Wiring – How to wire the WIFI thermostat to control the millivolt fireplace


I have a gas fireplace that has an millivolt on/off switch on my wall (and a remote that includes flame on/off, a countdown timer and a thermostatic flame control).

I want to be able to program this fireplace to automatically turn on and off based on different temperature thresholds during different times in the day. Just like I do with my forced hot air furnace using its programmable wall mounted thermostat. Then I started thinking programmable and I see the WIFI thermostats. Why not use a WIFI thermostat? Okay great!

With a WIFI thermostat I can make the wall thermostat a weather station too, showing the indoor and outdoor temperature. I can also cause the space to be heated from a remote location. For better or worse let’s assume I want a nice programmable WIFI thermostat.

Let’s assume too that the fireplace remote is useless. You can set the remote to a desired temperate, but then later you go to bed, having forgotten that it is set and it runs all night because my programmable forced hot air furnace thermostat lets a the house cool down to 60. All night the little fireplace, based on the remote setting, tries to heat up its space to 72. An annoying waste of gas and money.

The problem is the wiring for the WIFI thermostat. All I have is a milivolt circuit and no C wire. The fireplace on-off switch is millivolt. Also, the 24vac output from a standard WIFI thermostat, I assume, would fry the fireplace millivolt flame on/off controller.

How can I power and use this new WIFI thermostat when my fireplace wall switch is a millivolt circuit? There is house current to the switch box, where the fireplace fire on/off switch is currently located too.

I am thinking that I will mount the new programmable WIFI thermostat above the wall switch, as that is a good location for sensing the current temperate in the space.

It seems like the problems here are:
A) How do I convert the house current to 24vac, so that it can power the new thermostat?
B) How do I turn the 24vac thermostat on/off output from the new WIFI thermostat into a switch for the millivolt circuit?
C) Is there anything else I need to figure out?

How do I do this?

Best Answer

You'll need a transformer to step down mains voltage, to 24 volts that the thermostat can use. And you'll need a relay that can work with 24 volts.

You'll wire it up something like this.

millivolt thermostat control

When the thermostat calls for heat, the relay contact will be pulled closed. This will signal the fireplace to turn on.

NOTE: the three contacts used on the thermostat are R, W, and C. In the drawing the C may or may not look like a C, so I just wanted to make that clear.