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Hello I have a very old propane wall heater on a vacation home. It is a "millivolt" type that does not require external power (no battery, no 24 VAC, no 110 VAC). It is a simple 2 wire system. No C wire. You can see the current thermostat here: https://www.amazon.com/dp/B000BQWADS

I would like to keep this thermostat because we often lose power at this location. This allows me to control the gas heater even without electricity.

However I would like to install a smart thermostat in parallel that I can control remotely via the internet - but keep the existing one as back up. In case of a power outage, I can still control the temperature when I'm at the same location.

I understand that most smart thermostats like Nest don't support millivolt heaters out of the box, but I've read there are ways to rewire them to support them, especially the UK version of Nest. Regardless of this possible issue, my question is more basic:

Will two thermostats wired in parallel work well with each other?

Will heater run as long as one of the thermostats is ON (even if the other one is OFF)? Are there any conflicts or concerns is both thermostats are ON at the same time? Any concerns if one is ON and the other is OFF?

Thank you.

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The quick answer is yes: if both of your thermostats are the "dry contact" type, meaning they're a simple switch with no voltage applied, then they can be wired in parallel and connected to your millivolt gas valve. When either one or both together calls for heat the heater would turn on.

You're also correct that most thermostats designed for 24 VAC controlled furnaces etc wouldn't work directly with the millivolt system. It seems that such a thermostat could control a millivolt system, though, if you wire things so that the thermostat operates a relay and the relay interfaces to the millivolt system.

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Casting a spell so Americans in freezing zones cannot see this answer.

Fa-woomph in the dark

Yes, heaters with millivolt thermostats are wonderful. Because when the power fails, they don't care! They just keep on rollin'. That means your pipes don't freeze, and by "you" I mean UK people. (the only Americans who use these furnaces don't live in freeze districts; no idea why.)

Your idea of adding a Nest and leaving the original thermostat active is dead nuts on target. Set it low, like 50 degrees, so it kicks before the pipes freeze. Then, wire a relay's N.O. contacts in parallel with the millivolt thermostat, and have the Nest actuate the relay.

It will work properly in all modes. Neither the millivolt 'stat nor the relay contacts care if somebody else is also shunting the two wires. The heater will fire if either one calls for heat, so when power fails and the Nest's relay goes inoperative, the millivolt 'stat will save the day.

You absolutely cannot direct-wire the Nest to the millivolt 'stat because its operating voltage will fry the thermocouple and gas valve.

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