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Honeywell has a video that shows how to use the fan wire ("G") of the cooling system as the "C" wire for the thermostat. It is in the video - "Honeywell DIY wifi...substitute g-wire for c-wire" on youtube.

They say to move the g-wire in the thermostat and and move it to the c-wire terminal.
As shown in the picture below this part can be done. (I have a boiler and an a/c connected to this thermostat. The "R" and "W" wires are for the boiler. The "Rc" "G" and "W" wires are for the a/c.)

wires attaching to thermostat

The problem is in the a/c unit. In the youtube video they show wires attached to the "Y" "G" and "R" terminals. (I am ignoring the "W" because I think that is for the heat, and I have a separate boiler.)
As you can see in the picture below, mine has wires connected to the "R" "C" and "G" terminals. The a/c works fine and I am able to turn on the fan separately from the a/c.

How would I change the wiring to use the "C" wire on my thermostat?

inside a/c unit

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The Honeywell video describes the process, when the thermostat is used to control a forced air furnace. Since your situation is different, this may not work for you.

If you want to try it, you should proceed as follows...

  1. Disconnect the green wire in the second photo from the G terminal, and connect it along with the white wire to the C terminal.

  2. At the thermostat, move the green wire from the G terminal to the C terminal.

NOTE: This may not work in your situation, and may void the warranty of the equipment. Proceed at your own risk.

In this configuration, the thermostat will not be able to control the fan. And depending on the thermostat, the C wire may have to come from the same transformer as R. Using C from the Rc transformer may not work.


I've labeled the wires, to hopefully help you understand what's what.

Labeled Photo

This is what you have now:

Current wiring

This is what it will look like after you switch the green wire.

New Wiring

  • Thank you. Is the way I have it wired the normal way that an a/c is wired at the terminal block when there is no heating system? I don't want to mess around with the wiring at the terminal block. Since wiring the thermostat is only a temporary need I wanted to know if the wires connected to the terminal block are wired directly to the thermostat. In the pictures you are showing me the answer seems to be that the 'g' and 'r' wires are and the 'c' wire is not. If this is true then I will remove the 'c' wire totally and move the 'g' to the 'c' spot on the terminal block to play it safe. – ben Jan 6 '15 at 18:23
  • ...And then I will move the g wire on the thermostat to the c wire spot and remove the y wire on the thermostat. In the summer I will put back the wiring to where it was. – ben Jan 6 '15 at 18:27

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