I know this question gets asked all the time but I've spent hours staring at forums as well as my furnace and at this point I'm desperate. I just bought a nest thermostat that needs a C wire connection. My furnace doesn't have a C wire slot to plug in the wire and I'm trying to find out what I need to connect this C wire coming from my thermostat to.

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This is a strange assembly and is on the wall across from my furnace. I'm assuming the blue wire coming from the thermostat not connected to anything is the C wire. Only the red wires have voltage.

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I think this has something to do with the common wire but is grounded with no voltage enter image description here enter image description here

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    Hello, and welcome to Home Improvement. We'll need more info, e.g. make/model of furnace? even better would be a picture of the wiring diagram. May 19, 2019 at 16:26
  • Can you post a photo of the wiring at the thermostat end? It seems the last installer had a funny sense of color...\ May 19, 2019 at 23:24
  • Just added it @DanielGriscom May 20, 2019 at 1:20
  • Here is the thermostat @ThreePhaseEel May 20, 2019 at 1:21

2 Answers 2


Based off the first picture, it looks like the blue wire fell out from the wirenut next to it with the black and white wires going into it.

That should be the common side of the 24 volt transformer used for the AC, which is what you probably want to be using for C.

Test to see if that all makes sense first rather than just connecting it. I'd rather not make your problem worse!

I don't know what to tell you about the second picture though, so determine if adding the blue wire to that wirenut makes sense, and if it doesn't just down vote this answer.

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    The blue wire was just wrapped around the spare wires coming out of the thermostat. The black and white wires don't have any voltage so I figured that wouldn't help to include the blue wire but I'm really not sure. May 19, 2019 at 17:55

Connect your blue C wire from the thermostat to where the white wire from the A/C joins in with the dark wire from the furnace

There's a simple logic you can follow here, since you have an air conditioner. In just about all cases, the air conditioner's control wires must be connected to the Y and C wires in order for your air conditioner to work. As a result of this, we can trace the yellow Y wire from your thermostat cable to the red wire in a 2-wire cable at the junction in the first picture, which then must be the cable going to the air conditioner.

From there, we can deduce that the other wire (aka the white wire) in that cable must be connected to the C wire, so we can connect our C wire from the thermostat to the same place.

  • That worked! I appreciate your help! May 20, 2019 at 21:35
  • @MechanicallyDeclined we thank people around here by upvoting and/or accepting their answers :) May 20, 2019 at 23:13

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