I'm new to this forum and was wondering if I could get some help regarding installing a Honeywell WiFi thermostat that requires a "C" terminal.

I currently have an older thermostat that has only 4 wires (Furnace + AC) and my cable also only has 4 wires...so I was looking at adding a 5th wire for the C terminal, but can't seem to find that terminal...

If I follow the thermostat wires, I see them going to the G & R terminals close to the transformer, but there is no C terminal anywhere ( only R, G, W, Y and I think there may be an O)

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I initially thought the "O" would actually be a "C" terminal, but doesn't look that way (also hard to see).

I do have another pic below that shows some blue & yellow wires going into the the transformer, but I can't say for sure if it's the primary or secondary...I was thinking I might be able to use one of those as the C wire to close the 24V circuit, if I can identify which is which?

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Another thought I had, was whether I could tap into the 24V (GND) terminal from the ignition control module instead (see below) - would that work, or does it have to be directly from the transformer?

Any help would be greatly appreciated.

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The Black Wire goes directly to the AC (there is only one wire used from the other bundle)

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I went and got myself a multi-meter and checked the voltage ...this is what I found:

R (RED) = Hot 24V terminal/wire

W (WHITE) = Heating


G (GREEN) = Fan

Any combination between R + W/B/G = 27.4 V, which means the circuit closes ...so if I just take the G wire (Fan) and move it to the "C" terminal on the WiFi Thermostat, it should power the Thermostat, correct? I understand that I'll be loosing the stand-alone Fan functionality, but will the Fan turn on when Heat/AC is working?

  • Can you post a photo that clearly shows the wire nut that the black wire from the thermostat cable is connected to, as well as its surroundings? This is the likely key to finding your C wire given that you have A/C, from what you are saying... Dec 27, 2018 at 5:08
  • More precisely, can you post a photo showing where the white wire in the A/C cable terminates at? Dec 27, 2018 at 23:31
  • Hi - I don't see any white wire going the AC...there is only the black wire from the thermostat going into the AC
    – Dan
    Dec 28, 2018 at 21:14
  • OK, I did another run at the wires and this is what I found...while the Black wire from the Thermostat goes to the AC...the AC also has a White wire that goes to the O Terminal (see last picture in the thread marked AC)
    – Dan
    Dec 29, 2018 at 0:08
  • Please use the edit link underneath your post if you're updating it with new information. Don't post the updates as answers.
    – Niall C.
    Dec 29, 2018 at 16:29

2 Answers 2


The O terminal is C in your system

Because we know the black wire going off to the air conditioner is our Y wire (as it's connected to the black wire from the thermostat's Y terminal, according to what you have said and the photo's so far), the remaining wire in that cable must be a C wire, since you mentioned that there are only two wires going off to the outdoor unit. So, we follow that wire to where it nuts in with the white wire going off to the O terminal and the brown wire off to the rest of the furnace, and that is the point where we tap C at. Or in other words, in your system, O is what they call what the rest of us call C.


If you can see your transformer and the 4x terminal of that transformer which it appears you can, you can always find the common or the 'C' terminal. Half the transformer will have 110-volts going in. The other half will have about 24-volts coming out. The hot side of that secondary winding will go to the red wire going to the t-stat. The other terminal of that winding will be your common. Good Luck.

  • Hi thanks for your response...what about using the 24V (GND) terminal from the ignition control module instead?
    – Dan
    Dec 27, 2018 at 22:00

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