Old fashioned 2 wire pellet stove thermostat question

I have a pellet stove with a simple mercury switch thermostat. Can I add a 24 volt transformer and hook up the wyze thermostat? If so, what terminals would I connect to?
Could connect the 24v transformer to rc and c? And then could I connect my old thermostat wires to rh and w?

If I had a schematic of the thermostat terminals, I could figure this out on my own. Anyone?? Please help

TL;DR: the thermostat shorts RH with W1 thinking it will send 24V (which it does not as there is no C to the old millivolt heater). That short it what the 2-wire millivolt heater wants! It worked with my Wyze Thermostat.

Details

I had an WiFi thermostat from Emerson (model 1F87U-42WF) Sensi. Which is NOT supposed to be compatible with the older (1950) millivolt 2-wire heater control…
BUT it worked because …

Using a voltmeter, I determined that the Sensi thermostat would use the RH (which is supposed to be the 24V~ power source for the Heater) and just directed that pin (via mechanical relay which I can hear click) to W which normally controls the heater’s relay. Basically the thermostat was trying to send 24V into W by closing a relay, and expecting that the heater would the see 24V on W when compared to C (common). This relay closure between RH and W is what the old 2-wire millivolt heater wants!
I did install an extra 24V transformer as the WiFi would not work reliably because the 2x AA batteries were not enough.

For the Wyze Thermostat, I’m re-using that 24V supply and plugged it into C/Rc.
The Wyze Thermostat doesn’t allow testing on a bench without a heater connected to W1 as it is being smart and checks for connectivity.
But, as I am an engineers who dabbles in electronics … YOLO!
-> External
Don’t attempt if you plan on complaining and returning a fried Wyze Thermostat :slight_smile: or your heater blows up.

My old Sensi Wiring:

My working Wyze wiring:

Getting confirmation from a Wyze Electrical Engineer if the thermostat can tolerate 250- 750mV between Rh and W when it tries to heat would be awesome.
I checked my heater, and it uses a Robertshaw 7000 valve system which uses a thermopile with output between 250-750mV.

I can confirm that the Wyze thermostat works with 2-wire heat-only millivolt systems without the use of a relay. All it needs is a 24VAC power source connected to C and Rc to power the thermostat itself. It then provides the needed closure between Rh and W that activates the millivolt gas valve using the small DC voltage (250-750 millivolts) that’s continuously generated by the thermopile using the heat of the pilot flame. (Having read others’ experiences, I’d bought a 24VAC SPST NO relay, but didn’t need it.) With the Wyze thermostat connected as above and the pilot lit, selecting those four connections in the setup app satisfied the app’s connectivity test, allowing normal setup to proceed. My propane wall furnace (no blower) has been running in this configuration for about 20 hours so far.

  1. 24VAC connected to C and Rc
  2. Two thermostat wires connected to Rh and W

Including this possibility in compatibility specs would allow a substantial number of potential customers in climates such as coastal California, where these simple heaters are common, to benefit from the Wyze thermostat.

The one advantage of the old 2-wire thermostat was that it requires no external power, so I’d have heat in electrical outages without needing to arrange standby power to the thermostat. I got around this by leaving it connected in parallel with the Wyze thermostat. It’s set to a minimum temperature, so will have no effect unless I need it to.