Making a tealight/teracotta pot heater more efficient
December 14, 2013 8:44 AM   Subscribe

I've gotten caught up in the hype about those upturned-terracotta-plant-pot-with-tealight-candles-underneath-heaters, so I've made one. It's doing the job and keeping the chill off, and what can I expect, really, from something heated by 4 tealights. I want to make it more efficient, though.

The design I'm using at the moment is a 19cm X 9cm X 5cm loaf tin, inside which are 4 tealight candles, in the middle. Placed over the top of the 4 tealights is a 13cm X 13cm unglazed terracotta pot. The hole in the pot is stoppered up with some aluminium foil with a pebble from the garden on top. When the candles are lit, heat can be felt from the unit up to about 30cm away.

What variables is it worth me changing to get more heat out of the unit, or at least feel more of the heat that is currently being radiated?

Things I've thought of so far:-
More candles: more heat in = more heat out, I figure. This will mean using a larger pot, though, with a larger surface area - worth it?
Add more pots inside the existing pot: I've seen this done online, with steel bolts and such, but doesn't this just increase the amount of heat that gets absorbed and cause less heat to be radiated?
Some kind of shield to reflect heat back from the other side of the unit: what would work to do this?
Suspend the pot in the air: at the moment the pot is resting on the metal loaf tin, which gets quite warm - would contriving something to hold the pot in the air help with this?
Paint the pot black: I think this only helps with heat absorption, not radiation?

There's a device called a "kandle heeter" that is a very similar sort of device. The website isn't really that encouraging though...

Any thoughts or ideas?
posted by Solomon to Science & Nature (11 answers total) 6 users marked this as a favorite
Link for people who can explain the science but aren't aware of the hype you speak of.
posted by Houstonian at 8:59 AM on December 14, 2013 [3 favorites]

There is a good discussion of this on StackExchange that you may find helpful.

Are you trying to make the heater as cost-effective as a heater using candles can be, or are you looking for the most cost-effective way to get radiant heat? If it's the latter, buying an electric space heater will be more efficient unless your electric rates are v. high and your tealight rates are v. low.

Since you're in the UK, you should also be aware that the UK hasn't banned lead in candle wicks yet, at least as of 2012. IKEA is usually cited as the source of cheap tealights, and their candles specifically have had cadmium in the wicks, too (as noted in the first link). Candles are also a source of PAHs, particulate matter, etc. (Plus, huge fire hazard, especially if you have pets or children.)
posted by pie ninja at 9:11 AM on December 14, 2013 [3 favorites]

Having looked at this idea for all of 2 minutes, I'll give you my gut feelings about what I've seen so far and your thoughts on improvement.

We know that the heater can only have a maximum heat output that would be equivalent to the combustion energy of the candles. From the StackExchange thread that pie ninja linked, this seems to be on the order of 50-100 W per candle, depending on whose numbers you use (my fire science book, alas, is at work). That would be a total of 200-400 W for your application, which is about equivalent to the heat output of a desktop computer plus maybe some other minor electronics. So: you can totally get a space heater that would put out several times that amount of heat for about $15 (just from random Googling), plus the cost of electricity (which at the high end of the candle heat output in my area would be about a cent an hour). On preview, I guess you're in the UK so the numbers obviously aren't the same, but they are still illustrative.

However, I know you're interested in making your heater more efficient and not talking about electric heaters. So, your points:

- More candles: more heat in = more heat out, I figure. This will mean using a larger pot, though, with a larger surface area - worth it? More surface area might actually help you, since the heat transfer out is proportional to the area. That said, it's a tradeoff - if you make it too big, you'll have problems with gas mixing in the inside of the pot and you'll basically negate any heat transfer benefit you would have otherwise gotten. For all I know, four candles and a small flower pot is the sweet spot for this in terms of cost/efficiency tradeoffs.

- Add more pots inside the existing pot The video that Houstonian linked showed a guy using two pots, with about a half-inch or so of space between them. Is this kind of what you're talking about? In this case I think all you do is direct the flow of natural convection air through the hole at the top; this may help, but I would also wonder if you lose efficiency because you would be cutting down on the radiative heat transfer (the new outside surface is cooler).

- Some kind of shield to reflect heat back from the other side of the unit Now you're on to something! If you have it in a corner or against a wall, consider putting a sheet of aluminum foil behind it. I would put this heater in the center of a room so I wouldn't have to worry about this, but a shield of aluminum foil should probably do something.

- Suspend the pot in the air This is problematic because it will allow more of the hot gases to escape from around the lip of the pot, which would subvert the purpose of the pot in the first place. I get that you're worried about the loaf tin getting warm, but I think any benefit you'd see would be negated by other loses. You essentially want to cover up the candles while also allowing enough air to get in and out to allow the combustion to keep going.

- Paint the pot black Yes, this will help - dull, black bodies have higher emissivity, and radiative power emitted from the surface is directly proportional to the emissivity. Painting it black should help with the heat transfer, but I have no idea how the paint would interact with the ceramic pot, etc. You might consider painting the inside black too to help the interior heat transfer, but regardless you would want to use a high-temperature paint to be on the safe side.

Bottom line - they way these things would seem to work is that the pot stores the heat output by the candles and allows it to be released back into the environment in a slightly more controlled manner through radiation and convection. There might be some kinds of chimney effects you could induce with additional pots, and somehow the design might make the candle combustion more efficient, but you will never be able to get more than a few hundred watts out of the heater. Your best bet for using this to feel warmer is to try to increase the radiative heating aspects as much as possible, I think, and you already suggested the two best ways to do that.

pie ninja's StackExchange thread has some very good discussion that goes into some more detail on a lot of the factors that go into these heaters working or not, so definitely look there, too. Also, please do heed the warnings about safety - candles can be a major fire hazard, especially for people with children and pets, and combustion without a dedicated exhaust system has the potential to be very harmful. Even an electric space heater can be quite dangerous if left running in close proximity to flammable materials.

nb: I have a master's in mechanical engineering and currently work with thermal-fluid systems. I have some fire science education and some work experience with HVAC as well.
posted by malthas at 11:22 AM on December 14, 2013 [7 favorites]

If you want to reflect heat, why not wrap some cardboard with tin foil shiny-side out, like a solar oven? Or if that's too much of a fire hazard for you, find something else to stabilize the tin foil. Just make sure to go shiny side out!
posted by windykites at 11:23 AM on December 14, 2013

I think you should read about the history of masonry heaters.
posted by cda at 12:22 PM on December 14, 2013 [1 favorite]

First of all, this is not a great idea for indoor air quality (especially as your candles are starved for oxygen and so will not burn cleanly). There is also a fire risk.

Economically, it doesn't make much sense (electricity is much cheaper than tea lights). From a n energy perspective, it doesn't make sense either (the tea lights probably take a fair bit of energy to make and transport, plus there is the packaging and you are still burning fossil fuels as well - which is what wax is unless you use beeswax).

That said, from an energy perspective, you probably can't do much that is going to make a significant difference other than adding more candles.

Really, you should avoid all this and replace the whole thing with a warmer, cheaper to run, better for your air quality and for your carbon footprint electric radiant heater.
posted by ssg at 1:00 PM on December 14, 2013 [1 favorite]

I like playing with fire, and clay but these things are silly. I think that it is a safe bet that any improvised combustion heating approach that does more than this also has a good chance of leaving you cold and buried.

Candles aren't really all that cheap or clean burning, particularly with flowerpots impinging on their airflow. Wood scraps are cheap, but burning them cleanly is also a challenge. I have created some stoves out of tin cans that manage a seemingly hot, complete, clean burn, but i still would trust them indoors.

My suggestions are small radiant electric heaters or keeping an electric heating pad or hot water bottle in your lap. Also, fingerless gloves.
posted by Good Brain at 1:45 PM on December 14, 2013 [1 favorite]

There really is no way to make it "more efficient." You can only get a certain amount of heat from your four tea lights. The flower pot rig doesn't add to it or subtract from it. What it does is slow down the delivery of the heat, because it take a while to heat up the pot. Imagine putting your four tea lights inside a structure made of bricks, instead of the flowerpots. It would take a long time to feel any heat because the brick stricture would take a long time to heat, from the inside out, before releasing any heat on the outside. Do it with cinder blocks, and it's even slower. You get the heat fastest if you just burn the candles naked. You get it slower the more mass you surround the candles with. You could get it even faster if you burned the wax in a paraffin heater, aka kerosene heater, instead do with a wick. But in the end no matter what you do, the amount of heat you get into the room from burning the paraffin in four tealights is going to be exactly the same
posted by beagle at 2:03 PM on December 14, 2013

These candle stoves sound as if they are improvised by campers (who necessarily don't have electricity) and not meant to be used indoors.

Carbon monoxide problems indoors
posted by bad grammar at 3:53 PM on December 14, 2013

as a learning experience, this is a good thing to try. as an effective solution to anything, it pales compared to alternatives.

better approaches are to find waste heat and recover it, to find heat loss and prevent it, to localize heat production and application. personally, i think you'd be better off eating a candy bar and wearing a sweater.... you convert the food to heat, and the sweater localizes and conserves it. you could get a cat. cats are exothermic, adjustable, and convert cat food to heat, at some costs in convenience, admittedly. but then, i love cats and heater cats are my favorite. heater dogs, not so much. cats will actively seek you out to consolidate and exploit mutual warming opportunities. i suppose, if your living room could accommodate one, a heater llama might be good.

generally, there is no such thing as a free lunch. if this were a meaningful way of heating a volume, it would be what we use. you can glean a lot from studying why this is the case, and you can learn some thermodynamics from studying the generalities of this problem.

local burning is arguably more efficient than the electrical grid, if excited by coal, but not if supplied by hydro. it also comes with downsides. follow the money and energy losses for clues as to why.
posted by FauxScot at 3:47 AM on December 15, 2013

Adding some tin foil definitely helped. I had to keep it rather closer than I expected, which meant no cardboard attached, but it definitely made a difference. I've been on the lookout for some paint that will withstand the heat, but haven't managed to find any that will adhere to terracotta without a primer. Chiminea paint would likely work.

Fiddling with the design has been interesting. I now have a longer, slimmer pot on top of the shorter, squat one. The lower pot has the drainage hole open, while the upper pot is closed. This design seems to allow for more radiant heat, which was the point of this.
posted by Solomon at 2:09 PM on December 29, 2013

« Older I am a 53 year old woman. What should I be using...   |   I'm looking for the name of a kid's book series. Newer »
This thread is closed to new comments.