I have a Nest E thermostat, and like many others have lost wifi and getting low battery because I have no common wire connected.

After looking at my old Lennnox 78ugf3-75-1 furnace, I noticed that my AC Y wire is connected to the Common terminal(called 'T') on my furnace circuit board. Oddly enough, nothing is connected to my Y terminal.

The electrical cable has a green unused wire that I plan to use for my C wire, but before I do that, should I move the Y wire to the Y terminal or leave it on the C terminal? It must have been installed this way years ago as I've never swapped the wires before.

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    – Ack
    Apr 7, 2020 at 17:57
  • Y is normally use for cooling (compressor activation). Is this heat only?
    – JPhi1618
    Apr 7, 2020 at 17:57
  • Heat and cooling. AC wires are in a pigtail with the thermostat wiring then go into the furnace circuit board. I'll post more pics.
    – DJ2904
    Apr 7, 2020 at 18:03
  • That's weird, "Rc" or a "R" jumper to Rc if your AC unit doesnt have a transformer should run to one side of your 24 volt contactor on the AC unit with constant voltage, when the house calls for heat, Y sends power the opposing side of the 24 contactor thus pulling the pushpin in allowing the appropriate current to flow to the compressor, Have they used "C/T" instead of "R/Rc"?
    – hello moto
    Apr 7, 2020 at 18:11
  • The R wire is correctly connected to the R furnace input. No idea why the cooling is connected to C/T though. Would it be safe if I moved my Y cooling wire to Y terminal then connect my unused green wire to T? Or should I leave cooling as is and connect my green wire to that same T terminal?
    – DJ2904
    Apr 7, 2020 at 18:20

2 Answers 2


Your AC compressor needs two wires - Y and C. Y comes from the thermostat and C comes from the air handler or the common side of the 24vac transformer.

The picture you have with the mid-line wire splice is where the three conductor wire (green white, red) branches off and goes outside to the compressor. Since Y is the standard for "compressor, someone decided to cut the Y wire in that place and use either end of it. One side of the Y wire goes back to the air handler to pick up the C wire (T terminal), and the other side goes to the thermostat to pick up the switched Y wire.

So, the Y wire and it's connections seem fine. What I would do is move the Green wire to the G terminal on the air handler and the thermostat since that is the standard color, and then use the Blue wire for C. It can connect to the T terminal on the air handler and then the C terminal of the nest.

I think the Nest has a voltage check function, but a simple multimeter could be used to verify that you do indeed have 24VAC between the R and T terminals of the air handler. Based on the diagrams, the T terminal is the C wire, and it was working before, so there shouldn't be a problem.

C terminal on diagram

  • Okay so just to verify, I should leave the Y wire alone kept in the T terminal, then add my C wire(blue or green that matches thermostat) to the T terminal making it a total of 3 wires (C,Y,& humidifier R)connected to the T terminal?
    – DJ2904
    Apr 7, 2020 at 18:56
  • Correct. The C wire needs to go out to any separate equipment to complete the signalling circuit. One wire (like Y) provides the switched signal from the thermostat and the C wire provides the "return path" for the voltage.
    – JPhi1618
    Apr 7, 2020 at 19:04
  • Thanks. It's amazing that I couldn't find anything on the internet about the Y cooling split into two, with one end going to thermostat Y and the other end going to furnace C. Is that common practice?
    – DJ2904
    Apr 7, 2020 at 19:07
  • 1
    Most of the time the compressor wiring will come all the way back to the air handler and the splices will be there with the other connections. That's normally not a problem because the coolant lines have to make the same trip. Your original wiring could have been damaged (even during new construction), so they had to run a new wire. Or that's just the way they did it... No telling. HVAC guys are usually not electricians and don't really make the greatest decisions. If it was a previous home owner then ALL bets are off, lol. So, no, I wouldn't say its common, but it happens.
    – JPhi1618
    Apr 7, 2020 at 19:11
  • 1
    Not to disparage HVAC guys... Just saying that electrical work gets inspected and the wire colors are super important. HVAC wiring has "suggestions", but no real "rules", and it's not inspected other than to make sure it works since it's low voltage.
    – JPhi1618
    Apr 7, 2020 at 19:13

Moved to Y wire on thermostat connected to C on furnace

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