So, I recently moved. In my old place, I used my Nest without a common wire and never had an issue. However, in my new place, my Nest thermostat was working fine for about 20 days and started losing power. I put the old fashioned thermostat back for now. I know I need a C wire. My thermostat has 3 wires for the AC. I was thinking of buying a 18/2 thermostat wire and just fish it to the thermostat. That's the easy part. I take pride in doing DIY but I'm not so sure when it comes to electrical components. My next question is once I fish the new thermostat wire, what do I do next? I will attach pictures for reference.

Furnace wiring diagram

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There are a few more pictures here: https://www.dslreports.com/speak/slideshow/32447767?c=2392515&ret=64urlL2ZvcnVtL3IzMjQ0MTAxNi1IVkFDLXN3aXRjaGluZy10aGUtZmFuLWNvbm5lY3Rpb24taW4tSFZBQy10by1jb21tb24

  • Is replacing the existing thermostat cable an option? – ThreePhaseEel Jul 14 '19 at 22:25
  • Also, does that furnace control board have a C terminal on its terminal block? (Most do.) – ThreePhaseEel Jul 14 '19 at 22:26
  • I guess I don’t have any issue with swapping the existing thermostat wires but I don’t know what that would entail. I do not see any type of terminal block. My research has shown that most people have an actual block with the letters on that that you wrap the thermostat wire around but mine doesn’t seem to have that (see the pictures in the links I posted). – Pat Jul 14 '19 at 22:41

It seems that non-standard colors and mismatching connections are everywhere, so confirm everything and don't rely on color just because that's what the wiring diagrams say. From all of your diagrams, it appears there are several locations that a common wire should be accessible.

On the simplified diagram posted first, the second wire running to your A/C is common (brown if the diagram can be trusted). On the more detailed wiring diagrams, it indicates that yellow/black are connected to the primary side, while red/brown are connected to the secondary (24VAC) side of your transformer (I can see those colors in your pictures). In this case brown, again, is your common. If we follow this back, there should be a C terminal on a PCB where the G and R wires connect. If you can find it, I would connect your new C wire to this PCB location. You'll only use 1 wire from your new 18/2 bundle.

  • I've finally gotten around to trying to hook up the wires. I traced the brown wire which appears to be the common. The wire coming from the brown wire also had a red wire (looked like a thermostat wire) twisted together with a wire nut on it. I took my new thermostat wire and wrapped the new wire around that bundle of wires and then went and connected that new thermostat wire to the C connected on the Nest but the Nest did not turn on. Do I have to plug in all of the thermostat wires from the old thermostat into the nest for the Common to work? – Pat Nov 7 '19 at 22:29
  • C alone does not provide power, I've heard it explained more as a way for current to return. It will pull power from the R terminal (Rc/Rh depending on your configuration). I would plug them all in, otherwise you'll probably get an error that there is now power to whatever terminal isn't plugged in. – Matt Nov 7 '19 at 23:20
  • That worked! As soon as I plugged in the Rc thermostat wire in the Nest, the thermostat came to life. – Pat Nov 10 '19 at 12:15

Formerly, your Nest worked by leaking power through the furnace or air conditioner's operating relay. The relay was old and brutish enough that it flowed enough current for it to work. Now, you have an electronic control furnace that does not have those smarts. This is similar to the problem people have with lighted switches or dimmers when they switch to CFL/LED lights.

And there is a similar solution available. Put a device in parallel with the furnace or A/C control which leaks enough current to act like the big old relay. This can be done with a power resistor, or a capacitor since it is AC. We don't do product recommendations, but the HVAC market should have products (UL listed) made for this. I would not try to wing-ding it yourself with electronic components (ЯU listed) unless you know exactly what you are doing.

"parallel" means the device would go between R and C down in the furnace.

  • Yeah, that sounds a bit too complicated for my skill level. I've already run another 18/2 thermostat wire to the thermostat and to the AC handler. I just have to connect it. I'm thinking that I need to pigtail to the Y thermosat wire and the Y wire coming from the AC condensor unit outside? – Pat Jul 14 '19 at 17:52
  • Well, you need to be wherever the transformer is. You should be able to grab a C wire from somewhere around there. C is almost everywhere except the thermostat. I gather that Y is already hooked up correctly, so I wouldn't mess with Y. Unless your goal is to re-task your original cable's Y to be C and run a new Y. – Harper - Reinstate Monica Jul 14 '19 at 18:56
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    Yes, I opened the cover and can see the brown wire from the transformer. I've linked to the schematic and the transformer here. What do you think?dslreports.com/speak/slideshow/… – Pat Jul 14 '19 at 19:11

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