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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22510
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I have a Honeywell ST9120g 4012 furnace fan control unit.

Customer Question

I have a Honeywell ST9120g 4012 furnace fan control unit. I swapped out the thermostat a couple days ago. For the new thermostat, I had to connect the unused blue wire to the "C" terminal as was instructed to power the thermostat.
When I went down to the basement to the furnace, I found that even though there should not have been any use for the C terminal on the fan control unit, there was a wire connected (either white or green...I cannot remember, but I think it was white). I disconnected that wire and connected the blue wire. The thermostat came on and worked as expected.
The next day, I noticed that the air coming out of the vents was no longer cold, but only room temperature at best. I then noticed that the AC unit (Goodman) was no longer on when the fan was on inside.
Should I have kept the wire that was connected to the "C" terminal connected? The wire that was connected was coming from a small three wire bundle (red, white, green) that goes outside to the Goodman unit. The only wire still connected to the fan control unit is the red wire, which goes to the "Y" terminal on the 24VAC side of the fan control unit.
Should I re-connect the White or Green wire to the "C" terminal along with the blue wire going to the thermostat?
Submitted: 2 years ago.
Category: HVAC
Expert:  BillyHvac replied 2 years ago.

C is a meeting place for all common wires. You could have 2-3 wires on C.

The ac unit from outside will have a white wire on C...the thermostat will have a wire on C (commonly blue)

If you us a humidifier it will also use c.

So c is for more than one basically unhooked the cooling and hooked up the thermostat