I am about to install the Ecobee 3 lite thermostat. On the thermostat end, there is no C wire - just the typical 4 wires - R,Y,W,...

On the furnace end though, there is white C wire too, which I do not know where it leads to (definitely not coming to thermostat).

Given the situation, following are questions I am looking answers for:

  1. Can I simply remove the existing C wire and replace with the C wire from the PEK kit from Ecobee? Wondering what else may break in the house.
  2. Can I simply keep the existing C wire as is, and add the C wire from the PEK kit in addition, at the same terminal?

Any info or any tips that can help me out in this situation are highly welcome.

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  • 1
    Can you post photos of both ends of the thermostat wiring, and the wiring diagram (or at least a make/model) for your furnace? Sep 17, 2018 at 3:51
  • We need either photos of the wiring, or you'll have to include the make and model of the furnace. Also, do you have air conditioning through the furnace?
    – Tester101
    Sep 17, 2018 at 11:42
  • It looks like you have air conditioning connected to your furnace(y terminal has 1 wires). The c terminal is providing power to the air conditioner control circuit and turns on the air conditioner when the y terminal is connected by the thermostat. You need to run a wire from the c terminal of the furnace to the c terminal of the thermostat. That will provide 24 vac to the thermostat.
    – user68386
    Sep 17, 2018 at 19:33
  • Thanks user68386. To run the c wire from the furnace c terminal to the thermostat, the PEK kit can help, right? That said, as you can see, there is already a white wire on the furnace c terminal. Should I simply remove that and add the one from the PEK kit, or keep the one in the picture and add the PEK kit one too, to the same terminal?
    – Aditya
    Sep 17, 2018 at 20:55
  • Can you provide a photo of how the other wire in the cable going off to the air conditioner is connected to the furnace control board? Also, can you provide a photo of the terminal block that shows the labeling on the leftmost two terminals clearly? Sep 17, 2018 at 23:31

2 Answers 2


Hast thou pulleth upon the wires at thine thermostat to looketh upon the point thine c wire might be clippethed at the jacket? Thou should.

Thou should not double up thine wire at the c terminal upon thine circuit board without praying thine luck is true, elste thou shall unwittingly risk fryingeth thine furnaceths brain. Do so without specific knowledge of its ability to accommodate such doubleth penetrant at thine peril tho it may be fine, I doth offer with humility and humble deference that thou mighdts turneth luck to surety with some calling of the blacksmith who manufactured this magical baseboard. May luck be at your side on your quest to fancier temperature adjustment.

  • Well, I believe this is the first King James edition answer I've seen anywhere on Stack Exchange.
    – FreeMan
    May 17, 2021 at 16:38
  • blessed be thy comment. Doth thou wisheth to suggest a glorious tip for garnering thine upvote? May 17, 2021 at 21:22

You can connect two wires to this terminal

You can indeed connect the C wire from the PEK to this terminal in addition to the air conditioner's C wire. If you look closely at your Y terminal, in fact, you will see it is already double-tapped with the Y wire from your thermostat and the Y wire to your air conditioner.

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