I currently have a Honeywell Thermostat with 4 wires going to a Janitrol Furnace.

I am trying to install a Honeywell WiFi Thermostat RTH6580WF and I'm missing my C wire. My bundle has 8 possible wires, 4 of which are currently being used by the old thermostat (see attached OLD Pic). I went to the terminal block and saw a black screw above the W, G, R terminals. enter image description here

My question is: can I use the black screw above them as my C Wire? It has 24VAC burned into the board above it, so if my assumptions are correct, I would be fine with it; is this correct?

enter image description here

  • What is the model number of the furnace?
    – Tester101
    Nov 4, 2014 at 17:36

3 Answers 3


It does look like it might be a common, but only way to know for sure is to either look at a schematic of the board, or test it with a multi-meter.

Using a multimeter, set to VAC (Volts AC) mode, you should read 24V between the R and C (or in your case, "24V") terminals. You'll probably have to at least partially pull out the connector currently on the R to get the multimeter probe on it.

For reference:

  • R = Constant 24V ac
  • W = Heating contact (the thermostat connects this to R to turn on heat)
  • G = Fan contact (the thermostat connects this to R to turn on fan)
  • It will most likely only show 24v after defeating the safety switch, unsafely supplying power to the currently accessible unit. Proceed with caution.
    – Mazura
    Nov 5, 2014 at 1:00

That screw is for a 24v humidifier.

C wire is blue that connects to the transformer in these Janitrol furnaces.


It is blue, I had the same issue. Must remove the G wire on the terminal board on the furnace and attach to the chasis nut with the blue cord already attached. That will bring power to the unit. Then, in order to get the fan to turn on, connect a jumper from the Y wire already attached, which runs the cooling/compressor to the G attachment where you previously moved that original G wire.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.