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I have an A/C unit that is currently wired to my nest thermostat using 4 wires. No Common wire. there is an unused wire in the bundle of wires. Can I simply connect it to C on the A/C unit and then to C on the thermostat to add a common wire?
 

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I did the exact same thing when installing my ecobee. In fact, it was ecobee's suggestion to go this route. Made wiring it all a snap.
 

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A lot of thermostat manufacturers tried using a "power stealing" gimmick and the gas valve coil or circuit as C and not use the C wire and it failed miserably and damaged circuit boards and caused lots of problems.

Then they came up with this add a wire electronic cheater device which was just as bad.

Glad to see they recommend a proper C wire.
 

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I believe power stealing technology came from an era when the controls were all electro-mechanical and could tolerate a trickle current, no sensitive integrated circuits.
 
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