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I am trying to install a Nest Thermostat and I need some help locating which of the wires would be the common "C" wire. I have traced all the wires going from the thermostat, condensate removal pump, ac compressor, and furnace. Below are some pictures of what I am seeing and working with. I was able to also make a wiring diagram that I know is correct to how my furnace is currently hooked up, but I just can't seem to see which one would be the "C" wire. The only wire coming out from the control board from "C" goes to the compressor. So unless I am wrong, if I follow the path of the electricity, would my potential common wire be the Y?

My thought process.

  • "C" travels to the compressor on the red wire
  • Returns from the compressor to "Y1"
  • Travels to the Condensate Removal Pump from "Y1" on the white wire
  • Returns from Condensate Removal Pump on the red wire to the Wire Nut connection
  • Travels from Wire Nut connection on the yellow wire to "Y" on the thermostat.

Doesn't that technically make my Y the 24v common?

I tried testing the thermostat with a voltmeter for 24v by connecting to the red and testing all the wires one by one but they all gave about 26 volts.

What am I doing wrong? I appreciate any help I can get.

Thermostate Furnace Furnace Control Board Wire Diagram Wires from thermostate to furnace junction Wires from thermostate to furnace junction

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  • Do you have a spare wire in your thermostat cable? Mar 17 '21 at 1:29
  • I don't not. It's a 5 cable strand but all the wires are being used. That was the first thing I checked hoping I would have an extra wire there and at the furnace so I could just connect it. Mar 17 '21 at 1:37
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Your C wire is indeed the one that's labeled C

While your setup with the condensate pump is a bit more complicated than most, it doesn't change the fundamental way things work. Your thermostat still puts 24V on Y to call for cooling, which gets passed on by the condensate pump to the Y1 terminal on your control board. Your AC compressor is connected to that Y1 terminal, so it sees that signal and starts the compressor; we then know that the other wire coming from the AC is a C wire, and lo and behold, it lands on the terminal labeled "C" on the control board.

As to getting a spare wire, you could use an "add-a-wire" gadget of some sort, convert your furnace to single stage operation, or simply run a fatter thermostat cable, the choice is yours.

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    Look at the thermostat cable with the yellow wire. It looks like there's a brown wire wrapped around it near the furnace. If there is, in fact, an additional wire in that cable, you can hook both ends to the 'c' terminals.
    – gbronner
    Mar 17 '21 at 3:59
  • it's jut the cable sheath folded back, zoom the photo and look again.
    – Jasen
    Mar 17 '21 at 10:45
  • Firstly, thanks for all your help everyone. I updated the pictures. There is a 6 strand wire with a brown wrapped around it that goes from the inside to the outside of the furnace connecting the 5 stand wire coming from the thermostat. I can confirm that there only a 5 strand wire going up to the thermostat so gbronner you are not wrong. However it is wire capped to the 6 strand wire connecting it to the control board with the brown not be used at all. Mar 17 '21 at 16:03

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