The Myth of Clay Pot Heaters – Do They Work?

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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:

Compost Science for Gardeners by Robert Pavlis

“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.

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!

34 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

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