I have a Google Nest I want to install and I saw there was an unhooked blue wire at both ends, so no big deal right? Wrong, apparently. What the heck? We have a Climatrol 170-1-080-02-00 furnace and a Climatrol 940-161 A/C. See image below. What do I do??


I can't do the G wire bypass or hook up the C wire because of this wacky wiring going on.

I should mention that I tried connecting the blue wire to the other side of the common port (with the breaker turned off of course and turned off at the old thermostat) and then the entire system quit when I turned it back on (I turned breaker on first, then thermostat). Maintenance had to come fix.

Here is the thermostat:

thermostat terminals showing wire sequence

And furnace/AC wiring:

furnace terminals showing 3 wires connected


1 Answer 1


Thermostat wire colors are not standardized. Any installer can use any color they want for any wire.

However, it's obvious that the designers of furnaces were talking to the makers of thermostat cable. Cables are made in 2-wire, 3-wire, 4-wire, 5-wire etc. The colors are used in this order: Red, White, Green, Yellow, Blue, Orange.

If we sort furnace wires by popularity of use, we get this:

R = 24-volt supply to furnace
W = "Call for Heat" (short to R if you want heat)
G = "Call for furnace air handler to run its Fan Only"
Y = "Call for A/C"

See how clever that is? The terminal codes are chosen so the installer can simply match letters to wire colors.

"Can" being the operative word. There's no accounting for idiocy.

Also, sometimes a section of the wiring doesn't need all 5-6 wires - e.g. the branch wire to the A/C actually does not need R or W, it needs Y and C. However 2-wire thermostat cable is manufactured red and white. So expect to see red Y and white C in that location.

My theory

My theory is that you're looking at 2 completely different cables there. The cable that goes to the thermostat is not the same cable as the one going to the relay-transformer (if what's what that is).

Just because it's the first cable end you found, doesn't mean it's the only cable end.

It's clear that what's going to the relay-transformer is simply /3 thermostat cable. Go back and guess which colors /3 cable is. Right first time :) Et voilà, we see red-white-green wires. Remember what I said about branches might change the wire colors? A more fastidious installer might have re-marked the white wire black there to indicate that it's not W.

So you're looking in the wrong place. You have not found the other end of the thermostat cable.

  • Thank you! I suspected they may have done something up the line, because it doesn't make much sense. So the white wire coming out of it actually doesn't go to the box at all. It is connected to a black wire going somewhere else. The yellow wire (looks white in one of the pictures) is connected to the Control on the board. The Red and green are where they are supposed to be. There's nothing connected to White or Yellow on the board. It's very odd. I tried tracing wires as far as I was able, and I have no idea how they wired this thing. I decided it's probably best to leave to a professional D:
    – Melissa
    Jun 7, 2022 at 20:08
  • 1
    @Melissa "wired this THING singular". You're missing my point. There is more than one "thing" and you have only found one of them. You have not identified all the parts of your system yet. I think once you do find all the parts, this will be a forehead slap, followed by an easy win. Jun 7, 2022 at 20:41

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