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Would someone tell me how two flower-pots help four small candles to heat a room?

tea-lightscandle heaterA YouTube video showing how to heat a room for 8p a day has attracted millions of views.  A journalist named Dylan Winter posted the clip and says that the heat put out by his device, combined with that emitted by his computer is enough to heat his study and save on his energy bills.  Four tea-lights (which were known as night-lights in my childhood) are placed in a bread tin, and a small flower-pot is placed upside down above them.  Then a larger flower-pot, also inverted, is placed over the small one.  That’s it, except that the small pot has its hole, if any, filled in, whereas the large outer one is left with its hole open.  Since tea-lights last about 4 hours each, 8 of them cover an 8-hour working day.  Buying them in bulk brings the cost to about 1p each, hence 8p per day.

Now I can see how the covering flower-pots might change the convection flow of air in the room, but the explanation – that warm air rises and cold air flows in to replace it and is heated in turn – doesn’t seem to explain their function.  Four tea-lights put out a certain level of BTU, and the flower-pots do not add to that level.  Left uncovered, the warm air above the candles would rise, and cold air would be drawn in to replace it.  So what is the point of the flower-pots?  A friend who has tried this reports that they become very hot, as you would expect, and have to be handled with oven gloves when you replace the tea-lights.  He also says the small one becomes very sooty.  Would someone out there who has worked professionally in a scientific field please tell me whether the flower-pots add anything?  Do they make the device somehow heat the room more than four tea-lights would if left uncovered?  I can see how having warm flower-pots might feel like a heat source, but candle energy has to be used to heat them to that level.  As you might gather, I am skeptical about this.  Would someone enlighten me?  And if anyone tries it, please report if it works.

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5 Responses

  1. Not an expert but I did go to school and understand you cannot get more out of a system than you put in – as you point out.

    In sales it is called, no such thing as a free lunch.

    The only function of the flowerpots I can imagine is heat storage radiator, so the heat output from the candles will take place more slowly over a longer period of time than if the candles just emitted their heat into the air as it was produced to mix with the air.

    Candles move their heat mostly by convection, so you need to be above them to feel the heat, whereas the flower pot will do it mostly by radiation in all directions.

    In summary it is just a way of providing a source of stored radiant heat.

    The volume of air in the room is heated by the same amount whether or not plantpots are used… that’s physics.

    You could get a similar effect by filling a plant pot ( hole sealed ) with boiling water and covering the top with tin foil. Depends on the relative cost of boiling a kettle and candles.

    It may give the impression of being warmer because it is a focussed and not a dissipating heat source.

    My grand parents from the Victorian era, when I was very small, would put a house brick in the oven of the coal fired kitchen range for a few hours before they went to bed, then wrap it in a cloth and place it in their bed. They did eventually move with the times and get a hot water bottle.

  2. Well, from Wikipedia, a typical modern candle emits around 80 watts (light and heat), burning around 0.1g/min of paraffin wax.

    Typical tea-lights might vary from this, so some estimation is in order. The tea-lights in our house are about 10g each (out of the 11g I just weighed including case and wick) and, from Amazon in bulk, 8 would cost 19p (not Madsen’s 8p, though 5g tea lights are available and obviously cheaper). If the reported burning life is 4 hours, and that applies to a 10g one, this is around 1/24’th of a gramme per minute. So, assuming my tea-lights are typical, each would provide around 33W of power. The power from 4 tea-lights would be 132W.

    In my home/office study (modern well insulated house), the computer, monitor, modem/wi-fi and (standby mode) printer run around 120W (just measured), and there is an 11W compact fluorescent light. So that totals around 130W.

    Experience without the central heating on, for cool summer days and warmer spring/autumn days, tells me that alone the 130W from computer etc keeps the room quite warm when the door is closed; this is well above 21C – so the door is mostly left open.

    So I am quite happy to believe that similar computers, etc plus candlepower (totalling around 250W) would be OK during working hours, except for the very coldest times of year.

    As for the flower pots, I suspect this might include a bit of influence from better distribution of the heat, as suggested by John B (rather than it rising immediately to the ceiling, cooling a bit and then going round and back to the occupant). Also, some low level radiant heat, as distinct from convection, makes one feel warmer.

    However, I suspect that the main issues with the flower pots are ones of fire safety and of airborne soot reduction.

    Best regards

  3. Unless you move into quantum mechanics you cannot get more energy out than you put in and even Q/M has it’s ‘ event horizons ‘. In pure heating terms one BTU (British Thermal Unit) is more or less equal to 0.293071 watt hours. Flower pots do not feature in this known science concerning getting more heat out than one puts in. When you consider that the average size semi-detached house would require a central heating boiler with an output of 50,000 BTU’s and an efficiency rating of 90% which equals 14,000 watts or 14KWA (Kilowatt Hours). Four candles producing an optimistic 320 watts would just about burn your arse if you sat on them.

  4. Obviously the pots can’t increase the heat output, but they can redistribute it.
    Without the pots you only get warm air, but that would rise immediately to the ceiling without warming the lower part of the room. The pots however trap the hot air for a while and get very hot, so they radiate heat. That has the effect of spreading the warmth at a lower level and in a form that warms the items in the room and also feels warm to the occupant.
    That’s my 2p worth.

  5. We tried it, it didn’t work at all.

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