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? – ThreePhaseEel Sep 17 '18 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 '18 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 '18 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 '18 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? – ThreePhaseEel Sep 17 '18 at 23:31

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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