[mririe]

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?

 

[yuri]

Yes

Shut off the power to the AC first.

 

[mririe]

will that be enough to provide power to the thermostat?

 

[yuri]

Yes.

It uses very little power.

 

[sktn77a]

Apparently, the NEST thermostats need a common wire.

 

[simonnyc]

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.

 

[yuri]

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.

 

[user_12345a]

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.