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.
- 24VAC connected to C and Rc
- 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.