comments:
I’ve had that working since 2013 on two fireplaces (I have a log of major successes, that one wasn’t actually too bad once I got into it.) It turn out the fireplaces had circuitry to add a MV (Millivolt) wall thermostat. Originally I just used utility relays from Amazon https://a.co/d/05DtqBIg a year or so ago when one of the relays had developed high resistance again (bad with MV circuits) due to oxidation on the contacts (not visible…. But try calculating voltage drop on MV and you get the idea quickly lol) I replaced the relays and smart plugs with a Shelly 1 Gen 4 about a year ago. The Shelly is powered with line voltage and its internal solid state relay is switching the MV circuit from the fireplace. I use simple Hone Assistant Automations to create a pseudo-thermostat with a temp sensor to switch the fireplaces on/off. (Anyone attempting don’t forget hysteresis of at least 1 degree.)