Old furnace with no “C” terminal for wifi thermostat

furnacethermostatwifi

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

B (BLACK) = AC

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?

Best Answer

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.

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