nid15
    Nov 8, 2020 at 19:30

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