hvacthermostatwiring
Trying to install a WiFi thermostat. I have the unused blue wire which I believe is the common wire. It is unused both and the furnace level as well as on my current thermostat. However there is already a white wire in the COM: connection at the furnace. So I have no open terminal.
![Thermostat[![][1]](https://i.stack.imgur.com/zVHVr.jpg)
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Since in your case, one side of the transformer is grounded. You can simply use a fork or ring terminal, to connect the C wire to the chassis. Though it appears there's already a wire that's attached to ground, and comes right over near the thermostat wiring. I'd just put my C wire in with the other two wires, in that twist-on wire connector near the bottom of the photo.
Connect the blue wire from the thermostat cable, to the C terminal in the furnace. Then connect the blue with at the thermostat, to the C terminal of the new thermostat.
Best Answer
There's nothing wrong with adding another wire to the COM terminal at your furnace.
Just use the spare blue wire there along with the existing white wire from the other cable.