Are Candle Heaters Safe?

You've probably seen the 'clay pot suspended over a couple bricks with a candle under it' heater.  I used one a few years ago and it did a decent job of heating a small area.  Recently, I saw someone post that they are very dangerous and can blow up easily.  What are your experiences?

caveat: i haven’t used this sort of heater but i’ve lots of experience with all sorts of materials interacting with fire and heat.

if the bricks and pot are truly dry all the way through, it should be fine. the potential for danger is when there’s water hiding in there - water expanding quickly from heat can lead to the dreaded ‘blow-ups’. the pieces could be heated on a low heat in the oven for awhile to make sure they’re fully dry.

Wet terracotta is definitely dangerous. But it can also be thermal shock, if the flame gets too too close to the clay it can crack and spall.

ALL fire is dangerous. But I wouldn’t call those any worse than an oil lamp or many other flame based portable heaters.

I wasn't paying much attention, but I think the risk was something like...

Heater produces heat
Unmelted wax offs gass
This gas gets trapped in the system and builds up to a critical mass.
Heat increase, to the flash point of the gas.  
Explosion and unwanted fire

And the rest of the lecture was about how to modify the system to avoid the biggest dangers,  leaving the next biggest threat, unattended open flame and human nature.  My attention span was way shorter than the lecture.

My unsupported theory...

Use tea light with terra cotta pot only if drain holes not plugged.

Safe?

When I was a kid, in our teeny camping trailer we used a clay pot over a gas stove element for a heater. This worked over many winter adventures. No exploding pots. But no candle wax off gassing to make things bad either.

Any open flame uses oxygen and produces carbon dioxide and if there's incomplete combustion, carbon monoxide. Therefore, in a small space, one needs to be sure there's enough fresh air coming in.

Tents are leaky, so that's why Joylynn's family got away with it. I know of a family that burned some form of a coal burner inside their house and nearly died.

If it were me, I'd be inclined to use a fire of some sort outside to heat water for a hot water bottle. Water is an energy hog and will keep you warm for some time without any risk.

That said, a friend gave me one of those candle heaters that has several stacked clay pots. I will check to see how leaky its top is, as there is a bolt through it holding the pots. I had wondered if I could use it to heat a cold frame the odd time we get unusually for us cold weather. I suspect hot water bottles would do the job just as well, but I happen to also have been given far too many tea lights to ever use!

It would also be wise to use beeswax candles. They burn hotter (warming the clay better) and cleaner than other candles, so the "only" gasses to be concerned with would be the carbon dioxide & carbon monoxide. They also won't coat the inside of a tent(or whatever) with soot and/or petroleum residue.

I'm on the other side of this discussion personally.

A candle flame produces a tiny amount of heat. I've seen it claimed before that it produces much more heat energy in these "candle heaters" which is chemically and physically impossible. It's plausible that we experience that heat differently - a block of ceramic radiating infrared may make someone sitting very close feel different than that equivalent amount of energy going to heat the air directly. But these are very short range effects, more akin to using a personal electrically heated rug than to heating a space like a room.

When fuel prices were astronomically high in the early days of the Ukraine war my facebook feed was full of influencer types trying to persuade people that these things were a cheap alternative to running your whole home heating - a pretty dangerous claim in the case of vulnerable older people for whom a cold home is a genuine health risk. And a claim that was clearly being made by young comfortably well off people who have never needed to worry about heating their homes in the first place, and had certainly never tried to use a single candle to replace their home heating.

Bottom line - I see little harm in these provided they are used properly and the limitations are understood. Specifically:

Ensure that the ceramic does not interfere with the clean burning of the flame. If you see soot building up after a period of use then your ceramic is too close to the flame.

Don't depend on these to heat enclosed spaces - eg cars, camper vans, tents... They are an open flame with no dedicated air supply so there is a hypothetical risk of carbon monoxide build up. But that risk should be no worse than an equivalent candle used for light, provided the flame is allowed to burn cleanly.

*I* think ...    

Burning candles can be a nice thing for a half hour, once in a long while.  But I want to suggest that people do not attempt to compete with a candle for oxygen.

....   still my opinion ...

Candles are a fun novelty for very short periods.

I am concerned (still nothing more than my opinion) that people trying to get heat from something like this will run the candles for a long time.  And when that amount of heat is not enough, they will use more candles and more candles.  

The thing that baffles me is that the whole candle heater thing has gotten so much attention.

Paul, I am in full agreement. I've actually tried this - granted, it was in a room with a very high ceiling, and even sitting right next to it, the difference it made was negligible, with only 1 - 4 (beeswax) candles. That's just not going to do it, for me. I tend towards being cold, so I'm just better off bundling up or using an electric blanket or a hot water bottle or two.

There is a great thread around here somewhere about heating the person and not the space.  It's probably the most effective way.

I think a single hot water bottle will do 20 times more than the whole candle thing.