UPDATE: the red and the white in the separate sheath do not connect to the thermostat, trying to find where they split. more info to come and thoughts on debugging ...

I am trying to get a Nest to work. My old setup looks like this:

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My furnace control panel looks like this: Note wiring is not correct, see updated diagram

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I thought this was the wiring, from looking at the cables. Anything not mentioned at the control panel seems to go elsewhere (AC? other part of furnace?)

NEW WIRING DIAGRAM - MAYBE ACCURATE?? enter image description here

UPDATE - The white wire (sheath) that I thought went to the thermostat went to the AC. I've updated the diagram. This seems to suggest exactly how I should switch things but: 1) I thought the red small wire and the large white wire as R and C didn't work with NEST (testing in a bit) and 2) I am confused why things work currently because it looks like the thermostat R and W both connect back to W1?!?

Old Text I tried putting the thick white wire in my Nest as C. That didn't work so I tried swapping the red wires as R (thick one for thin one), leaving white wire as C. That didn't work either.

After measuring the voltage I found there was 27V between the two red wires. I thus plugged in the thin red wire into C. This worked!

However, after turning the heat off at night the thermostat died. Any ideas on why that might happen (i.e., why did it work, then it didn't). The furnace was making a chugging sound like it was going on and off repeatedly. Any other recommendations on how to debug the wiring diagram at the point of the thermostat? I was considering disconnecting the wires at the furnace and then doing a connectivity test between two wires, like this:

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Any other suggestions on how to debug?

Edit: I followed the wires as far as I could outside the furnace. The white wire that I thought was going to the thermostat was in fact going to the external condenser. A possible lesson here is that if you have exposed wire going to the outside AC unit, you can perhaps follow that easier and eliminate a wire from consideration. There is a second dark wire going to the thermostat that I need to now trace back to the furnace. Here's the junction point:

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A closer look: enter image description here

Here's a closer look at the initial wires which suggested to me that the large white wire bundle was the same one seeing at the thermostat. Apparently that is not the case!

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  • 1
    Make sure you positively identify the ends of the cable sheath. All your pictures just show individual wires and require a leap of presumption that they all come from the same cable. Also, blue is usually the fifth wire in a thermostat cable, so where there's blue there should be yellow too. R W G Y B is enough to do it all in 1 cable and not need to mess with the other cable. Commented Nov 22, 2021 at 19:58
  • Thanks for the feedback. I have verified that the cable sheath appears to be the same at the wall and in the furnace. The cable from the control panel to the splice point (about 2 feet away) is a bit hard to follow. I guess that would give me a shorter/easier route to test since I could reach both ends (at control and at splice). The cable from splice point to thermostat is only 4 conductor, no yellow. The other cable is two conductor and I'm seriously baffled as to how the red cable inside that sheath is not a Y1 to R connection
    – Jimbo
    Commented Nov 22, 2021 at 20:21
  • @Jimbo can you post photos of the furnace wiring that show the wires as thye exit the cable? Commented Nov 23, 2021 at 4:38
  • Figured it out, I think. More info to come later ...
    – Jimbo
    Commented Nov 24, 2021 at 2:04
  • @Jimbo -- are you OK with cutting the cabletie on the junction? That might help you (or us, with a photo of course) trace out the wires there Commented Nov 24, 2021 at 3:31

1 Answer 1


Note, this solution worked or had a hope of working because I had six wires at my thermostat. Below is my quest to figure out how to make use of those six wires to get a C wire to my new thermostat. If I had only had 4 wires I probably would have called for help.

There were a couple of big hints that were not immediately obvious to me regarding how to proceed.

First, visually inspect both sides of wiring (at thermostat and at control panel). Unless you have obvious wiring I would think you would need to look at both ends. I have a new furnace that was recently installed. I also probably should have asked at the time of install whether they thought I could have a smart thermostat, but I didn't. Anyway, here's the front panel:

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The 4 black things on the corners are hinges that open/close using a hex wrench.

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Without seeing the control panel, there was no way I could have figured out what was going on.

Second, it was critical to visually inspect the wire on both ends. I didn't even do this until I had finished fixing things but had I pulled out the wire at the thermostat a bit further, I would have noticed a completely different wire at the thermostat for even the 4 conductor wire (note it is white, not brown).

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Here is the brown wire at the control panel.

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It gets spliced into another brown wire in the HVAC box, which I thought was the same wire I was seeing at the thermostat:

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Second, everything works. This should have given me clues. The red wire didn't look correct, but I should have assumed it was and figured out why, rather than assuming it wasn't. I still can't track all of the final wiring, but one 2-conductor wire is going to the AC unit outside and I think the other is going to a humidifier unit that I didn't even know I had.

This photo (rotated slightly, me looking up at the ceiling) is the white wire that hits the HVAC control panel. I can easily follow it (if I try) to the copper pipe that goes to the exposed basement part of my house and eventually to the outside AC unit. This is what is connected to Y1 and C (cooling and power).

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The other wire, brown at W1 and blue at C goes into my ceiling and I can't trace it, but I think it goes to a humidifier unit.

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So, in other words, if you have a W1/C and a Y1/C you might have separate wires going to the AC unit and a humidifier unit.

Much of my basement is finished with a dip that happens from my HVAC ducts. Close the the HVAC unit were some panels:

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When removed, they showed this: enter image description here

Physically this location is on its way to the thermostat so I was pretty confident this was the last junction point. This shows the red from the 4 conductor HVAC wire (R) going to the thick red wire in the 2 conductor thermostat cable. The red wire from the thermostat is going no where (it had a cap on which I temporarily removed in this photo). The thick white wire from the thermostat 2 conductor cable is going to a 4 conductor cable that I could physically trace back to the closet with the HVAC:

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That cable was doing nothing, just sitting on the wall! It was actually hidden a bit more but I could easily find it once I tried looking for it.

At this point, because I had put the effort in to tracking the wires, I had a pretty good idea of what was going on. Or rather, I had a pretty good idea how to fix the problem, I have no idea why the house was wired like that.

Finally, a third point: voltages. I had checked the voltages and for some reason I had a 27 V AC drop from thick red to thin red at the thermostat, suggesting that the thin red was somehow the C wire. As can be seen above, thin red was just floating, not connected to anything. I'm not sure why I was getting the voltage between the two red wires (no connection would imply 0 V, as you see if you touch the red wire and hold another end of a multimeter into the air). I'm wondering if I actually had a scope to look at the trace if it would have been obvious that it was just noise and that somehow using a multimeter confused the situation. Anyway, this proved to be misleading.

The R connection, to the G, W, and Y connections (as measured at the thermostat) should all be at 24+ V AC when nothing is on which I think is guaranteed to happen when the thermostat is physically disconnected. This should have been a hint that indeed R was R, and I hadn't stumbled upon some magic thermostat.

In one last final attempt to be an electrical engineer/electrician I turned off the HVAC, tied the thick white wire and thin red wire at the thermostat together, and at the ceiling panel in the basement did a connectivity test between the two wires, and it passed/beeped, confirming the wires were what I thought they were.

This is the old image from above. I did run the test and it passed. enter image description here

The final step in getting this all working was to take the 4 conductor wire sitting on my wall and to wire it to the C on the HVAC control panel. From the ceiling photo I could tell the thick white wire at the thermostat was going to the white wire in this cable. I took that cable, fed it into the hole in the HVAC system where all the other wires came out, and connected the white wire to an existing junction point where the blue wire from the control panel just spliced with a white wire that again I think is going to the humidifier.

Here's the original splice point with blue and white mating: enter image description here

Here's an image with all 3 mating. You can't really see the individual wires but you can see my 4 conductor cable coming in sort of awkwardly and the white cable going somewhere. enter image description here

I'll note I did this wiring with the HVAC off (used the switch by the unit). Once I had added the C wire I wired it up at the old thermostat and confirmed that my old unit now worked without batteries. It did. At that point I turned the system off again, wired up the Nest, and after turning on the HVAC system again had a working Nest thermostat. As a final step I put some zip ties around the new cable (one that had been on the wall), tying it to the piping like was done with the other wire.

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