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I am thinking about changing thermostats for my ICP END4X36T17A2 furnace, but my options are pretty limited due to my current thermostat hookup lacking a C wire. It currently has four wires hooked up to the Y, G, W, and Rh terminals. I followed the wire bundle back to the furnace and looked at how it's connected to the circuit board. I see five terminals there labeled G, C, W, Y, and R; all of them except C are currently running to the thermostat. (There is a separate two-wire bundle running from C and Y to my external air conditioner) It would seem to be a pretty straightforward task of running a wire from the C terminal to my thermostat, except checking with a multimeter I don't detect the expected 24VAC from it. I'm guessing this means it wouldn't work. There are also two tabs next to the Y and R terminals labeled "TEST/TWIN" and "24VAC", but I don't detect any AC voltage from them either. Is there a way to make this work? Is this a job for a professional?

Here is a close up of the terminals (click to enlarge). The four wires running off the right edge are the bundle going to my thermostat.

Closeup of the thermostat wire connections Circuit board details Furnace details

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The furnace control board is in the blower compartment. Nearly all furnaces have a safety switch which cuts power to the furnace and its controls when the blower compartment is opened. You'll have to bypass that safety switch while measuring AC volts between R and C. (While doing this, take the usual precautions to ensure that long hair, clothing, etc would not be pulled into the blower if it should come on unexpectedly.)

For mains-voltage AC it would be a clear violation of electrical code to run one extra conductor separate from the cable. Not so for low voltage like a thermostat, but still it has the potential to be confusing to others later. If it's possible to run one conductor from the furnace to the thermostat, I suggest to run a new 7-conductor cable instead. The work is the same but the result is much more professional.

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  • I feel foolish now; there is an obvious switch that gets pressed only when the compartment is closed. When I press it manually I measure about 28VAC between the R and C terminals; is this within the acceptable limits? Good point on the wiring. I'll get a multiconductor cable to replace my 4-conductor one later. Dec 4 '20 at 20:08
  • Look into the 'add-a-wire kits. You can move one of your wire bundle to the C wire, and then multi-plex the signals from the other wires ==>-~-~-~-<=== and demultiplex them at your circuit board. Much easier than running another wire.
    – gbronner
    Dec 4 '20 at 20:27
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    @DavidPitchford Yes 28 VAC is great.
    – Greg Hill
    Dec 4 '20 at 21:03
  • Only suggestion I'd have would be to use 8-conductor thermostat cable (or fatter, if you can get it) instead of 7-conductor Dec 5 '20 at 0:23
  • Actually, the thermostat I'm eyeing now apparently doesn't require a C wire, but I'm pretty confident I could run a fatter cable to my thermostat if I need to. Thanks! Dec 5 '20 at 18:02

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