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enter image description hereI am attempting to install a nest but do not have a common wire on my hvac unit, perhaps it is slightly dated. I have read that on older units the O terminal is the same as C and I can remove the green wire (fan) and use that on my O as my common wire. Since I am unable to add a net new wire, could someone kindly let me know if this is a viable solution and the best approach I should take (see pic of my setup)? Thank you.

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  • It seems that your photo did not post...try hitting "edit" in the "share edit...." links on the bottom left, then using the mountain-and-sun button in the editor to upload your photo Mar 22, 2020 at 4:25
  • Hello, and welcome to Home Improvement. Unfortunately, we'll need more info before we have any chance of helping you. Please take our tour so you'll know how best to participate here. Mar 22, 2020 at 12:11
  • What make and model is your furnace/air-handler? Mar 22, 2020 at 18:54
  • Goodman. Model #GMNTE080-4 (03100004307). Thanks @ThreePhaseEel Mar 22, 2020 at 23:40

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Try the terminal between G and Y on your control board

It looks like the C terminal on your control board is between the G and Y terminals, so I would try there as your first port of call, provided your thermostat cable has spare wires in it. (It's labeled /C in the bottom row of labels, and has a wire from what likely is your air conditioner compressor running to it already.)

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  • How did I miss that - I took a closer look and it is B/C. I guess I was looking for a standalone C. I don’t have any spare wires so do I move my green over and connect it to that B/C terminal? Thanks Mar 23, 2020 at 1:29
  • @MatthewMcKenzie -- we thank people around here by upvoting and/or accepting their answers :) Mar 23, 2020 at 1:32
  • ThreePhaseEel or anyone else, I just read that if I use my G as my C since I don’t have any extra wires in my harness I should or need to fix a jump wire between my Y and G on the furnace. Thoughts? Thank you very much Mar 24, 2020 at 0:25
  • @MatthewMcKenzie -- yeah, if you do that, you need to jumper Y to G so that the furnace blower runs during cooling Mar 24, 2020 at 0:53
  • Hi ThreePhaseEel - Just moved the green over to B/C and added a jumper between Y and G. Could you please take a look at the first pic I just posted and let me know if all checks out? I turned everything on and sounded like everything fired back up without issue. Thanks again. Mar 25, 2020 at 20:03

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