Have a propane gas fireplace, controlled by a hand held remote control thermostat. Wish to install a wall mounted wifi controlled thermostat, heat only. Seems that all available thermostats operate with a 24V system and I have been told that my fireplace requires a milivolt system. How do I resolve this problem. It is a country home which I only visit occasionally during the winter.

4 Answers 4


No problem.

First, the furnace must be designed to have an external millivolt thermostat. A millivolt thermostat is a very simple creature, it has 2 wires going to it, and it shunts those to call for heat.

Second, the smart 'stat will need a 24V power supply. This is available from any $15 thermostat transformer, such as many designed to mount in junction box knockouts or on the cover of a junction box. The 24V wires go to R and C on the smart 'stat. It goes without saying, this smart stat will not work with the power out.

The smart 'stat cannot directly talk to the millivolt furnace. However since a millivolt 'stat simply shunts the two wires, a relay can be used to shunt those same two wires. The NO contacts of the relay go to those two wires. The coil winding of the relay goes to W and C on the smart 'stat.

And that's it.

Now one more thing. A wonderful feature of most millivolt furnaces is that they are able to run normally when the AC power is knocked out. It's the difference between shelter in place, or needing a hotel room. ** Now, the smart stat will lose its 24VAC when the AC power fails, so it will not be able to command the relay to turn on. For that reason, leave an analog millivolt stat in parallel with the relay. Set it to 40F so it stays out of the way, then when you need it, you can set it to your desired temp and it will shunt the wires and make the furnace kick.

** Difference between your pipes freezing or not. (As if anyone would install a millivolt furnace in the snowbelt, ha ha, they are all used in Florida and California of course. You can't even buy them in the snowbelt, nobody carries them and your HVAC guy doesn't know what they are).

  • The other thing you need to a power outage proof heat system are no pumps or blowers. Many years ago, I was renting a place near Boston, with gas fueled double ended steam system, with a millivolt thermostat and igniter. Never lost heat with that.
    – DaveM
    Oct 21, 2017 at 3:09

It IS a millivolt system. It does not require it.

You need a thermostat that does not require 24V to operate. I have a Honeywell Lyric that works great and does not require 24V or a C wire. It comes with a replaceable lithium battery.

This would need to get wired into your fireplace. There is a gas valve just like a furnace with TT terminals. Are you familiar with this kind of wiring. If not then calling in a pro to wire it is by far your best and safest bet.


24 volt thermostats will not handle millivolt. There is a company that has a millivolt wifi coming.


This is a very late answer but anyone with a millivolt, standing pilot gas fireplace can try the LuxGeo wifi programmable thermostat. Use it with the battery option rather than the C hardware option. We have a propane millivolt gas fireplace, and it works great; we can turn the fireplace on/off and program via wifi or thermostat on wall.

  • Does LuxGeo link to Apple Home?
    – Lance Kind
    Nov 9, 2020 at 6:29

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