The first law says that energy cannot be created or destroyed within a system. Heat is a form of energy.
So if you’re putting a candle in a terracotta pot, lighting it and not doing anything else, you can’t generate more energy than was originally there. On a very basic level, the terracotta candle heater can’t “amplify the heat” because you’re not adding any energy.
“In layman’s terms, you cannot create energy. So, a ‘free lunch’ is not possible,” Erich A Muller, professor of thermodynamics at Imperial College London, told the Big Issue.
It’s only natural the “heater” will make the room hotter – but it’s insulation that will keep it warm, Muller added. “This combustion generates heat and will raise the temperature of the space,” he said.
“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,” Muller added.
“If you really want to save energy, there are no silver bullets: double glaze the windows, draft-proof the doors, insulate the walls cavities and ceiling, and use efficient heaters – for example, ditching the electric heater for gas or, better yet, for heat pumps.”
You might also think that by heating up the terracotta pot, you’re allowing the heat to be released over a longer period of time than you would with the candle, and allowing the room to heat up properly.
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Not so, Muller said: “There is no difference in the final result if you heat the space slowly or rapidly.”
So, pretty comprehensively, the terracotta heater method isn’t actually very effective as a heater.
And here’s what literal firefighters say about doing that…
Are plant pot candle heaters safe?
No – fire services have strongly warned against using them.
Charlie Pugsley, the London Fire Brigade’s Assistant Commissioner, said his service sees devastating results when things go wrong for people trying to heat their homes on the cheap.
“Candles are one of the most common causes of fires in the home and they shouldn’t be used for anything other than their intended purpose. Although life hacks can be handy, so many of them have the potential to be dangerous and we would urge people not to follow videos online if they could put you or others in danger,” Pugsley said.
“We would always recommend people swap to using safer LED candles rather than flame ones, but if you are using them they should never be left unattended.”
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Pugsley also recommended making sure you have working smoke alarms and getting clued up on what to do if there is a fire.
He added: “If you’re struggling to pay your energy bills, Citizens Advice can give you information on grants and benefits that can help you.”
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It’s not just the risk of fire with plant pot candle heaters. A Scottish Fire and Rescue Service spokesperson warned that: “We would always strongly discourage the use of any homemade heating device. This could present the risk of accidental fire within the home and the potential of Carbon Monoxide poisoning.”
So all the evidence and expert advice points to quite a simple conclusion. Don’t do it.