Can anybody figure out where a C-wire would be connected in this old forced air electric furnace? How do you test it with a multimeter to make sure?

  • Big 240V cables are at 1 (pink circle) in picture; ground wire is just below, kind of underneath, connected to white wire going to motor (not shown, towards left of picture)
  • 240V to 24V transformer is at 2 (pink circle) with two yellow cables coming out on top for the 24V circuit
  • R cable for the 24V circuit is in a red circle and connects directly to the "R" screw used by the thermostat. The thermostat orange wire is currently connected to... nothing. Don't know why. Ignore that. There is also a floating gray cable containing 3 wires connected to nothing on both ends. Ignore that too.

There are 4 diagrams inside the box, but I think this is the correct one. Other diagrams seem to be for different outputs (kW) and voltages (240V or 208V).

Obviously trying to connect a smart thermostat (Ecobee Premium) to a very old furnace. The irony!

Thanks! 😉

Furnace wires

Thermostat connections

Wiring diagram

3 Answers 3


I have colored the "C" wire cyan, also a link wire and an unused terminal

enter image description here

I suggest running a wire from the unused C terminal through to an unused space on the the barrier junction block. labeling that space, and the connecting the c wire on the thermostat cable there.

  • To add: You can use a wire from the unused grey cable (preferably the blue or brown one) to make that connection from the C terminal to the barrier junction box. Commented Apr 11 at 8:18
  • I forgot to ask in the original question, but where can I connect a ground wire to the 24V circuit? Commented Apr 11 at 14:15
  • Why do you want to do that?
    – Jasen
    Commented Apr 11 at 21:36
  • Well, most thermostats now have R (Rc and Rh), C, Y and G. There must be a reason why they include G (?!). If I can easily add the G wire, might as well! Commented Apr 12 at 14:42
  • "G" is the wire that commands the fan, it's not a ground.
    – Jasen
    Commented Apr 15 at 2:06

Look closer.

You have 24 Volt Transformer clearly labeled.

On one side of it there is the C-wire and 24 volt coming out

Two yellow wires. One goes to the R on the thermostat and other relays, the other is the C-wire.

Do not confuse it with the incoming 240 volt power on the other site of the transformer.


I don't think that this has been clearly explained:

In most thermostats (and according to this wiring diagram), the R wire is hot and provides power to the thermostat.

The W wire goes through various switches, and when continuously connected (e.g. when the thermostat is calling for heat), it connects to the neutral on the 24V.

It looks like your thermostat was wired with an 8 wire cable. I'm curious what the other end of the orange wire in the cable is connected to at the thermostat.

What I would do is as follows:

  1. Verify that the R wire is hot, and that the W wire (the other yellow one) is unpowered when the thermostat is open (e.g. set to 0 degrees)
  2. Run a wire (and possibly a cable if chafing is an issue) from the terminal block below where the 'W' is (i.e. where the orange wire is sitting) through the grommet and over to where the top yellow wire is near the transformer.
  3. Cut the yellow 'W' wire near the transformer, and wire nut the two ends as well as the new wire together.
  4. Find your thermostat. Verify that there are 8 wires in the cable behind it. Pick a color (standard is generally blue or black) and connect that to the 'C' terminal on your new thermostat.
  5. Connect the other end of that wire to the opposite side of the 3rd terminal block.
  6. Write a 'C' in marker next to the terminal block.

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