The Myth of Clay Pot Heaters – Do They Work?

Robert Pavlis

Homeowners and DIYers call them clay pot heaters while commercial outlets make them sound fancy using terms like Tea Light Oven, Ceramic Radiators or Terracotta Candle Heaters. Fancy ones on Etsy will run you $300 US? DIY options can be made for $15.

This idea may have started on TickToc a number of years ago and it is still being promoted to heat rooms, trailer homes and greenhouses. If you need to heat a space at a low cost, clay pot heaters might fit the bill – but do they work?

The Myth of Candle Clay Pot Heaters - Do They Work?
The Myth of Candle Clay Pot Heaters – Do They Work?

What is a Clay Pot Heater?

There are many different designs and as usual everyone claims their design is best – without providing any proof to show it is best. There are two main parts. A set of small candles and many people use convenient tea lights. On top of that you place one or more clay pots. These are raised up a bit so air can enter from below to keep the candles burning. The hole in the clay pot is usually closed to “help trap heat” – although that makes no difference at all.

The Claims for a Clay Pot Heater

At a basic level it is claimed that they heat a room but most people also claim that these heaters are much more efficient than candles alone. One site claims the heater is 4 times more efficient than just candles. This is a typical claim:

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“Candles can help provide heat transfer, but they aren’t very efficient. Enter the clay pot heater. Yes, they’re little heaters, but that doesn’t make them ineffective! You don’t need a large fire to produce a lot of heat.”

You get the point.

Deluxe clay pot heater with three pots - great idea if you have lots of washers and bolts lying around
Deluxe clay pot heater with three nested pots – a great idea if you have lot of extra washers and nuts that you don’t need, source: Survival News Online

Do Clay Pot Heaters Work?

I love it when people ask if somethings works without defining what they mean by “works”. Without a definition, any explanation is useless.

If we define works as “producing heat” then clay pot heaters work, but so do candles on their own! This is easy to test. Stick your finger in the flame of a burning candle and you will convince yourself as you sit in the hospital waiting room.

If we define works as “producing more heat than the candles alone” then the answer is a resounding NO!

First Law of Thermodynamics

The first law of thermodynamics says you can’t create energy. Energy can be converted from one form to another, but the total amount of energy in any closed system is fixed.

This is a fundamental law of science that is well accepted and taught in many entry level science courses. It was first published by James Watt in 1774 as part of his work on the steam engine but others helped refine the wording.

A burning candle converts the chemical energy in the wax to heat energy. One tea candle will produce around 30 watts of heat. The way you burn the candle does not change the amount of heat produced, provided it is fully burned. A candle outside a clay pot or inside a clay pot still produces 30 watts of heat.

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The clay pot is the same before you light the candle and after the heater cools down. Since it does not change, the first law of thermodynamics says it can’t produce heat. Adding more clay pots or using different sizes of clay pots or any of the other suggested designs will not affect this.

Anyone who claims that one clay pot heater design produces more heat than another is deluding themselves because none are more efficient than just using the candles on their own.

Super deluxe clay pot heater with chimney and fan,
Super deluxe clay pot heater with chimney and fan – it still does not work any better, Source: desertsun02

Does a Clay Pot Heater Heat Differently?

This is a different question than the one I discussed above. The clay pot will affect how a candle heats the room.

When you light a candle on its own it gives off heat instantly. If you start the same candle under a clay pot you won’t feel any heat right away because the generated heat is absorbed by the pot and none goes into the room, initially.

After a few minutes the clay is heated enough and it starts to radiate heat into the room.

When a candle is finished burning the heat emanating from it stops almost immediately. However, if the flame goes out under a clay pot, the clay pot heater keeps radiating heat into the room, until the pot is the same temperature as the room. A single pot will cool off faster than three pots nested inside of one another.

I suspect that this delayed cooling has convinced some people that the heater produces more heat. Others are convinced because they believe anything they read on social media!

The bottom line: the total amount of heat is the same with or without a pot on it.

But …. But ….. But …..

You have probably seen some of the YouTube videos showing guys (and gals) testing these heaters with all kinds of gadgets. They measure the temperature of the room and the temperature of the clay pots and at the end of it all they claim “the heater works”.

One guy measured the clay pot before starting the candles and found it was  7.9 C (it was a cold room). After 2 hours of heating with 3 candles he measured the pot again and exclaimed, “WOW they are now 70 C, Holly man …. that’s crazy”! The temperature of the room went up 2 degrees. He declared that “the heater worked”!

I hope that you can see the flaw here. To verify that the heater worked you have to test two identical rooms, one with 3 candles and another with a heater housing 3 candles. You then measure both rooms over time to get a more accurate result. The second room is the control since it has candles but no clay pot.

Numerous videos and blog posts claim these heaters work and even show experimental results to verify their claim. None of the ones that I looked at had a control, which means they can’t reach a logical conclusion.

Update Feb 22, 2023: I did find a video that used two very tiny cabins and used one as a control. The temperature a couple of feet above the heater was warmer for the clay pot heater. The problem here is that they never tested the air away from the heaters. As mentioned above you can expect it to be lower. They also did not switch the heaters around to see it the heat loss of the cabins was the same.

Before declaring success you would repeat the experiment with either a lot of identical rooms at the same time, or with the two identical rooms used repeatedly.  Clearly homeowners can’t do this easily and it is why we need to rely on the science instead and in this case the science is REAL clear. The first law of thermodynamics has not been busted in over 200 years.

YouTube video

Can You Heat a Room with a Clay Pot Heater?

The candle does produce heat and it does heat the air in a room. The ability to measure the change in temperature depends on the accuracy of your instruments and the parameters of the room. Small rooms heat faster than big ones. An insulated house heats better than a greenhouse. A well sealed room heats better than a drafty one.

Does a Clay Pot Heater Heat a room? Yes, we know that from the first law of thermodynamics, but that does not mean you can measure the change with your instruments.

Are Clay Pot Heaters Safe?

Clay pot heater causes fire and evacuation of 50 people, Source: Derbyshire Fire and Rescue Service/PA
This clay pot heater caused a fire and the evacuation of 50 people, Source: Derbyshire Fire and Rescue Service/PA

Fire chiefs in the UK are warning people not to use these heaters because they can catch fire? There is even a case were 50 people had to be evacuated from a housing complex after a clay pot heater caught fire.

I have not seen any good explanation as to why these heaters would catch fire. Some say the wax melts and forms a pool of wax which can then burn. So I guess if the candles were not in a suitable tray this might cause the fire. Most of these heaters use a terracotta tray which should work fine. The picture above shows the one that caused the above mentioned fire. The tray had a lot of candles and may have gotten too hot, cracked, leaking out burning molten wax? Or it might have cracked when water was poured on it to put the fire out?

In any event, since the heater does not work, there is really no reason to take a chance.

Science can be so powerful – if only people understood it better

and believed it!

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

32 thoughts on “The Myth of Clay Pot Heaters – Do They Work?”

  1. it is more efficient since a lot of the heat is wasted in an open flame. However, the volume of air inside the pot also has an impact as air is an isulator and a terrible conductor, so heat transfer is very poor. However, it has more performance than without a pot. Ideally, a better method would to have the flame in direct contact with a metal substrate like copper, which can than directly transfer the heat to to clay which will then retain the heat.

    Reply
    • “heat is wasted in an open flame” – it is not wasted – it heats the cold room.
      “it has more performance than without a pot” – what is performance? The candle performs the same way.

      Reply
  2. I’m going to destroy your work on the topic in just one question: Have YOU done the experiment the way you said it should be done to get the correct result? No, so whatever you said is crap. Also, you played footsie with all this and, obviously, omitted to mention some facts. So I’ll fix that right now:

    First, you said
    – the concept is delusional.
    – none did the proper testing to get the correct answer.

    BUT.. you didn’t do it either, so you can’t brag to have the right answer. You have absolutely no positive proof of what you said.

    Second, you intentionally omitted to mention the RADIANT effect. The candle itself gives a certain amount of heat. Now, if you had solid objects like screws, bolts and terracotta pots, those objects will gain heat and retain it for a certain amount of time. Are all those components increasing heat by addition?that has to be tested. Did you? NOT

    Third; Let say we light a candle and measure the heat. We’ll get a certain value. That heat will almost dissipate instantly the moment we extinguish the candle. Now, if we do the exact same thing with the exact same candle, but adding one, or several clay pots. Then extinguish the candle after the same amount of time as previously.

    What will be different? The difference is that the clay pot(s) will still release heat for a while. This means adding clay pot(s) will generate heat for a longer time for the same burning time. Therefore, much more efficient than a candle alone. And I think that’s what most means when they say it’s good to heat a room. This concept has been validate thousands of times and it’s called “mass heater”.

    This being said, will it make a BIG difference? and should one rely on it? i don’t think so. It’s a good emergency option that’s 100% sure. But it may cost you more in candle that the savings on your energy bill.

    Bottom line: Does the concept of candles + clay pot is working? The answer is a resounding YES and it’s the principle of mass heaters. Is it working well enough to heat your place when no emergency situation? No.

    P.S. I saw your comment where you say “Does this provides more energy? The answer is no and that was my point”. This too proves that you’re playing footsie just to try to gain more people on your web site. NOBODY says it’s producing more energy. Most says it’s an optional way to heat a room. And as said above, you will get heat for a longer time with clay pots than with none. So stop bragging about the thermodynamics laws because you took what suited you and omitted everything else including… MASS HEATING.

    Reply
    • 1) The idea that I can’t report on known science without doing the testing myself is ridiculous. You clearly do not understand how science works.
      2) Mass heating changes the way heat is distributed – which I acknowledged in the article – it does NOT change the amount of heat produced.

      Did you fail to read this part? “Does a Clay Pot Heater Heat Differently?
      This is a different question than the one I discussed above. The clay pot will affect how a candle heats the room.”

      You seem to have missed the main point of the post!

      Reply
  3. I’m on Long Island in NY. I have a cheap plastic covered greenhouse that I lined with bubble wrap, the cover and then 6mm thick plastic. Since the winter has been mild, it has not made it below freezing yet. The space heater I do have blew out yesterday. I built a heater from a terra Cotta tray, a small tub of crisco, 4″ high candles stuffed into the crisco, then put a large then a smaller Terra Cotta pot on top. An infrared thermal gun reads the top small pot to be 138.9°.

    This morning it was 4° outside and the inside of the greenhouse was 26° with one candle. I added 3 more candles this morning and the inside of my 8×8 greenhouse is at 30.7°.

    I agree that the pots don’t change the hear that the candle produces. But they do help disperse the heat and definitely helped to keep my greenhouse must warmer. Pretty cheap at $20 for supplies.

    Reply
    • 1) The pots don’t disperse the heat – they do the opposite. They concentrate it in the pots until it is released.
      2) Without a control you don’t know if the pots made any difference.
      3) You agree that ” the pots don’t change the hear that the candle produces” and then go on to contradict this statement by saying “they (the clay pots definitely) the pots don’t change the hear that the candle produces”.

      Reply
      • Candle wick heats the air immediately around it, transferring most of its energy (some going into melting wax) into a small volume of air using the big three of radiation, convection and conduction. This causes air density to lessen and the hotter, more energetic air rises rapidly.

        Put a pot in the way and the pot absorbs that heat from the rapidly rising air, over a wider surface area. I don’t pretend to remember the sum total of the thermodynamics courses I took at university, but the heat transfer to the ceramic pot will be a different ratio of R/C/C because the clay is denser than air.

        The pot will largely radiate the heat out into a wider surface area of air, which being warmed less than the directly heated air, will rise more slowly.

        Add in a bit of air circulation and instead of the heat rising immediately to the ceiling and wafting off into the atmosphere from there, depending on insulation, the pot heats the space more equably than the candle alone.

        So while the thermal battery created by the pot doesn’t increase the amount of heat produced, it makes better use of it.

        It’s why we have storage heaters. Also, use a low wattage storage heater instead.. Less likely to catch fire in a typically untidy and full of flammable things greenhouse. I guess the candles are a little more romantic, depending on how literally you take a “passion for gardening”.

        Reply
        • “The pot will largely radiate the heat out into a wider surface area of air” – I don’t think that is true. The flame heats air which quickly rises. The pot disappaits heat more slowly than the flame – that is why it stays hot after the flame goes out.

          Reply
        • This is an accurate explanation. The pot(s) act as accummulators. The function of accummulator is to maintain the temperature reducing suddent fluctuations, The candle alone would heat the air in close vicinity which would immediately make way for fresh air and a continuous vertical heat flux is created which would merge with a larger atmosphere outside the room (since any room is ventilated). The pot acts to hold the heat longer in radiate slowly.

          Reply
  4. I think most people understand that it is not amplifying or creating energy, but it is certainly redirecting energy to the person right next to the burner, opposed to going straight to the ceiling. I think Loggic makes a good point saying that’s why space heaters have a reflective surface and metal mesh instead of just being a Bunsen burner. I feel like the point I am making makes sense and does not break the laws of thermodynamics. Do any of you disagree with this specifically?

    Reply
  5. One tee candle produces 30W, if I use 100=3000w, I think this will clearly heat a not too big room. But 100 candles burning is expensive and will eat up oxygen! Maybe it’s enough to keep a greenhouse frostfree.

    Reply
    • You missed the point of the article.

      Yes 100 candles gives more heat than one – but 100 candles under a flower pot does not give more heat than 100 candles without a flower pot.

      Reply
  6. Interesting, I’d never heard of these. Of course they couldn’t produce more heat than the candle would. But they probably do divert the heat sideways, instead of letting it rise straight up to the ceiling. I think that’s what Loggic was saying. It’s working the same way as an umbrella, but in reverse. An umbrella can’t affect how much rain falls on a person, it just diverts it. Same here, the terra-cotta cover just deflects the heat sideways, creating the illusion of more heat. Not a safe or efficient device…but better than nothing if you’re depending on electricity for heating, and there’s a blackout.

    Reply
  7. Oh, and this sentence did not make sense to me…is there a typo here? Wouldn’t 70C be extremely hot? “One guy measured the clay pot before starting the candles and found it was 7.9 C (it was a cold room). After 2 hours of heating with 3 candles he measured the pot again and exclaimed, “WOW they are now 70 C, Holly man …. that’s crazy”! The temperature of the room went up 2 degrees.”

    (feel free to delete this comment, I hate publicly calling anyone out, but know you care about accuracy)

    Reply
  8. Putin’s gas & Saudi oil will keep you warm, forget clay pots & candles. Failing that wrap up in bed with an electric blanket the cheapest way of keeping warm at home

    Reply
    • Agreed, though poorly monitored and maintained they can also be a fire hazard. The closer the heat source is to you, the more value you will get, and the amount of heat for the wattage is phenomenal (wrapped up with mine on low as I write this). Make the space you heat smaller, only heat when it is needed, and keep close to it is the way to save the money.

      The flagrant waste of keeping every room in a house totally warmed when there is actually no one in a room is just insane. The expenditure of zoning our heat and having separate thermostats (with small wall mount space heaters in small rooms like the bathroom for “as needed” heat) has been well worth it. Analyze how much time you spend where in the winter and plan accordingly.

      Reply
  9. This shows the problems associated with picking a definition of success. “Produces more heat” is an easy definition, but I would contend that it isn’t the right one. Does it produce more heat energy? No, of course not. Does it make the room *feel* warmer than a candle alone? Probably.

    Why? Because that’s how basically every space heater out there works. It isn’t just about the total heat *released*, it is about the total heat that travels to any person occupying the room. A bare heat source, whether it is a natural gas burner, a candle, or anything else, will inevitably lose a lot of heat to convection – the heat travels straight up with the warmed air rather than warming a person. By placing something in the path of the convective heat, that object absorbs the heat, increases in temperature, then radiates that heat out to anything within line of site proportionate to T_hot^4 – T_cold^4. That’s why a space heater isn’t just a Bunsen burner: the heater uses an object (typically a metal mesh) to absorb & re-radiate heat sideways or even downward toward the people who want to warm up.

    So does it create more heat? No, it just allows that heat to be used more effectively – more heat ends up warming people rather than warming the ceiling.

    This provides a very different answer to the question “does it work?” When you provide a specific & technical answer to a broad and non-technically crafted question, you’re in danger of answering something that wasn’t asked. I don’t think many people are specifically asking about the total number of joules of heat energy produced by burning a specific quantity of wax. They’re asking if this contraption helps people warm up more than a candle alone. In that case, the answer is a resounding yes.

    For historically proven examples you can look to masonry heater designs. Masonry “bell heaters” have been used for centuries to warm people’s homes by employing this same idea. Rather than venting heat through the chimney, the bell captures & radiates heat into the living space. Less heat goes out of the chimney, more heat goes into the home.

    Reply
    • “Does it produce more heat energy? No, of course not.” – that is the point of my post.
      “They’re asking if this contraption helps people warm up more than a candle alone. In that case, the answer is a resounding yes.” – no it is not. If you believe that show us some data.

      Reply
      • Loggic gave very good reasons why the clay pot would help, and your response is simply “No.” Why don’t YOU provide some data to back up your claims?

        Reply
        • 1) I have not seen any “good logic” why clay pots would increase the heat level.
          2) I did not invent this idea. The onus is on the people who claim it works to show proof it does.

          You seem to think it works – where is your proof?

          Reply
          • The idea of the clay pot heater is not a lot different from the old trick of carrying a hot brick to church to keep in your pew box. If you are very close to the heater, say, keeping it near your feet, or holding your hands up to it, it could add to your comfort. It’s not a big enough deal for a clinical trial. I figure anyone who wants to should just give it a shot and see what they get. Tea candles are 3 bucks for a bag of 100 at dollar stores, pots will cost a bit more. Sounds like fun.

          • Yes – being close to the heater will make you feel warmer – the post acknowledges that. But that is not what the internet is claiming.

          • > I have not seen any “good logic” why clay pots would increase the heat level.

            Instead of the heat energy instantly rising to the ceiling, it is held and radiated closer to the user.

          • Correct – which is what the post says. But that does not increase the heat level in the room nor does it produce more heat.

    • I think your analysis is correct. The only way to know whether terracotta stoves work is to actually use them in a greenhouse setting. That’s the only way to account for all of the variables. Lots of things look like they should (or in this case, shouldn’t) work in theory. And by “work” what I mean is- do they heat the air around the plants enough to help keep them from freezing? Well, yes, depending on the greenhouse, they definitely can.

      I’ve done it, and the pot heats my greenhouse about 15 degrees F where the plants are in the lower center of the greenhouse. It was 20 degrees inside, 1 degree outside. Candles alone were negligible- about 2 degrees (believe me, I did not want to build a terracotta stove in the middle of a cold dark Montana night, so I did try candles alone). I also had frost covers on the plants, and so they survived.

      I used a small can of Crisco and four candle stubs, which burned all night. I had enough left for the next night, also, when it was 9 degrees. Actually I was shocked at how well it worked. I used four pots, successively smaller ones set on the larger ones. The largest being supported by three small empty cat food cans on gravel. It looked like a little rocket, with an orange glow coming out of the bottom.

      I think candles alone are less effective because the heat from the candle is in a highly concentrated convection current, and it plus heated candle gasses and particulates go straight to the ceiling, where there’s a greater temperature gradient leading to faster heat loss out of the ceiling, and also the vents are there, which do not seal perfectly..

      It’s a question of how the heat is distributed and most importantly, retained. Stoves and candles do that better than candles alone. I also have a five-wall polycarbonate greenhouse. With less insulation, one may not notice any difference between the two- they both may not work well. But radiant heat distributed and retained in a larger space (including the gravel the stove is on and the beds around it) is detectable by the thermometer, and helps my plants. A hot ceiling is neither measured by my thermometer, nor helps my plants.

      It was probably toasty warm up there, though.

      Reply
      • “by “work” what I mean is- do they heat the air around the plants enough to help keep them from freezing? ” – then you missed the whole point of the article. It only works if the clay pot keeps the plants warmer than without the clay pot.

        Reply
  10. My first response? Ha!
    Do clay pot heaters work? At some low efficiency too low to raise a room temperature to comfort level – minimum 70 degrees F.

    Yet, to accomplish that temperature rise with a few candles under upended clay pots? Don’t think so – unless you’re lining your walls with clay pots. You’ve Got to create sufficient heat energy and that’s going to require a lot of clay pots and candles?

    Cost? Very expensive! Go price clay pots and candles! Even the rich can’t afford them!

    Reply
  11. My father said that the three laws of thermodynamics were 1) you can’t win, 2) you can’t even tie, and 3) you can’t get out of the game.

    Reply

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