Like so many others, I am looking to install a new wifi thermostat with an older furnace. I've searched across the Internet and I have a pretty good idea of what is needed and why; however, I have a couple of questions and I'm hoping someone out there will recognize my setup and provide some clarification. Please note that I purchase this house in 2017, so I was not around for the current furnace or thermostat installation.

So, with regard to the photos below:

  1. Why is the "Y wire" connected to the "Y" port on the thermostat, but the "Com" (C) port on the furnace? Shouldn't it match "Y" <-> "Y"? (Note: You can't see it on the photo, but the yellow wire is twisted so that it connects to the "COM" port on the furnace, along with the red and white wires from the other cable.)

  2. I see that there are extra wires that are unused (blue, orange, and brown). Should I just connect one of those to the "COM" (C) port on the furnace along with the other wire that is there?

So, I guess I'm just wanting to confirm the best/easiest way to create a "C" wire for my new thermostat. My guess is that I use one of the extra wires that aren't being used and just connect it to the "COM" (C) port on the furnace panel, correct?


Thermostat Wiring

Furnace Panel Wiring

1 Answer 1


From your picture it looks like in reality that the yellow (Y) wire from the fatter thermostat cable connects to the red (R) of the smaller two conductor cable. All I can see is that the white (W) of the smaller cable connects to the COM (C) of the furnace board. (That wire may look yellowed because of age but I suggest that it is actually white).

The yellow (Y) wire from the thermostat is typically the conductor that is energized to call for cooling. From your picture it is almost assured that the two conductor cable goes to your air conditioning compressor unit to cause it to turn on. The return for the current going to the compressor has to return to the 24VAC and that is what is the white wire of the two conductor cable.

The way that compressor control wiring is hooked up looks quite normal.

You should be able to easily use one of the spare wires in the fatter cable to feed a common connection from the furnace back to the thermometer. The blue or black wire is often used for this depending upon what is in the cable bundle.

  • 1
    Thanks! I just used the extra blue wire and connected it to the COM ("C") on the furnace board and the "C" connection on the thermostat, and it seems to be working.
    – Amadeus
    Commented Nov 24, 2019 at 23:00

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