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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8326
Experience:  Furnace, A/C and heat pump specialist.
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Rick, I just purchase a new thermostat from Honeywell and I

Customer Question

Hi Rick,
I just purchase a new thermostat from Honeywell and I am trying to install is. There is no c wire connectivity so the Honeywell tech asked me to look on the furnace for the thermostat connection and I do not know where to look
Submitted: 2 years ago.
Category: HVAC
Expert:  airheatman replied 2 years ago.

Remove bottom panel on furnace and tell me if there is a control board mounted to lower housing.If so, there should be a C terminal on that board.

Customer: replied 2 years ago.
I see a connection to w c r g y
Expert:  airheatman replied 2 years ago.

OK we need to run a wire from that C on the board to C on your thermostat.That is 24 volt common to power the thermostat.

Customer: replied 2 years ago.
Ok. I am being told to inquire if it is ok for me to run the green wire which is a c wire from the therm to the c on the board which already has a red wire attached
Expert:  airheatman replied 2 years ago.

If you do not have an extra wire in the bundle between thermostat and furnace, you can substitute the G wire but you will lose fan only capability.

To do this, move the G wire from G to C at both thermostat and furnace ,Then you will also need to install a jumper at furnace between Y and G so that we energize fan on call for COOL.

Customer: replied 2 years ago.
Jumper? I'm lost. Sorry- not my field.
Expert:  airheatman replied 2 years ago.

OK the G wire is currently signaling fan to come on .If you steal the G wire and use it for C, WE have nothing to signal blower to come on. So you will need to take just a short piece of wire at the furnace control board and connect Y and G.

Customer: replied 2 years ago.
Makes sense. There is a switch that says emergency off/on. If I turn it off will that stop all power from running so that I am not electrocuted
Expert:  airheatman replied 2 years ago.

When you removed the door panel. we broke power through door button but you can kill power at switch it you feel safer. Below shows jumper. We just need to send a message to G so blower will come on with AC:

Customer: replied 2 years ago.
Can I use this
Expert:  airheatman replied 2 years ago.

AS long as it is not able to touch anything that might short.I prefer insulated wire like the thermostat wire. If the thermostat wires are long enough in furnace, I would steal a piece a couple of inches long from one of the existing wires.