I have a fireplace wired up to a switch (simple off/on), and I want to throw a Nest thermostat on there.

It has a black and white wire coming out of the wall. I think the white goes to the W1 and the black goes to the RH?

I may have to run a common wire as well I read.

But is there any risk replacing the switch? Do I have my hook up correct?

  • how much current is seen by the switch?
    – dandavis
    Mar 6, 2020 at 20:41
  • 50mA through the switch
    – sci-guy
    Mar 6, 2020 at 21:04
  • i'm just some dude on the internet, but it sounds like it should be fine. use a relay buffer if in doubt.
    – dandavis
    Mar 6, 2020 at 21:05
  • Most fireplaces are millivolt actuated, if you do not have power at the switch it may take quite a bit more as nest thermostats are not millivolt devices. More info will probably be needed. If the switch goes directly back to the gas relay is is probably millivolt running off a thermocouple, or thermopile in the pilot flame for the voltage to open the gas valve.
    – Ed Beal
    Mar 6, 2020 at 21:38
  • @EdBeal the leads do come from the thermopile. Is that bad or good?
    – sci-guy
    Mar 6, 2020 at 22:24

1 Answer 1


You need both a transformer and a relay to do this

The Nest cannot switch a millivolt system on its own, even when supplied with 24VAC, as it has solid-state output switches instead of mechanical relays. Hence, you'll need both a 24VAC transformer and a 24VAC fan relay in order to control your fireplace from your Nest. The transformer secondary goes to Rh and C, while the relay coil is connected to W and C, and the relay contacts connect to the existing switch wires.

  • Should have read your answer and upvoted prior to commenting, +
    – Ed Beal
    Mar 7, 2020 at 15:03

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