I just purchased an ecobee4 and I found that I need C wire for wifi which is not connected to my current thermostat. However, there 2 extra wires I found in the wall. Could you use one of them for the C wire? Also the board has something already going in to C screw but I think its for something else? It's coming out of the separate brown cable. There is also one wire coming from brown cable going to yellow. I leave these 2 alone?

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    You don't need C for WiFi. You need C for additional power to the thermostat. You need the additional power for... among other things, WiFi. Dec 16, 2017 at 0:53
  • Where does the brown cable go? It is powering a doorbell? Or what?
    – Bryce
    Dec 17, 2017 at 5:09
  • @Bryce -- my answer covers that. (it's the cable to the air conditioner compressor)\ Dec 17, 2017 at 10:53

1 Answer 1


Go ahead, hook that blue wire up to C and use it for your Ecobee

As the terminal strip on your furnace control board is typical in that it has room for 2 wires on each screw, you can simply land the blue wire on the furnace control board's C terminal alongside the white wire that's already there.

BTW: the brown cable you're asking about can simply be left alone -- it's the cable that connects your thermostat's yellow wire (Y terminal) and the furnace control board's C terminal to the compressor contactor in your air conditioner so the thermostat can turn it on and off when things get sweltering in the summertime.

  • Thank you, Quick question, the brown cable has 2 wires coming out of, one is Yellow which is also going Y and C which is going C. So I leave the extra yellow as is correct? and just use the blue coming out of the white cable as my C.
    – Sam
    Dec 16, 2017 at 0:40
  • @Sam -- that is correct. the white wire from the brown cable needs to be left as-is as well Dec 16, 2017 at 1:02

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