I am upgrading my thermostat to a Nest, and would like to add a common wire. My current furnace controller doesn't have a C terminal, but the current thermostat wire has 2 unused wires (black and blue)

old thermostat wiring diagram transformer

what's the best way to do this?

On the 24V side of the transformer there is: 1 Red Wire 3 Blue Wire - 1 Blue is ground - 1 Blue goes to gas valve - 1 Blue goes to furnace controller

Can I cut one of the blue wires, lets say the one to the controller, strip the ends and connect both ends to the c-wire on my thermostat wire?


  • For most installations the Nest doesn't require a C wire. Do you know that you need one?
    – Hank
    Nov 17, 2014 at 23:31
  • it doesn't require it but I'd prefer to have it. Rather than have the best turn on and off the furnace to get a charge. spring and summer are times the nest wouldn't get much power. I figure if the wire is already there I just need to hook it up, why not?
    – user28078
    Nov 18, 2014 at 8:04
  • Did this work? I ask because when I did the same with the Ecobee thermostat, it started blowing up the 3 Amp fuse on the control board.
    – user47848
    Jan 15, 2016 at 18:31
  • That's not how a Nest works. It doesn't need to turn on the furnace to get a charge. It just lets a small amount of current leak through the relays it controls, but not enough to pick up the relay and start the device. If that doesn't work properly for you, you need a C wire. Jan 16, 2016 at 0:20

1 Answer 1


Since in your case, one side of the transformer is grounded. You can simply use a fork or ring terminal, to connect the C wire to the chassis. Though it appears there's already a wire that's attached to ground, and comes right over near the thermostat wiring. I'd just put my C wire in with the other two wires, in that twist-on wire connector near the bottom of the photo.

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