I've looked at just about every post on adding a C-wire to power a WiFi thermostat but remain uncertain on how to proceed. The current installation on a Rheem RGAC-080A is two wire as here:

XFMR - White -> Thermostat - Red -> Valve TH terminal, Valve TR terminal to silver Honeywell box (capacitor?) -> XFMR

(Tried to take a photo but it's too dark to be informative.)

I can run 3-wire from the thermostat to the furnace and reproduce the Red & White connections. But where does the third wire go? Valve TR?


Edit: Furnace schematic

  • Couldn't find any documentation for the unit. Is there a schematic printed inside (possibly on the access panel cover)?
    – Tester101
    Commented Dec 29, 2015 at 15:58
  • What is XFMR? Your wiring description is very confusing - try to not use any abbreviations at all. The "C-Wire" is just one of the wires coming from the transformer.
    – JPhi1618
    Commented Dec 29, 2015 at 16:04
  • @JPhi1618 XFMR = transformer. My assumption is that, according to the graphic immediately below "Welcome to the future" at this answer, the C-wire goes to the transformer terminal opposite that to which the thermostat wire (in this case a white wire) goes.
    – geoB
    Commented Dec 29, 2015 at 16:32
  • @Tester101 Yes, there is a schematic. For me it's a lot of noise, very little signal. Am I correct in thinking that the C-wire is to provide a complete circuit to the wireless side of the thermostat regardless of the continuity for the furnace valve?
    – geoB
    Commented Dec 29, 2015 at 16:35
  • In that image the transformer has a red and a blue wire coming from it. The red wire normally goes to the Thermostat terminals labeled "R", "Rh", and/or "Rc". The blue wire gets hooked up only to the furnace somewhere (the internal connection between the green box and purple box in the first picture). You need to run the blue transformer wire to the thermostat as the "C" wire. C stands for "Common", and is very much like the white wire in home electrical wiring.
    – JPhi1618
    Commented Dec 29, 2015 at 16:39

1 Answer 1


Based on your description of the wiring, and the schematic you've provided, this is what I think is going on.

enter image description here
Click for larger view

It sounds like there's a white wire and a black wire, connected to the secondary side of the transformer. The white wire carries 24VAC to the thermostat, which means the black wire is connected to the C terminal.

So the new thermostat should be wired as follows:

  • White -> R
  • Red -> W
  • Extra (blue in diagram) -> C

You can verify this by measuring the voltage between the blue and white wires, in the diagram above. You should get ~24 volts AC, though it might be a bit higher or lower.

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