We have a gas stove that can be either on or off. It has two wires going to a thermostat that has more capability than the stove, such as control of non-existent air conditioning and fan. The wires from the stove connect to the thermostat's W and R terminals, with the R jumpered to RC.

The stove is not our main source of heat. It is mostly used to supplement our heat-pump system and for ambience. So we would prefer to turn it on or off at will. The ultimate goal is to control the stove with Alexa. Smart thermostats seem to offer too much capability at too high a cost. We want a simple solution. One thought I had was to use a smart plug to operate a relay if a simple switch is all the stove needs.

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    Can you clarify what you mean by "stove"? When I read the headline, and even the first sentence or two, I was thinking "food cooking appliance", you obviously are talking about some sort of "home heating appliance". Maybe include the make/model and pictures of the mfgr plate, the wiring block (and wiring diagram if one's included). – FreeMan Jun 27 at 15:56
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    @FreeMan, I understand the confusion--I have the same problem. Evidently, that is the correct term when it's not a fireplace. It is not a cooking stove. It is a free-standing, glass-front, fake-log type. It is located in our living room. The make is Napoleon. The model is GDS60-1P. We believe it was installed circa 2016. I can add a circuit diagram from the manual, but need to scan it. – BigBlonde Jun 27 at 16:19

Disclaimer: I am not a professional. I am an electronics hobbyist who has researched and tinkered with my own home heating.

Yes, a relay is almost certainly all you need.

In conventional thermostat systems the device being controlled supplies 24 VAC (if it has an electric supply line to it) or a small DC voltage generated from the heat of the pilot light, for gas appliances not otherwise powered (a so-called "millivolt" system). In either case, the circuit consists of the power source, the gas valve, and the thermostat contacts.

(Side note for completeness: electric heaters are sometimes controlled by thermostats which directly switch the power to the heater. It's still true that a relay can replace the thermostat, but in that case a relay capable of handling the full voltage and current must be used, and installed safely. I have not heard of line voltage control circuits being used for gas appliances.)

You can cautiously confirm this for your system step by step:

  1. With a multimeter, check the voltage across the thermostat terminals with the heat off or the thermostat disconnected. It should be 24 V AC or something like 1 V DC.
  2. With the thermostat disconnected and the meter in resistance/continuity mode, check that the thermostat is closing a circuit across R and W when it is on and not when it is off.
  3. Having established it won't do harm by step 2, touch together the R and W wires going to the stove and expect that it turns on.

If all of that is as expected then you can add your relay without fear.

By the way, it is also entirely safe to hook up two thermostats in parallel to the same control wires (two closed relays are the same as one). In particular, you could add your Alexa-controlled relay and also connect the stove to a thermostat so that the stove is on if either of those controllers calls for heat.

You could also consider controlling the stove with the "emergency heat" / "second stage heat" terminals of the same thermostat that controls your heat pump, but that's trickier to do right because you're connecting two separately powered electrical systems and to make sure there are no consequences of that you'd want to use another relay as interface, with a 24V coil powered by the from the heat pump.

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  • Sadly, the [tag:thermostat-c-wire] shortcut only works in comments. (Either that or the space after the colon (:) is throwing it off. – FreeMan Jun 27 at 16:23
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    @FreeMan Fixed, thanks. – Kevin Reid Jun 27 at 16:36
  • @KevinReid, Thank you! This is precisely the help I needed. It sure looks like the thermostat is wired as a switch. I mostly needed to make sure. I like the idea of two ways to close the circuit. I have done that with humidistats in bathrooms (when I had kids who invariably forgot to turn on the fan when they showered). A thermostat might be useful during the coldest nights of the year. Mostly, though, we just want to run the gas stove on demand. – BigBlonde Jun 27 at 17:50

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