Smart millivolt thermostat

I have an old furnace that doesn’t provide power to the thermostat. My current thermostat uses batteries. I’ve looked at many smart thermostats that I’d like, but they all require 24v. I’d like to see one that runs on batteries and millivolts. You have an outdoor cam that runs on battery power. Why not a smart thermostat?

Yes! The new doorbell will work with my old wiring and it would be great if the thermostat did too. It won’t.

I agree with the above. I have a gas wood stove at our cottage that heats the cottage. There is no separate furnace. The thermostat is wireless and runs on battery. I also have another unit in my home in the basement as auxiliary heat. if your thermostat would work on these units it would be amazing.

Usually these are used with a wall furnace or something of that ilk. The millivolt power is actually generated by a thermopile (thermocouple?) that sits in the flame of the pilot light. It travels to your thermostat and when the thermostat goes on it completes the circuit back to the furnace and that current allows the gas valve to open. It’s a safety mechanism as much as anything, if the pilot goes out, the gas cannot turn on. If you have a gas water heater, it is basically the same thing, just self-contained. So all the thermostat is doing is closing a circuit provided by the furnace, the batteries just run the clock.

Anyway, no one does smart thermostats for this. Not that I have ever found. The reasons seems to be that keeping the “smart” stuff working and (mainly) keeping connected to wi-fi, eats a lot more battery than old programmable type thermostats use, which are basically just alarm clocks/timers. Even with primary cells (non-rechargeable) the wi-fi connection would only last a few weeks or couple months at most. A smart thermostat generally needs a more timely live connection than other smart devices, even cams, not because it is doing a whole lot, but as you want it to be extremely responsive, and also because your HVAC is more vital than a cam, you need long runtime between changeout of batteries. It could most certainly be done, the basic concept is nothing special, but the underlying engineering and performance requirements are more complicated than they seem.

If you dig online, you can find ways of doing it with a plug-in 24v transformer to provide power to the thermostat and installing a relay switch to open/close the millivolt circuit. But as it’s a furnace, and as it’s gas, one should probably not be attempting it if they aren’t familiar with this stuff. That said, once it’s wired up, it’s done, and changing to any smart thermostat in the future, if you wanted a new one per se, would be super easy.

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