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Nest thermostat

Community Member

Has anyone used fan wire for c wire

i did and might have caused furnace damage as fan will not shut down and furnace does not fire

no voltage across thermostat hot wires


Silver Product Expert
Silver Product Expert

Mikemilt, The basic control system uses a step down transformer to supply power. the make your blower fan work, the Power In on the R terminal, is connected to the G terminal thus passing the Power IN to the Blower Fan relay. This energizes the relay and turns on the power for the fan motor. What you did by putting the G wire on the C terminal, is connect R to G without a call. Remove the green wire from C and put it back on the G. By doing this the thermostat will properly control the on and off of the blower.

Ken, The AC Cooling Wizard.

NestPro and HVAC company owner

Hey folks,

We appreciate your help here, @CoolingWizard.
@Mikemilt, I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.


Hey there,

Checking back in should you still have some questions here. Let us know by replying to this thread.


Hi everyone,

Just one quick final check in here since activity has slowed down. We'll be locking the thread in the next 24 hours, but if you still need help, I would be happy to keep it open. If there's more we can do, just let me know.