Thermostat wiring - C wire - How does it work?

I’m having a confusion moment while I anxiously await the shipping and delivery of my thermostat. ( Mid December is here! it should be any day now)

My understanding of the wiring is it wont work on a standard 2 thermostat wire system ( which is simply a switch connecting the 2 wires together) because it needs a 3rd “c” wire added which supplies 24vac to light up the thermostat( and memory I would assume)
so My question is, Only the 3rd 24vac wire is needed? it doesnt need a ground wire? how does that work?

Did you run the compatibility test? I didn’t think a 2 conductor wire was enough. Regardless, if you don’t have a C wire the Wyze thermostat includes a C wire adapter. I’m no expert but I don’t think the C wire actually provides the power, I think it’s the return that works with the power wire.

now Im more confused, I previously had asked what the C wire was for and was told it provided power to the thermostat ( VS using a battery which isnt an option) and other reading I thought I saw it was 24vac. Is that wrong?
As far as the actual wire, I have 18/5 but currently only 2 wires are used and at the current thermostat end its labeled Y and RH. A new furnace was just installed and I had hoped the new thermostat would have been here in time to be installed at the same time, but it wasnt. The new furnace does have a 24vac transformer and I was wondering if I could just hook to that as a C wire?
So now I guess Im back to being totally lost since none of the terminals in the furnace itself have any of the label options in the compatibility test.

If you have a new furnace with the transformer than the control board likely has the c connection along with the others. There should be a wiring diagram attached to the furnace and the terminals for C, R, RH, Y, G, etc should be labeled. Mine is old and weird and there’s 5 terminals right on the transformer itself, including the C, but that not common. My guess is that since you only had a simple 2 wire thermostat when the furnace was installed, they only hooked up the two wires you needed at the time and you can hook up the rest now.

I checked mine with a multimeter and as I thought, the C terminal is a ground/neutral. The actual voltage is on the R terminal (at least on mine), but you need that ground (C wire) to complete the circuit for power at the thermostat.

As Ray said, when you had your new furnace installed they just used the 2 wires since that’s what your thermostat needed. You can just connect the C terminal to one of the available wires.

Initially, I did not have a C wire and had to use the C wire adapter, but I ran a new 5 conductor wire and used the C terminal to eliminate the C wire adapter.

Here’s a picture of my control board.

Ok thanks for the replies. I knew it did t make sense for C to be 24v and no ground. The ground being the C makes much more sense. I must have read previous discussions about it wrong. I think I should be able to figure out something now… although mine looks nothing like yours *


have you traced the thermostat wiring back to that UTEC?

they actually go to the Y and G labels on the transformer picture. If you can zoom in close on the transformer, it has slight indentation lettering at each of the terminal… AND there is a C showing at the black wire. I’m thinking thats my C wire? ( I never saw these letters until I saw them in the picture, without the flash they are not visible)
BUT I just checked the compatability chart and it says NOT compatable… ugg, But the guys that installed it said they have people with Nest hooked up so I would assume this can too?!?

so those 2 wires for Y & G are the only wires at the thermostat? I think you would need both a C wire and a R/Rc for constant power. Do you have AC too? From what I’ve read, Y is usually to call for cooling & W for heat. That’s how mine is too.

here’s a article with some good info: What is a C Wire? And Why's it so Important for Your Smart Thermostat?

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“C” is not “ground”.

The C wire is simply the “common” side of the transformer 24V output (there are two wires coming out of the 24V transformer, “hot” and “common”). Just tap into the “common”. The R (red) wire is really the power source. The C wire closes the circuit and allows continuous flow of 24V energy to power the thermostat.

Thanks- I did just double check and the wires labeled Y and G go to wire nuts 6" past the labels, those 2 wires are then labeled (together) 24V and they go to the thermostat. So I will just hook up one of the unused wires to the C on the transformer and hope for the best.
Thanks for everyones help!!

ahh I see, sounds like a difference in terminology. Pretty much the same as a “ground” to complete the circuit.

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