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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.
Lou Grinzo
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  • You should add 3A: buzz out the wires w/ ohmmeter to ensure proper setup before turning on power. – Carl Witthoft Apr 28 '16 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. – Carl Witthoft Apr 28 '16 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 '16 at 17:39
  • @Tester101 no, it is not a hack solution. In fact, it's the recommended solution that the thermostat (Emerson among others)manufacturers recommend for use with older furnace systems which do not have anything other than 2-wire feeds to a make/break thermostat. Your suggestion to downvote implies rather a lack of understanding on your part. – Carl Witthoft Apr 28 '16 at 19:30
  • @CarlWitthoft having a wall wart plugged in, with a power cord running up to a thermostat looks pretty hackish to me. The more professional method, is to pull a new cable between the furnace/air handler and thermostat, and wire in a proper C wire. – Tester101 Apr 28 '16 at 19:57
  • @Tester101 and I repeat: there is no C-wire or C-terminal on older furnaces – Carl Witthoft Apr 29 '16 at 11:20
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    @CarlWitthoft There's always a "C wire", it's just not always labeled. – Tester101 Apr 29 '16 at 12:40
  • Sounds like you've got it all sorted. Write up an answer, and give yourself the checkmark. – Tester101 Apr 28 '16 at 16:09
  • Do not hook up the white AC (neutral) wire to the C post of the furnace transformer. – Stavr00 Oct 30 '17 at 18:11

2 Answers2

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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.

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

j c
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  • 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! – ThreePhaseEel Dec 06 '16 at 03:02