enter image description hereCurrent thermostat is using 4 wires (W,Y,R,G) C is present but unused. Would like to update to smart thermostat (Nest Learning). Junctions coming from furnace pictured with unused C wire wrapped around main thermostat wire towards lower right hand side of pic. We hooked up the Nest as is and it wouldn't power on. We tried disconnecting the blue and white bottom wire junction, and connected the blue to the unused C from the therm wire leaving the white out but the AC wouldn't start. Any ideas on how to connect? Do I bundle the C in with the blue/white junctionenter image description here

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    You need to trace all these wires starting from furnace board where they connect before you start connecting anything. Colors don't mean a lot -- white is usually W (heat), and blue is usually B (heat pump reversing valve). If you just start connecting things, in the worse case you can damage your thermostat, furnace control board, or both. From your picture it's not even clear what cable goes to the thermostat, let alone what the colors are connected to.
    – gregmac
    Jul 11, 2022 at 22:19
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    Top junction- Yellow from furnace is connected to Y from thermostat and red wire from white/red bundle, 2nd junction is red from furnace to red from thermostat, 3rd- white from furnace to w from therm, 4th- green furn/green therm, 5th- white from red/white 2 bundle wire to blue from furnace.
    – KMTH1
    Jul 11, 2022 at 22:41

1 Answer 1


You are correct as to where your C wire is

We know from the existing junction that the red wire in your 2-wire cable is the Y signal to the compressor contactor as it's tied in with the yellow wire from the cable to the thermostat, which is connected to your thermostat's Y terminal. Given that the compressor contactor needs to be connected to C to complete the circuit, and that we know the white wire in the 2-wire must be that return path, we can then infer that your C wire is whatever wire is connected to said white wire in the compressor cable. As a result, you can simply connect your unused blue wire into the junction of the existing blue wire and the white wire in the compressor cable, then hook up your new thermostat, turn the system back on, and enjoy!

  • Yay!!! I was hoping you would respond (and confirm that)- I've been trying to piece things together based on some other feedback you've given... thank you so much!
    – KMTH1
    Jul 12, 2022 at 1:32

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