Milivolt Systems w/ Modern Thermostats

Jesse Grandbois is a tech who reads the tech tips, and he wrote a few tips that he wanted to share on some gas furnace control basics. This tip is about how to use a fancy digital stat on a millivolt system.

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 a couple of 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 wiring diagram while reading the information below.
You're going to feed 120v to the transformer. This is what will be used to create the 24v for the thermostat.
Wire your 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.

—Jesse

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4 responses to “Milivolt Systems w/ Modern Thermostats”

  1. Though the article addresses adding a transformer and operating the gas valve through the contacts of an isolation relay there are digital thermostats that can be setup for milivolt systems just with the thermostat battery.

  2. There’s lots of digital thermostats that will work with millivolt systems, but you’ll need the 24v for wifi thermostats which is what this is about specifically in the article.

    • Can you recommend a good relay to use? My preference is DIN-rail or panel mount. Solid state would be good too, but mechanical is fine.

    • Is there a way to operate a minivolt wall-mounted gas furnace (no fan) with a WIFI thermostat just with a 24 volt AC transformer and no isolation relay?

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