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?
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:
“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.
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.
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.
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?
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.
Did Donald Trump invent the theory of the Clay Pot Heater?
I wonder if there’s a difference in temperature, at the ceiling, directly above?
I’d imagine the bare candle would pass more heat vertically by convection and the clay pot would radiate more.
Maybe the candle heated ceramic releases more energy as infrared than the bare candle.
No one claims that both options heat everywhere in the room the same way.
Warm air rises so yes, the roof area will be slightly warmer.
The clay pots act as heat brick the terracotta heats up and the air is warmed around the pots.
As Robert states, area heated is warmest around the pot due to the terracotta heating up but it will not heat a whole room or greenhouse to 40 C in extreme cold temperatures. You just cannot get more energy out of he candles, the pots just help focus the heat by heat transfer to the air.
Of course as the candle heat diminishes the terracotta pots will cool down quickly.
Of course the amount of energy stays the same.
There’s just a huge difference between a candle/candles heating the air above them and that heat then distributing evenly to all of the air and all surfaces of the entire room and a heated pot that emanates heat to it’s vicinity during the burning time of the candles and after, when the goal is to keep a close-by human warmer from cold more efficiently.
The goal of a clay pot isn’t to “increase the heat” (though yes, for debunking some specific youtube videos you have seen that say so, it is of course true to say it doesn’t) but to reserve and localize the heat produced. It’s hard to keep yourself warm in a cold house with 4 candles. Easier with a big object emanating heat that was warmed by 4 candles.
True – but that is not what most people claim.
In the gardening world, the heaters are being used to keep a greenhouse warmer. just see some of the other comments.
It is true the the plant sitting right next to the flower pot will be warmer, but not the plant at the edge of the space.
I have a two terracotta pot ’emergency’ frost heater in the greenhouse and it just keeps the chill off.
I only have a 6 x 6 frame and it twin walled polycarbonate with a bubble-wrap insulation fitted. The greenhouse is a nursery for continued growing of seedlings until strong and then planted outside and the cycle starts again however, I was given some pepper plants and put my hanging baskets in there to keep from the frost. I do have a tube heater and all the electrics are done with waterproof sockets etc unfortunately, I am awaiting for parts to then take the cable to my outside junction box. So…what to do with the temp dropping.
So, I decided to make a terracotta heater, just a simple no bolts, washers etc. It sits on the floor on a flag surrounded by three bricks, one side left open for air, a stainless steel tray acts as my 8 hr tealight holder and I have a smaller pot with a coin above the hole and a larger second pot over this one with a air space-top hole is left open. 4 8 hr tealights spaced 3 inches apart are used as the heat source.
Now, I too have a science background and I do not envision for one second that this will heat my greenhouse to 50 C and be all toasty and tropical-as explained in this article, thermodynamics at work cannot be changed. For me, it is about keeping frost at bay not creating a cheap way to grow all year round-it ain’t going to happen.
As a note-I have three thermometers around the greenhouse, one on a shelf around three quarters way up on a back wall (A), one on a outside wall rail just below the roof line (B) and one on some wooden staging I made that is on the opposite side near the door (which has seals) which sits around a quarter the height from the floor (C). These are the results so far after 8 hrs of use and one candle change.
Outside temperature reading is 2 C, with some sleet/snow at the moment.
Greenhouse temperature :- Thermometer (A) 9 C
Thermometer (B) 6 C
Thermometer (C) 8 C
The amount of sunlight today was around 30 mins not a lot to generate a lot of heat in the greenhouse. A thing to remember is that you will need to monitor and change the candles as the although terracotta has good thermal properties it will loose its stored heat quickly-I am looking at another candle change around 4:30 AM…oh joy.
So, in conclusion, for me, do they work, yes and no. Yes, they can help to take the edge off a frosty night or so if the outside temperature is not going to drop to major minus figures-I reckon a outside drop to minus 2 would be the maximum that a largish pot heater could cope with, anymore and just do not bother. The negatives are:- Changing candles around every 4 hrs, and of course condensation; warm-ish interior, plants expiring water and a cold frame causing water droplets to form and drip. So, is it worth it? Probably not long term, risk of excess moisture, mold, damp issues etc. Coupled with small amounts of soot, water vapor from the burning process, various chemical, Toluene, Benzine etc if you are using paraffin wax candles-make sure you have good ventilation-which of course brings me onto the last part. If you have to have good ventilation then the amount of heat produced and stored in the terracotta to be released is cancelled out by the cold air of the ventilation-so, bear in mind. I have a small circular vent made from a car dashboard vent with stainless steel fine insect mesh over the outside and a plastic potting pot over the top fitted using a circular hole borer-that was nerve wracking cutting into the polycarbonate, slow speed cordless drill worked a treat though, ten just some good grade exterior caulking used to secure. This is situated above the door in a small window and can be closed or open at any angle so that is my ventilation along with a couple of small holes near the bottom of the door.
If you do want to to try and have a go, please be safe and use open flames with the regard they deserve, use proper foundations to place your candles and pots properly.
Thanks for reading.
Si.
“it just keeps the chill off” – yes – that is what a candle does, it produces heat. The clay pots do not increase the amount of heat produced!
Indeed Robert, what the pot does is provide a overall spread of radiated heat, plus you are containing the open flames of candles in a controlled area and not just left exposed.
The effect is like a storage heater that uses electricity to heat bricks that are then used to radiate that stored heat. Now, the problem is, that although terracotta has good thermal properties and radiates the stored heat from the candles underneath, it will also loose heat rapidly in the same way.
Again, if this is the only way you have of just keeping a small area frost free then it does work, as for long term heat control, not a chance. To keep a radiated heat source going and a temperature high enough you would need at least three of these largish pots going-that isn’t feasible in a small greenhouse. Coupled with the added potential of a fire risk if something went wrong to me, isn’t worth it.
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.
“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.
Hi Micheal, I did some tests in the greenhouse last night.
First a ring of new tealights, in holders V a fresh batch of tealights in the terracotta pot heater.
Results: The ring of 8 hour tealights went out within 3 hours or so and left a considerable amount of unused paraffin wax around the edge of the tealight. The tealights in the heater had completely melted and so continued burning for roughly the 8 hours.
Conclusion:- The heater controlled the heat, confined it as the terracotta soaked up the heat which then radiated out into the greenhouse and kept the tealights burning longer.
Did I get more heat output? Yes, as they kept lit longer so the heater does work.
R.E. Using the metal as conductor I have a feeling you could maybe have a heightened hot spot that could cause a potential crack. Good idea though, maybe a frame with the copper sheets, thinking a 1/2 inch in width reaching out from a center point, the candle/candles sit beneath heating them-hmmm, maybe something to look into.
So what you are saying is that tea lights don’t work on their own? That is not my experience. The ones I have used burned right down.
Hi Robert, for me the tealights were in a holder, that could have something to do with them not fully burning down. They burnt down from the wick and left a ring of paraffin wax around the edge. The ones in the heater 95% burnt down and left hardly any paraffin wax.
The heater worked better in that instant as it provides more local heat to fully melt the candles and the wick just burns down using more of the fuel.
I agree with you that you cannot get more energy out than you put it, one candle power gives around 30-ish watts of heat so stands to reason that more candles will give up more heat however, as you have stated you cannot get more heat out of the source-the candles. The pot works by trapping the heat source and heating up the terracotta like a heat brick this is them radiated via the larger pot and through the hole was warm air. Was it more efficient, yes regarding burn time and so a longer period of providing some heat.
One thing I will say is that although not expensive, I was going through around 15 candles in a 24 hr period and that was just the heater with 6 candles and then 8 as the temperature dropped to – 3 + for a extended period. I also used the open tealights in the ring holder a couple of times, that takes 6 at a time. So, was not cheap-cheap and not suited, I think as a long term solution. Okay as temporary heat source or just as frost protection. To be honest, if you have no access to power, then I would think about giving up using the greenhouse in the winter months when cold air and dropping temps are a given.
UPDATE: I now have my electrics wired in and used the 85w tube heater, was not as good as I had hoped, this is down to positioning as it against a wall area fitted onto the wooden staging I made. Despite foil for heat reflection the thermometers showed 3 degrees C on the lowest point thermometer, fitted to the wooden staging near the door and heat source. The opposite thermometer, which is higher was at 0 degrees C and the highest thermometer on a shelf showed 1 so I was getting worse results than the pot heater. This needs addressing as I will have to move it at some point. To combat this I used my oil filled radiator, taken from my work shed, and found that worked super well. On the lowest heat setting I am able to have a stable warm environment.
Do I think I can grow in the winter months instead of just using the greenhouse for garden plant storage? I have planted some carrot seeds as a test in six pots so we will see if anything develops over the next month or so.
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.
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!