I would appreciate a sanity check/advice from this community.

I plan to install a Wifi thermostat (likely a Honeywell) that requires a C wire in a house built in 2005. It's propane forced air plus AC. There is a blue wire running from the furnace to the (old, dumb) thermostat, but it's not connected to anything on either end.

On the furnace end, there is clearly a C terminal on the circuit board, with one wire attached to it, going to the AC compressor. The transformer hooked to the circuit board is labeled 24VAC 40VA.

From everything I've read online, I think I can simply:

  1. Shut off power to the furnace and AC at the breaker box.
  2. Hook up the blue wire to C terminal on the furnace so both the blue wire and the white AC wire are attached.
  3. Hook up the blue wire to new thermostat as part of installing it.
  4. Turn breakers back on and enjoy the vast wonders of 21st century networking technology.
  • Sounds like you've got it all sorted. Write up an answer, and give yourself the checkmark.
    – Tester101
    Apr 28, 2016 at 16:09
  • You should add 3A: buzz out the wires w/ ohmmeter to ensure proper setup before turning on power. Apr 28, 2016 at 17:25
  • BTW, for anyone else who does not have a C-terminal available, most thermostats will accept a local 24 VAC feed. I did this - plugged a 120--24 transformer (output regulated) into the wall and fed its wire pair to C and whichever "hot" leads the thermostat wanted. Apr 28, 2016 at 17:27
  • @CarlWitthoft That's a hack solution. If there was a way to down vote comments, you sir, would have been down voted.
    – Tester101
    Apr 28, 2016 at 17:39
  • 1
    @CarlWitthoft There's always a "C wire", it's just not always labeled.
    – Tester101
    Apr 29, 2016 at 12:40

2 Answers 2


find the Transformer inside the furnace. One wire connects to the R terminal 24 v other wire is common or neutral.

  • Modern furnace control boards make the connection from the C terminal on the transformer to the C terminal on the terminal block on the control PCB -- it's a wonder that they don't PCB mount the control transformer itself! Dec 6, 2016 at 3:02

You are correct -- the C terminal on the control board thermostat terminal block is the correct terminal for the blue C wire to your thermostat. The wire from the AC going to it is another clue -- the cable to the outdoor unit goes to the Y and C terminals, always.

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