I am upgrading my thermostat (Have AC & Heat) and found that it doesn't have the "C" wire that I need to power my new thermostat. The furnace does have a terminal for C, but only condensation pump is connected. Can I simply run an 18 gauge wire from the furnace C to the thermostat C. to have a fully functioning, powered thermostat? There are already 5 wires connected. I have attached a picture of my furnace wiring Furnace wiringin case more information is needed.

  • Yes that C in the control board will give you full time 24-volts at the t-stat. If that is easy to do, that would be best. Or if you can give up your independent fan control, you can jumper the yellow to the green at the board which would cover the air conditioning. Then you would have the green wire to use as the common. P.
    – Paul Logan
    Nov 30, 2017 at 5:08
  • Thank you for the fast response. Yes, it would be easiest for me to just add the separate C wire, as the furnace and the thermostat are just on the other side of the wall. I'm glad that is an actual option for me.
    – David
    Nov 30, 2017 at 5:12
  • Similar question has answers with massive detail about this problem, posted starting in 2011. diy.stackexchange.com/questions/10482/…
    – Upnorth
    Nov 30, 2017 at 13:10

1 Answer 1


Technically, to do it properly, you should run a new cable with more wires.

But yes. If you connect the C terminal on the thermostat, to that C terminal on the furnace control board. That should power your new thermostat.

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