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I'm seeking a fireplace that can have two thermostats installed in parallel, a "smart" thermostat that requires power and a "dumb" mechanical thermostat that allows the fireplace to provide heat without power or batteries.

My research tells me they are common but I need help in cracking the "code" that can hide them among fireplaces that can't use the "dumb" thermostats. There doesn't appear to be any consistent naming convention, hopefully someone here can expand my vocabulary so I can communicate what I'm looking for when talking to installers and suppliers. It is in lacking this vocabulary that is making it difficult to even ask the right questions.

There's still plenty of overhead power lines around here, and it snows plenty on occasion, meaning power outages tend to come in the winter. To combat this I'll see plenty of homes that have a fireplace with a common light switch next to them on the wall, if the power goes out in the home then the occupants can flip the switch and have the fireplace produce heat. Those with a bit of DIY in them replace the $1 contractor special light switch with a simple $30 mechanical thermostat so that they can maintain the temperature without manually operating the switch. It appears not just any thermostat will do, they must be "millivolt compatible" or they will quickly fail. Why this is the case is explained below. I can find these thermostats all day around here, no doubt because they maintain popularity with all the fireplaces and seeing increased popularity after every winter storm. It's finding a new fireplace like these that is the problem.

I believe these "dumb" thermostat compatible fireplaces didn't disappear, they are just hiding behind "smart" thermostats that are included with them. If I understand their function correctly then I should be able to buy a new off-the-shelf fireplace and wire a mechanical "dumb" thermostat in parallel with the "smart" thermostat so I get the best of both, I can use the "smart" controls in normal times and the "dumb" fail safe takes over in case of a power outage. Given the number of people here that appear to have done DIY thermostat work before, and presumably bought fireplaces before, there's likely some expertise to help verify my research on peeling back the veil on which fireplaces are capable of having a "dumb" thermostat put in parallel and which are not.

My first problem is that I haven't seen a consistent naming convention for these fireplaces. For lack of a better term I'll call them "power out" fireplaces as they continue operating even if the power is out.

What appears to make these power out fireplaces work is a thermocouple that is heated by the pilot light, it produces a small electrical current that can open a gas valve when the circuit is closed. Open the circuit the valve closes again and the flame goes out. I've seen these firsthand in other people's homes, and they are (or were) quite popular. The thermostats used to control these apparently must be "millivolt compatible" since the typical 24VAC thermostats will have the contacts quickly burned up by the DC power, even a 120VAC contact will be quickly worn out, including the contacts on the light switches that contractors keep putting in with new fireplaces. These fireplaces aren't called "millivolt fireplaces" they are called something else, but what is that?

One feature a power out fireplace must have is a "standing pilot", I learned that much in my research. This is opposed to "intermittent pilot" fireplaces, those definitely need power to operate, just like a high efficiency water heater needs power to light the pilot but the simpler and less efficient water heaters have a continuous (or "standing") pilot. But a standing pilot alone apparently doesn't break the code on what is inside the fireplace, there are those that will need external power to open the valve even with a standing pilot. Is there a name for this distinction?

After cracking some of the code on a power out fireplace I was able to dig into the installation manual on one of them to find that the maximum wire length for the millivolt control cannot be more than 20 feet. Has anyone seen different? Are there thermocouples rated for "high power" or "long wire" applications? Apparently these thermocouples are pretty standard in their form, fit, and function but there could be some variations. These are also parts that will fail on occasion and so are made to be replaced quickly by a technician or DIY homeowner. I'm hoping I can find a thermocouple that is able to handle more than 20 feet of wire as I'd like to put the new fireplace thermostat next to the existing furnace thermostat, and I can't do much about where the fireplace goes. I believe I need maybe another 5, 10, or worst case 20 feet extra.

I don't need a smart thermostat on the fireplace but it looks like I'll get one regardless, that is unless I get a very "old school" model of fireplace with all mechanical parts but I don't really want that. The smart thermostat may be required to operate features like lights and fans but they will provide heat without all the extra bits running.

I expect to be asked if the only allowed solution is a fireplace, if a wall mounted heater would be acceptable. That would be acceptable if there's some issue in finding a fireplace that fits where the old fireplace was. Even if I get a heater instead it appears that all the same questions apply. Is there a consistent name for what I would call a "power out heater"? Is there something in the product description that would indicate "power out" operation? It appears these heaters use the same, or highly similar, thermocouples since the thermostats are considered compatible with "millivolt" heaters, furnaces, fireplaces, and whatever else might heat up a space. What is the maximum length for the thermostat control wire?

I've seen guides on converting 24VAC thermostats to control "power out" heating appliances but that defeats the purpose of seeking out a "power out" appliance. I may use this information for adapting a "smart" thermostat later but that comes after the purchase and install of the "dumb" thermostat.

MacGuffin
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4 Answers4

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There are plenty of millivolt wall and floor furnaces which use a pilot light, thermopile and millivolt thermostat. They are typically called "Empire style" and are made by Williams and Empire, of course.

That kit is readily available in the sun-belt, because such units are adequate for house heating. They are persona non-grata in the snow belt, because they would impinge on generator sales, I guess? So you will have to smuggle one in.

As far as thermostat wire length, the problem I believe is voltage drop on the wire, since voltage is low and current is high. But you can just use larger wires e.g. Romex.

As far as synchronizing two thermostats, it's a simple matter. Thermostats can be paralleled. Any thermostat shorting the millivolt wires will call for heat. So you install the millivolt thermostat normally, and set its temperature to lower than you would normally set. Then you also have the 24V thermostat acting on a high current 24V coil relay, whose points short the millivolt line. Thus either thermostat can call for heat.

If AC power fails, the smart stat will be inoperative. Temperature will fall to the set point of the millivolt stat, and it will kick on. You can also work the millivolt stat manually.

Harper - Reinstate Monica
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A gas appliance that can run without any house power supplied has a "millivolt" gas control. It has an always-burning pilot light, and a thermocouple or thermopile that sits in the pilot flame provides internal power to run the control. Two screws on the gas control, when wired together, will make the control turn on.

Any thermostat which uses a switch (example: tilting mercury switch, bimetallic switch) or a relay (makes a little click noise when the thermostat turns on or off) can control this gas control. It merely connects the two wires together through the temperature-controlled switch or relay. Bimetallic or old-school tilting mercury thermostats require no power or batteries.

More than one thermostat can be wired in parallel to the control. If they're both on, the wires are merely bridged together in two places. Both thermostats have to be off for the fireplace to go off.

Find a fireplace with a millivolt gas control and a two-wire thermostat which wires to two screws on the gas control, not to a computer board somewhere else in the unit. Add a dumb thermostat to the two wires in parallel to the fireplace's supplied thermostat. Set the dumb thermostat slightly cooler than the smart thermostat. The dumb thermostat will take over when the smart thermostat fails to switch in due to no power.

isherwood
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Triplefault
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The failure mode of ordinary switches in a millivolt application is actually not anything like what you'd expect. No melted contacts, no arcing and sparking. They're neither burned up nor worn out. It's actually very dull: they oxidize. All switch contacts oxidize in fact, but when a switch carries a more substantial amount of current, the sparking that occurs each time it opens actually cleans the oxidation off the contact!

Because the fireplace is controlled by shorting a pair of wires together it's very easy to make a logic-or function to turn it on. You simply use as many switches or thermostats as you please and wire them all in parallel. If any one (or more than one) calls for heat you'll get heat.

The old mercury-bulb type thermostats had a sealed glass vial into which two or three metal electrodes were inserted. A small drop of mercury rolled around inside; the whole thing was mounted to a bimetal coil spring. The bimetal piece moves as it changes temperature and this causes the glass vial to tilt, which allows the drop of mercury to roll down and short pairs of electrodes together.

If you can get one of those somewhere, I'd guess its contacts might stay clean and reliably operate that millivolt system for many years. Maybe look on eBay I guess. Or on the walls of homes that haven't been remodeled in 30+ years. The "Honeywell Round" was one such type. There's still a model by that name on the market but I don't know whether it's still the mercury bulb type.

Greg Hill
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(UPDATE - this is a bad answer and is only left to show why this won't help in this situation.)

Thermostats aren't a thing here, but powering redundantly can be done by a UPS or battery. Use one thermostat with an external power source, because two is going to make it complex and confusing.

Two 12V SLA batteries in series for 24V DC, charged by a trickle charger on a timer so thermostat runs for a week on battery then gets topped up for a day. You can even re-use old car batteries that can't start a car.

You could power other 24VDC items, perhaps a doorbell too.

However this won't work because many AC units supply 24V AC power to the thermostat, and sending it 24V DC will likely not work.

Some units might be AD/DC compliant and switchable, but that's uncommon; check your manual.

Criggie
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