Tag: millivolt

Jesse Grandbois is one of the techs who reads the tech tips 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


There’s still a ton of millivolt furnaces out there and a lot 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 basics.
You’ll be using an isolation relay. It’s just a SPST (Single Pole, Sngle 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

Scroll to top
Translate »

Daily Tech Tip

Subscribe to our daily tech tip to receive daily tips and advice!
Email address
Name