I absolutely need to use a common wire to install my Nest...which is the common?

[Amateurplumber1]

There's a lone blue wire upstairs where the thermostat is that is unused; the other four wires are used. Downstairs, at the furnace, there are three blue wires. I've labeled 'em for you. I have no idea which blue wire is the wire that's going upstairs. I'm assuming it is wire 1, because the insulated bundle of wires that wire 1 is in are all going to terminals, so that makes me think those are all the wires going upstairs to the thermostat. It could be wire 2, but for some reason I feel like it's not because it appears there are three unused wires in that bundle. I dont think its wire 3 since that one goes into a wire nut, but I don't know. I just can't make heads or tails of this. Is there any possible way to figure out which wire is the one I need to connect to the C terminal? Here are a bunch of other assorted pics of the furnace in case it helps. THANK YOU SO MUCH!

 

[Stuff]

I think you are right with wire #1. The 4 wires attached to thermostat are red, white, yellow, and green and you can see that only one "cable" has a green wire hooked up at the furnace so this would be the cable going to your thermostat.

Do you have a multi-meter? You could measure the voltage between red and blue at the thermostat end before hooking up the new thermostat. It should be zero until you attach the correct blue at the furnace.

 

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[Amateurplumber1]

I think you are right with wire #1. The 4 wires attached to thermostat are red, white, yellow, and green and you can see that only one "cable" has a green wire hooked up at the furnace so this would be the cable going to your thermostat.

Do you have a multi-meter? You could measure the voltage between red and blue at the thermostat end before hooking up the new thermostat. It should be zero until you attach the correct blue at the furnace.

That was exactly my thought as well! Glad we came to the same conclusion. Yes, I do have a multimeter. Should I put the blue wire directly on the screw along with the other two wires or splice them? Thank you!

 

[Reach4]

Looking at the photo again, it looks like the first terminal has a C label on the terminal strip.

 

[Stuff]

It is low voltage so multiple wires under a screw is allowed. It does make it easier to troubleshoot as well. You strip the wires long (I go overboard and do an inch) and then twist them together before wrapping around the screw clockwise.

Put your meter directly on the C and R terminals to verify your meter and furnace works. Should be over 24 volts AC. My nest is reporting that it sees 36 volts.



FYI - Nest is odd in that it shows peak AC in its menus so you need to divide by 1.414 (square root of 2) to get RMS (root mean square) that is "normal" voltage that you would see on a meter. S0 Nest 36 volts peak = 25.5 Vac rms

 

[Amateurplumber1]

It is low voltage so multiple wires under a screw is allowed. It does make it easier to troubleshoot as well. You strip the wires long (I go overboard and do an inch) and then twist them together before wrapping around the screw clockwise.

Put your meter directly on the C and R terminals to verify your meter and furnace works. Should be over 24 volts AC. My nest is reporting that it sees 36 volts.



FYI - Nest is odd in that it shows peak AC in its menus so you need to divide by 1.414 (square root of 2) to get RMS (root mean square) that is "normal" voltage that you would see on a meter. S0 Nest 36 volts peak = 25.5 Vac rms

Thanks a lot! I actually tested it before you wrote this reply and everything was a-ok! Great. Only thing is, i just put the wire under the screw instead of twisting them together (but thats what I wanted to do--just wasnt sure if I should have since I didnt read your comment haha).