For those of you who don't know, a millivolt system uses a thermocouple/thermopile with a standing (constant) pilot flame to generate a tiny “millivolt” signal used to control the furnace rather than the 24v signal modern thermostats use.

There's still a ton of millivolt furnaces out there, and many of the owners want the new and fancy Wi-Fi thermostats. If you're not sure how to get the 24v out of a millivolt system, this tip is for you.

But first, let's go over some basics:

You'll be using an isolation relay. It's just an SPST (single-pole, single-throw) NO (normally open) relay to separate the different voltages in a system.

Millivolt furnaces operate using a thermopile to power the system. A thermopile is simply multiple thermocouples. Some millivolt systems will also use a thermocouple for flame safety. Under load, a thermopile should operate at no less than 190MV.

Please refer to the following wiring diagram while reading the information underneath it:

You're going to feed 120v to the transformer. This is what will be used to create the 24v for the thermostat.

Wire the hot side of the 24v transformer to R on the Wi-Fi thermostat.

Connect W on the thermostat to one side of the relay coil. On the other side of the coil, you'll add C from the thermostat and common wire of the transformer.

You're going to take the thermostat wires off your old millivolt thermostat and add them to the NO terminals on the relay.

On a call for heat, the relay coil will be powered, closing the SPST relay powering the millivolt system (NO terminals) for heat.