I'm trying to utilize a spare wire as my C wire for my new thermostat. A lot of what I've read online assumes you have these simple, lettered terminals in your furnace. However, that's not the case in my system.

Here's the previous wiring:

previous wiring

I noticed a spare couple of wires in the conduit, so I looked inside the furnace and found the yellow one I'd like to make my C wire:

spare wire

Here's the inside of the system. No nice, user-friendly terminal screws, just a bunch of wire nuts!

enter image description here

Here's the schematic.

enter image description here

It's so old it obviously doesn't reference a c wire, but I'm wondering if it's possible for this system. I tried joining into wire nut with the G (green) wire, because apparently that's an alternative power source for C. However, when I tried that, the thermostat would power on at first, but it seemed when the fan tried to turn on, the system would trip and the thermostat would lose power altogether.


In the photo that shows the insides of the furnace. On the left side of the photo there's a yellow and a red wire connected together. That should be where you want to connect the C wire.

you can confirm this by tracing the wires. From the thermostat cable, the red wire should connect with the blue and orange wire. While the blue wire connects to a white wire, with a blue twist-on wire connector. There's another blue twist-on wire connector where two red wires connect. One of the red wires is grouped with the white wire (in the same cable), those go to the A/C. The other red wire should run over and connect to the yellow wire, using a yellow twist-on connector.

If all this is true (it's difficult to trace the red wires through the photo), then the yellow wire from the thermostat cable (new C wire) should connect with the red and yellow wires.

enter image description here

  • 1
    Wow you must know these things very well! You were right about everything. I added the spare wire to that C wire you pointed out in the photo and I'm in business - heat and cold! Thank you!! – Donnie Ray Jones Jan 31 '15 at 21:12

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