Powering C Wire in Fireplace install with 24VAC transformer

[douglasahelm]

Out of the fireplace I have Red and White millivolt wires leading to the tstat. Red is connected to RH and White to W. I have a 24VAC transformer I was planning to use to power the wireless module. From the transformer, was planning on running the "C" wire, but I am unsure whether I need a common or neutral wire between the tstat and transformer. I need to hook both wires to transformer to get a full 24 volts. Thanks for your help!

[blaine]

Wires from millivolt control to RH and W no polarity
Wires from adapter to RH and C no polarity
That puts 2 wires on RH and that is OK

[douglasahelm]

Thanks for your response Blaine. I am a little anxious about hooking into the RH with 24v from the transformer because my understanding is is that the RH and W from the fireplace are dry contacts. I would think hooking 24V into that would fry either the tstat or the control board down on the fireplace. It won't?

[blaine]

You are not connecting 24VAC to the millivolt control system. There is no such thing as any voltage on a single wire; voltage is always between 2 wires. You are sharing one terminal and that is all. The 24VAC is between RH and C. If you connected the adapter to RH and W, you would cause problems but you are not.

[douglasahelm]

Thanks Blaine! Got around to trying it this afternoon and it works just as you described.