I am a bit confused with my furnace wires. I did some research and watched a bunch of YouTube videos on how to install the Nest thermostat. I currently have a normal digital Honeywell thermostat.

I popped the cover off and it seems like there is no C connected to the thermostat as it is currently running on batteries. However there is a blue wire, which is disconnected.

I went up to my furnace to see the wiring and there is a thicker brown wire connected to the C, but I don't know where it goes. I don't see it by my thermostat but I do see the other end of the blue wire.

I was planning on stripping some of the blue wire and using that as my C for my smart thermostat? Please advise on what I can do and what this brown wire on the C can be?

wiring at thermostat and furnace

  • 2
    Is the cable at your thermostat the same one that is shown at your control board? The one at the thermostat has more wires and they look heavier. It has six wires, which is plenty to include a C wire, but at your control board there are only four. Is it the same cable? Are the other two wires available at the control board? Are there in fact two cables spliced somewhere? Where? Do you have access from that place to run a new cable to the control board?
    – jay613
    Commented Feb 23, 2022 at 13:20
  • Yes. It looks like two different cables.
    – gnicko
    Commented Feb 24, 2022 at 0:31

2 Answers 2


Yes. What's happening there is that R and C are coming from a 24-volt transformer. (you can think of C as "common" in electronics or "chassis" in car wiring, if you want.)

R is 24 volts, and goes to the old-school thermostat. The thermostat is like a switch, it connects R to either W, G or Y, depending on what it wants.

  • R to W to "call for heat". This tells the furnace to do its furnace thing, and the furnace gets local control of the fan/blower.
  • R to G to "call for fan". That tells the furnace to run the fan, even though you're not calling for heat. This is needed for ...
  • R to Y to "call for air conditioning". That tells the air conditioning compressor to run. The furnace is not involved in that. Most likely, the "Y" terminal on the furnace is only there as a courtesy splice block.

Meanwhile, actually at the air conditioner, Y and C go out there. They connect to a fat relay which feeds power to the A/C compressor motor. The circuit completes:

Transformer - R - thermostat - Y - A/C relay - C - transformer.

Note the brown wires on Y and C. Those go to the A/C unit, just as you'd expect.

(yeah, you'd think it would be more sophisticated. That's only starting to happen.)

  • Good write up, Harper R.M.
    – gnicko
    Commented Feb 24, 2022 at 0:33

Connect the blue wire to C

Simply connect the blue wire to the C terminal at the furnace end, in addition to the wire which is already there (which goes off to your air conditioner's outdoor unit/condenser in order to provide a return for the compressor control signal, look at the Y terminal on your furnace's control board for how it's done), and then you can hook that blue wire up to the Nest's C terminal when installing it.

  • 1
    I don't see a blue wire at the furnace end. My guess is the cable from the thermostat and the one we see at the furnace are spliced somewhere. Hopefully somewhere accessible.
    – jay613
    Commented Feb 23, 2022 at 21:06

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