Energy Is Expensive, So Free Heating Hacks Abound. But Are Flower Pot Candle Heaters Safe Or Dangerous?

Every winter day I see people talking about how flower pot heaters can keep our greenhouses frost free. But are they safe?

Are Flower Pot Candle Heaters Safe Or Dangerous?

Let me start by saying I’m not an expert on fire safety. I rely on the Fire Brigades for fire safety advance and ignore what non-experts claim on social media … especially TikTok and Facebook. However I’m sure some gardeners will claim they know better and rubbish the rest of this article!

Let’s start with the science.

Flower Pot Candle Science

The Laws of Thermodynamics are very clear. The law of conservation of energy is clear. In a closed system energy cannot be created or destroyed. In other words if we have say, a candle, that is capable of producing 40 watts of energy, it can’t suddenly produce extra heat. Of course you can produce more heat if you burn more candles, but none of them can produce more than the 40 watts they posses. The following g quote is from someone that understands the law of the conservation of energy. This is what he says.

“In layman’s terms, you cannot create energy. So, a ‘free lunch’ is not possible,”

“The terracotta is just forcing some of the warm air from the candles to stay in place and not dissipate so rapidly, but it is doing nothing additional other than the illusion of a little heater,” 

Erich A Muller, professor of thermodynamics, Imperial College London

Of course you’ll see videos on social media showing how extremely hot the terracotta flower pots get in these systems.

They don’t lie. The flower pots get very very hot. That’s to be expected when, instead of the hot air rising from the candle, the heat is trapped in the flower pot. And when this happens it converts from convection heat to radiant heat. BUT there is no extra heat. That is physically impossible.

When this happens any moisture in the flower pot expands very rapidly and the pot can explode, throwing out sharp pot shards and burning candles.

If you are lucky, your pot will be dry enough internally and this will not happen. But there is a greater risk.


One of the most dangerous situations in a house fire can also happen in a greenhouse. When things get hot they give off fumes. After a while the fumes build up and when hot enough can cause a flashover. It’s when the fumes explode into fire and fire rips through a house.

If you have several candles packed closely together in a flower pot candle heater the heat can cause all the wax to melt, give off fumes and suddenly explode. According to fire prevention experts, flashovers in greenhouses cause several fires in greenhouses every year.

There are three examples in the newspaper reports below.

Buying Flower Pot Candle Heater Online

You can buy a flower pot candle heater online for less than a tenner. Some cost more. You could pay over £100 for one.

But whatever price they are fire officers warn against them because they cause fires.

Apparently it’s not enough to ensure they are on a stable surface, that there’s nothing flammable nearby. If one of these flashes over then the flames can jump a long way and find anything flammable. And if nothing flammable is nearby, your plants will be. They might not burst into flames, but they will not survive a dangerous flashover.

Here’s what the experts say.

It’s not just the risk of fire with plant pot candle heaters. “We would always strongly discourage the use of any homemade heating device.” 

Scottish Fire and Rescue Service spokesperson

And here’s a newspaper article.

Finally, the problem isn’t just one for greenhouses. Here’s a video from a boat owner where his flowerpot heater caused damage to his boat. To be fair, his situation is one of the good ones. Both he and his boat survived to tell the tale. I’d not bet my life on that sort of luck!

I’ll say it again. I’m not a fire safety expert. I merely repeat what they warn.

Image Attribution: Doyle Doss, Public domain, via Wikimedia Commons
Tag: Are Flower Pot Candle Heaters Safe Or Dangerous?

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