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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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    Can we have that in a executive summary format – asinine Aug 23 '23 at 22:53
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    I read the whole thing, and I'm not sure what your actual question is. – KMJ Aug 23 '23 at 22:58
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    I'm "front loading" this question with plenty of background to avoid what I expect to be questions of an "X vs. Y problem" or some such. Questions like... Does it have to be a fireplace? No, it can be a heater. Would battery power be acceptable? No, it should run off the thermocouple or something else that derives power from the natural gas. What kind of "smart" thermostat do you have in mind? The one included with the fireplace. I'm still figuring out how to ask a good question. If I keep it short then I get a lot of unrelated answers, too long and I get no answers at all. – MacGuffin Aug 23 '23 at 23:20
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    Often folks will make a question with a TL;DR at the top and a expanded data section below, for those that need it. This may get you to have the concise question that's usually needed, as well as the extra info if the concise question isn't fully graspable. – Triplefault Aug 23 '23 at 23:38
  • The question title apparently does not match what the body "question" is supposed to contain, per comments on my answer below. – Triplefault Aug 24 '23 at 00:10
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    I did extensive edits to the question in hopes this clarifies things for people. It appears part of the problem is a general lack of familiarity with millivolt HVAC systems. It seems to me that these are pretty common but it must be a regional thing. They are getting less common, or so it seems. There's the "budget" mechanical fireplaces and those with "smarts", but it is the kind in the middle is what I'm looking for. The kind that **can** have a thermostat attached but **don't** have one that comes with it in the box. – MacGuffin Aug 24 '23 at 03:28
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    Edited title to put the word "fireplace" before "thermostat" since it seems people processed the original title to mean the problem was the thermostat, not the fireplace. New title with edited question should now make the problem to be solved clearer. As pointed out in the question I'm lacking in the precise nomenclature to even phrase the question in a clear and concise manner. With some added research and the feedback so far I am at least closer to just asking the right questions. – MacGuffin Aug 24 '23 at 05:29
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    Still far too wordy, but the first sentence and the new title can be considered a TL;DR. Edited for clarity. If the edit is not what you meant, you can roll it back. – Triplefault Aug 24 '23 at 14:00
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    "Still far too wordy" I felt I had to explain that I know the kind of fireplace I was looking for existed because of commentary that claimed the opposite. I've seen these fireplaces with my own eyes so I know that they exist. I didn't want the commentary that I'm asking for the impossible as I am clearly not doing so. The problem is that there does not appear to be a consistent name for these fireplaces. They didn't need a name until something different came along. Best guess is the "old" style is "standing pilot" and the "new hotness" is "intermittent pilot". Still not sure. – MacGuffin Aug 24 '23 at 19:21
  • @MacGuffin -- yeah, "standing pilot millivolt" is what you're after – ThreePhaseEel Aug 27 '23 at 02:52

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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  • "But you can just use larger wires e.g. Romex." Any idea how much longer that allows the wire to get? Any guides, operator manuals, specification sheets, etc. to be found online that could show what to expect? Thanks. – MacGuffin Aug 24 '23 at 03:13
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    @MacGuffin -- with appropriate pigtailing, you could go all the way to 6AWG but that's probably not necessary for the purposes of this exercise – ThreePhaseEel Aug 24 '23 at 11:44
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    I'd like to avoid using anything but brown jacketed thermostat wire if possible, there's just a lot that can go wrong with that. Using Romex could leave the impression later (possibly even to me should I be forgetful) that this is a 120VAC wire. Anything bigger than 16AWG or 18AWG might not fit the thermostat, and a pigtail could introduce an unacceptable resistance. This 20 foot length limit appears to be vendor specific and/or assumes light gauge wire as I've found another install manual showing 40 feet is acceptable with 18AWG wire, and 64 feet with 16AWG. I will continue my research. – MacGuffin Aug 24 '23 at 21:47
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    @MacGuffin It's about wire resistance. Figure out the wire resistance of a 20' roundtrip of #18 wire. Wire it any way you like that has lower resistance than that. If you're having trouble with high resistance in pigtails, then work on your technique. That stuff will arc and melt down if it's 120V! – Harper - Reinstate Monica Aug 25 '23 at 06:35
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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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    Perhaps I was not clear earlier in the wording of the question. The question has been edited to (hopefully) clarify the concern is finding a fireplace, not a thermostat. I can find a suitable "dumb" mechanical thermostat, one rated for millivolt DC and not contain mercury on the shelf locally. I'm not looking on eBay for a thermostat since that's full of fly-by-night outfits that scam people, I can find the right thermostat at the local big box hardware stores. We can debate why they fail but there is agreement that they do fail, no? Pretty sure no new thermostat has mercury in them. – MacGuffin Aug 24 '23 at 18:14
  • @MacGuffin Sounds like you're all set for controls then. You could ask installers and suppliers for an appliance that supports hard-wired millivolt switch control. Seems like that's all there is to it. All the conversation about thermostat compatibility just confuses them. – Greg Hill Aug 24 '23 at 18:35
  • "You could ask installers and suppliers for an appliance that supports hard-wired millivolt switch control." Judging from previous conversations with installers I'd have a hard time communicating what I'm looking for unless I'm using some industry standard nomenclature. I need to know the name of the feature, function, or device or I'd likely just frustrate and confuse them. If I'm paying by the hour for this conversation then I'd want it short. I'm not even sure "milivolt switch control" is the right name, and even if it were there may be something of similar function by a different name. – MacGuffin Aug 24 '23 at 18:56
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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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    Two issues with that. First, adding batteries means adding complexity and cost, the goal is to keep both to a minimum. Second, running thermostats built for 24V***A***C on 24V***D***C is not supported by the major thermostat manufacturers, I already researched that as an option. Running 24VDC into a simple ringer doorbell might work but few to none of the "smart" doorbells (those with a camera and/or intercom) support 24VDC. 12VDC, like for in an RV, doesn't seem to help, they appear to use millivolt DC like in homes. I like how you think though, I went down that same path before. – MacGuffin Aug 24 '23 at 20:19
  • Oh yeah - 24V **AC** my apologies. Ant the power wastage of running a mains-voltage UPS for a small wall-wart adapter would be significant. I'll edit question to show its a bad idea, for future readers. – Criggie Aug 24 '23 at 23:48
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    I believe I should walk back my comment on 12VDC thermostats being rare to nonexistent, upon further research to verify my comment I find that 12VDC thermostats are widely available. A trickle charger on a 12VDC SLA battery would provide reasonably reliable power for HVAC in a power outage but that leaves two issues much like I stated before. I can likely find a thermostat for 12VDC but that means finding a fireplace compatible with 12VDC, so not exactly the same problem but similar as it shifts the burden from search of a thermostat to search of fireplace. RV fireplace might not meet code. – MacGuffin Aug 25 '23 at 00:39