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I want to install a smart thermostat and it's recommended that I have a common wire in place to supply power to it for full functionality.

I checked the wire coming into the existing thermostat and it has three wires: red, white, and green. Only red and white are connected, green is unused.

When looking at the zone controller, the thermostat ports have space for three wires: R, W, and C. C is not connected, but present in the wire bundle leading into the controller, just like for the thermostat.

I know that Blue/Black is the common color for connecting to C, but since I only need C, can I just connect the green wire to C at both ends and just label it appropriately?

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Use the green, and make a note on each end of the cable telling which color is what. This can be placed behind the thermostat and on a tag in the furnace. Consequently green becomes a repurposed C wire. Next time there will be no confusion.

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