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.