Looking to connect a newer WiFi thermostat which requires the connection of a c wire. My old thermostat didn’t use a c wire however there is a c wire present in the bundle of wires ( unconnected at thermostat and inside air exchanger/furnace. However I am a little lost on where to connect this loose C wire inside the air exchanger compartment. My furnace doesn’t use the typical screw terminals I’ve seen while looking for the solution to this. It uses a clamping type of connector ( see pics) there is a C terminal however this is already used. The wire from the c terminal goes to some black block ( I believe is a heat sequencer through some googling) then another connection off of that “heat sequencer” connects to the red wire from the 2 wire cable that I believe goes to the air conditioner? Any assistance on where I need to connect this c wire would be greatly greatly appreciated!!!! If any additional pics are needed I can do that ASAP enter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description here

  • I take it you have central air conditioning, no? May 15, 2020 at 1:38
  • Yes that is correct
    – RobJL94
    May 15, 2020 at 1:40
  • 1
    Youre actually the person I was hoping would answer this haha. Ive browsed a few other posts on this topic where you've answered. I just wanted to double check before doing it all. If I have it right, I should be putting the C wire (the loose blue wire coiled around thermostat cable in last picture) in with the two wires (teal blue, and red) that are connected with a twist cap in the last picture, correct?
    – RobJL94
    May 15, 2020 at 1:54
  • That is indeed correct! May 15, 2020 at 2:03

2 Answers 2


This is easier than it sounds

Since we know that your air conditioning is turned on by the thermostat putting 24VAC on the yellow (Y) wire that's connected to a wire in the 2-wire low-voltage cable to the compressor contactor (the white wire in that cable, in your case), we know that the other wire in that low-voltage cable (the red wire, in your case) must be connected to C. If it wasn't connected to C, your air conditioning could never work, as current needs to flow in a closed loop from the 24VAC transformer, through the thermostat contacts, through the compressor contactor's coil, and back to the transformer in the furnace in order for the contactor to be energized by a closed circuit.

So, since we know the junction of the thick blue wire and the thin red wire is where the C wire from the compressor connects, you can safely add your new C wire to that wirenut junction, install your new thermostat, and enjoy!

  • Okay awesome, that is what I was thinking. What threw me off was that the thick blue wire wasnt coming straight off the C terminal, and instead was going through the "black block" I referenced. I wasnt sure if this changed anything as far as what I needed to do. Thank you so much!
    – RobJL94
    May 15, 2020 at 2:25
  • @RobJL94 -- we thank folks for their answers around here by upvoting and/or accepting them :) May 15, 2020 at 2:38

Your C connection is as depicted below in the furnace electrical diagram as I highlighted by the blue line. As you will note that highlighted connection in the lower right corner is to connection #5 of the transformer. Note also that that connection also connects to the frame ground of the furnace.

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It is very typical that your air conditioner compressor is driven from the furnace control wiring with a two wire cable. One of those two wires will also be connected into this C wire connection. If you carefully trace your wire connections from the AC unit you will find the wire of the two that ends up being tied to furnace frame ground. (With power turned off to the furnace a digital multi-meter in continuity testing mode can be a handy way to find the wire that connects to the frame ground).

Once you identify the wiring harness that attaches directly to the frame ground (or transformer connection #5) you can tie your spare wire in the thermostat cable into that harness wire.

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