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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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DJ2904
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  • Hello, and welcome to Home Improvement. Please take our [tour](https://diy.stackexchange.com/tour) so you'll know how best to participate here. – Ack Apr 07 '20 at 17:57
  • Y is normally use for cooling (compressor activation). Is this heat only? – JPhi1618 Apr 07 '20 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 07 '20 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 07 '20 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 07 '20 at 18:20
  • Fyi - the additional red and white wires coming from the top go to my humidifier. – DJ2904 Apr 07 '20 at 18:22
  • Looks like someone messed around with it, "G" is suppose to be a green wire which goes to a relay which goes to a fan, for "fan" only. – hello moto Apr 07 '20 at 18:27
  • Here's a good post https://diy.stackexchange.com/a/115083/102572 – hello moto Apr 07 '20 at 18:31
  • Thanks. I actually read that post this morning about connecting a new C wire, but I'm still not sure why my Y wire is in the C/T terminal. Is there any valid reason to do that? As for the G fan wire, looks like they used the blue wire for that instead. – DJ2904 Apr 07 '20 at 18:38

2 Answers2

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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

JPhi1618
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  • 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 07 '20 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 07 '20 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 07 '20 at 19:07
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    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 07 '20 at 19:11
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    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 07 '20 at 19:13
  • Yesterday, I made the changes you suggested above. However, my fan was literally running all day, even when cool wasn't running. My only option was to disconnect the G wire at midnight to finally get the fan to stop. Would it make a difference if I make the blue wire connect to the G terminal, then use green for C? Maybe using a new G wire caused it? – DJ2904 Apr 08 '20 at 13:36
  • @DJ2904 did you cross the wires? Regardless of color, what's connected to the G terminal on the air handler should connect to G on the Nest, and whats connected to T should connect to C on the nest. – JPhi1618 Apr 08 '20 at 15:29
  • Yeah everything matches. Remarkably, I reconnected the G wire at the thermostat this morning, and it didn't kick on automatically. I just ran a cool cycle and fan kicked on, then off when the cycle ended. No idea why it was running nonstop yesterday, but it works as intended now! I remember yesterday after the rewire, the fan kicked on as soon as I flipped power back on, but never stopped until disconnected. Perhaps the fan needed a manual on/off cycle to adjust to normal. – DJ2904 Apr 08 '20 at 16:27
  • Maybe the fan was set to ON in the Nest and you just assumed it was wiring trouble? Glad its working now! – JPhi1618 Apr 08 '20 at 16:28
  • You would think, but I had the fan disabled and then thermostat completely off, which is when I pulled the G wire as a last resort. – DJ2904 Apr 08 '20 at 16:30
  • After rewiring on Nest and the control board/airhandler, you probably needed to do Factory Reset on Nest and start over. Thats what I read in the instructions. Btw, my unit is setup exactly like yours. Y from Nest is connected to C on the control board. Good thing I found this thread, will try the solution later today :) – sql-noob Aug 10 '21 at 17:52
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Moved to Y wire on thermostat connected to C on furnace

Javier Martin
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