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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 – ThreePhaseEel Mar 22 at 4:25
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  • What make and model is your furnace/air-handler? – ThreePhaseEel Mar 22 at 18:54
  • Goodman. Model #GMNTE080-4 (03100004307). Thanks @ThreePhaseEel – Matthew McKenzie Mar 22 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 – Matthew McKenzie Mar 23 at 1:29
  • @MatthewMcKenzie -- we thank people around here by upvoting and/or accepting their answers :) – ThreePhaseEel Mar 23 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 – Matthew McKenzie Mar 24 at 0:25
  • @MatthewMcKenzie -- yeah, if you do that, you need to jumper Y to G so that the furnace blower runs during cooling – ThreePhaseEel Mar 24 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. – Matthew McKenzie Mar 25 at 20:03

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