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I'd like to replace a mechanical millivolt (2-wire) thermostat for my propane space heater stove with a programmable one. I'd like to do this to ensure that the space heater doesn't heat overnight, which amounts to nontrivial propane and hence $$$ savings during winter time.

However, I can't seem to find thermostats that are at the same time battery powered, programmable and suitable for 2-wire millivolt setups. You semingly have either nonprogrammable 2-wires, or powered programmable ones, or smart ones that don't do millivolt.

Are there such thermostats for sale at all?

Here's why I need this:

  • Battery powered: since I don't want to contort to give it a 24V supply.
  • Programmable: so I don't need to remember to set the temperature manually. Doesn't need to be smart, doesn't need to connect to anything. I just need the ability to set a schedule.
  • Millivolt: because the stove uses the millivolt thermostat circuit.
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  • all you need is a relay that breaks the circuit at night ... bistable relay would require power only when switching
    – jsotola
    Dec 3 '20 at 20:06
  • Note, I don't want to break circuit at night. I live in the mountains where it gets very cold, and I want to keep a certain temperature level at night as well.
    – filmil
    Dec 3 '20 at 22:28
  • You might look at the Honeywell RTH series. We have a non-programmable, battery-powered unit that would appear to work with a 2-wire circuit. (Dunno if it will work with millivolt, though.)
    – Hot Licks
    Dec 3 '20 at 22:55
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Yes Honeywell makes 2 wire battery controlled thermostats, some that look just like the old round ones of the past. I have a newer one on my pellet stove when the stove doesn't kick into high when below the set point I know the battery is dead.

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  • Can you share make and model, perhaps?
    – filmil
    Dec 3 '20 at 22:29
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    @filmil I have a Honeywell RTH6450D which has worked well with my millivolt gas wall heater for 3 years. I hooked it up in parallel with the existing mechanical thermostat so that if the battery powered thermostat fails there's a backup. The model I specified is "5-1-1", but in hindsight I'd choose to buy the "7 day programmable" version so that Friday (or other weekdays) can have different schedules. [I hope this comment provides material that can be a useful addition to this answer; I didn't post it as a seperate answer since this is getting into product recommendation territory.]
    – Kevin Reid
    Dec 4 '20 at 2:06
  • I had to pull the cover at home because I could not remember. It is a TH1100dv the battery life is at least the heating season I think I got 2 years once it can be hooked up to 24v I had forgot that but my stove is millivolt and the battery works fine, you can see the displayed temp in the day light half way across the room but at night you have to tap the buttons and the back light comes on the battery last longer if turned off when not heating like summer time. I went with the more modern style I think it was around 30$ a few years back.
    – Ed Beal
    Dec 4 '20 at 14:31
  • TH1100dv is battery powered, but not programmable, apparently. RTH6540D has a battery backup but requires 24V power long term. Sadly neither fulfills my requirements.
    – filmil
    Dec 4 '20 at 21:54
  • Yes it is a replacement for a 2 wire round style any of the millivolt models would work. Not sure how long the battery’s would last, if memory serves mine mentioned dual power in the cover battery or 24v
    – Ed Beal
    Dec 4 '20 at 21:58
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Most "first gen" programmables do support battery operation and millivolt systems

Where you're getting tripped up is that while most non-"smart" ("old school"/first generation programmable) thermostats support 24V power either using "power stealing" or a C wire, they also support battery power, and use relays for switching, which allows them to support millivolt systems just as readily as anything else. You can even have them set up to support an external transformer, such as a 24VAC "wall wart", while retaining the batteries for power backup during outages. (This is done by wiring the stove to Rh and W, while wiring the transformer to Rc and C.)

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  • May be. I'd prefer a thermometer which is designed to use batteries as a primary power source, rather as a backup. I can imagine the design constraints being different for these two cases.
    – filmil
    Dec 4 '20 at 19:13
  • @filmil -- it's more that the thermostat is designed to work just fine for a variety of applications (transformer only, battery only, "power stealing", one vs. two transformer as well) Dec 4 '20 at 22:40
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I want to report back with my Nest learning thermostat and millivolt escapades. In case it is useful to someone.

tl;dr: I made it work. But indeed, Nest doesn't support millivolt use cases well enough for it to be the only thermostat.

Quick run-down of facts:

  1. Found this nice presentation which, if you look down the pages, contains a reasonably detailed block-schematic of the switching and power connections for the Nest. The block diagram is detailed enough that you can draw meaningful conclusions about which approach may work. The schematic says, for example, that when heater is actuated, Rh and W1 terminals will be shorted. This is exactly what we need to actuate a millivolt system - the only concern is whether the short circuit that the Nest circuitry makes is "millivolt enough" to cause the correct reaction. Turns out, it is.

  2. Connected a 24V AC adapter to the C and Rc terminals. This provides power to the Nest device. The block schematic says that the C-Rc combination will allow Nest to get power. It also seems that this power will not be electrically coupled with Rh in this particular wiring configuration, which seems important.

  3. Connected the millivolt wire terminals to Rh and W1 on the Nest backplate. The wires will be shorted to actuate, so either way of wiring wil be is OK. The block schematic says that there is a FET switch which will short the two terminals when actuated, but otherwise keeps them open. Apparently the voltage drop across the FET is low enough that our stove considers it as a short and actuates.

  4. The schematic seems to indicate that Nest senses the wires mechanically. This means you need to use either solid core wire, or tin the terminals that go into the backplate so that they compress when clamped down by the latch. Stranded wires will spread and flatten the conductor cross-section so much so that the mechanical switch will think there's no wire. I was lazy to tin the terminals, but I instead removed a longer piece of insulation from the wires than minimally needed, and then folded about 1/8in of the wire onto itself and jammed that in. It was enough to make nest think that the wire is there.

  5. Nest is battery powered, so in theory it could use battery power to actuate the heater during power outage. However, Nest's software prevents this - you get some error code which says that power is out - and it may well be for a good reason. The end result is, you can't use a Nest learning thermostat if power is down in your home. This matters a lot to us, since we live in the mountains where power outages are not unheard of, and the propane stove that Nest controls can and should work without power. I ended up leaving the old mechanical thermostat connected in parallel to the Nest, just in case. So when power is out, you simply use the old thermostat. For the "regular" Nest operation, when it controls an electrical heater, a power outage is irrelevant, since when power is out to the Nest, the power is out also to the device that it controls.

  6. I still worry about the Rh terminal floating but haven't yet measured what happens.

  7. YMMV, based on the type of the device that you are trying to control. Some people have reported what seems to be this exact setup as non-working. I also bought a 24V AC operated mechanical relay to insert in case it is necessary, and as advised elsewhere. I tried to find a solid-state relay but couldn't find one which is correctly rated - they seem to start at over 40V and all I found are DC - which makes sense, but is worth knowing. I ended up not using the relay. Note that in case of relay operation, your wiring must be different.

Once the above is handled, the rest of the Nest setup went basically without surprise.

Hope this helps. I think this is more detail about this setup that I could ever find on the Internets at one place.

Some colleagues at work advised me that tinning stranded wires isn't the best idea since the tinning is soft and can flatten over time and then confuse the mechanical wire sensing in the Nest learning thermostat, and they recommended putting an actual connector terminal with a wire crimp. I was too lazy to do that now that our setup works fairly well, but this is something to consider if you are following the instructions above.

Be very, very aware that the instructions above are specifically for the thermostat that is called "Nest learning thermostat". This is, as it were, the most expensive Nest thermostat out there. There are other, cheaper models, for which my colleagues reported failure to control a millivolt thermostat. The Nest learning thermostat is some $250 at the time I write this; but given how much propane it saves us, it has already paid itself off during the winter. Nest-E won't work this way.

Note also, the Nest learning thermostat does not officially support millivolt systems. I think this isn't related to whether it is possible to do, but rather related to the fact that Nest can not work without external power, so your Nest would be useless in a power outage. Since millivolt devices can usually operate without power (like a propane stove for example), and are installed in places where power may be flaky, it is expected that the thermostat can operate them without power too. But Nest can not deliver on that - the battery power simply isn't enough for this. As you can see above, I worked around that issue by keeping the old mechanical thermostat connected in parallel.

Hope this helps.

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  • About wire tinning: after a few months of operation, the tinned wires were mechanically flattened, and no longer worked reliably. I had to redo them, but the intervention was simple.
    – filmil
    Sep 29 at 5:05

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