Introduction: Off-Grid Tealight Slow Cooker

Picture of Off-Grid Tealight Slow Cooker

This idea is about as simple as they come, and a few people
will doubtless read this and think “duh… bit obvious”, but the idea for this came from a conversation about whether it was possible to power a slow cooker from an off grid 12V setup (the answer to that being “yes, but it needs a lot of batteries”). Which got me thinking of alternative off-grid ways of slow cooking, which in turn led me to the question “what would provide a low level of heat over a 6-8 hour period?”.

The answer is of course: Tea-lights – those pesky little things that clutter up a draw or cupboard for 99% of the time, until that one occasion you have a power cut. They burn steadily and reliably, and they come in all manner of burn duration times from 3 to 10 hours.

Anyone who’s done any hiking or camping in the UK will have come across the Trangia – it’s a brilliant all-in-one stove & pan setup that runs on meths; when cooking the pans nestle in a sort of container-cum-windbreak, and I thought that type of design would be a good place to start for building something that was solid, capable of handling a large stock-pot (with about 7 litres of capacity), and would provide sufficient shelter for what boils down to a candle-powered cooker.

I know this is by no means the only way of doing it, and I suspect this isn’t the best way of doing this, but the principle is sound and I’ve used it to make a slow cooked meal so I can claim it does genuinely work

So on with the guide….

What you’ll need:

· A stock pot to cook in

· Timber

· Candles

· Screws / nails / wood glue

Step 1: Size & Plan Your Build

What size stock pot you go for informs your build. My stock
pot had a 24cm diameter, so I settled with a design that would be about 30cm x 30cm and about 8” deep in total. I was also wanting to use materials I had lying about in the shed so decided to use some butchers block workstop offcuts, and a length of 6” wide dressed pine; though what you use is of course your own business.

Step 2: Build a Box

Picture of Build a Box

This can be as simple or as complicated as you want to make it. For me I used butchers block worktop as the top and bottom of the box, and used the 6” timber as the sides, and glues & screwed the whole thing together. From there I cut out a 24.5cm diameter (the diameter of the stockpot plus a little extra) circle in the top of the box, and screwed rubber feet to the bottom for reasons I’ll go into next. The height of the box was specifically set up to ensure that once the tealights were inside the box, the base of the stockpot would be about 2-3cm above the tealight where the flame was hottest (the stockpots handles rested on the top of the box to stop it dropping all the way through – see the pictures if you’re not sure what I mean) . Once the box was built, I sanded down the rough edges and gave it a coat of woodstain to pretty it up a bit

Step 3: Make It Candle Friendly

Picture of Make It Candle Friendly

Remember the fire triangle from school? To function, a fire needs 3 things: Oxygen, Heat & Fuel; and the same is true of a candle-powered cooker. Setting aside the heat & fuel elements, the key thing we need to ensure this works is a good flow of oxygen, handily provided in every-day air.

To ensure good ventilation I pulled out the drill and put a series of 2cm holes in the underside of the box along 2 sides of the box, because the box would be off the ground due to the rubber feet I’d fitted, this meant air could be pulled into the box from the underside. I also got the holesaw out and put a 38mm hole in the side of the box, in the corner “opposite” the centre of the right angle of holes on the underside. Inside that hole I fitted a 40mm 12v pc ventilation fan, the logic being that once running that fan will draw old stale air out of the box and allow fresh air to be drawn in via the holes on the underside. The fan was then connected to a 12v power source and was good to go (and before anyone asks, I went for 12V as it would be easier to get off grid, also it would be a slow draw of air and less like to cause the candles to go out).

In fact on my first test cooking I had 6 tealights running and there was no need to run the fan, but possibly if you run a lot of candles it may become necessary

Step 4: How Many Tealights?

This is a little bit of trial and error – a slow cooker will normally cook at anything from 70 to 85 degrees C. so you need to have enough candles to get up to that temperature quickly, and then keep it at that temperature. For that reason I went – for my first slow-cooked meal – with a mixture of 3 hour and 10 hour tealights; all lit at the same time. The logic would be that the use of additional shorter burn tealights would help get the temperature up to 70-80, and then they die out and the remainder keep the stock-pot hot and slowly cooking. However how many you need will depend on what volume of water you’re using, so a little bit of trial and error may be needed (or you can just cheat and boil the water via some other quick method!)

Step 5: Cooking on Candles

Picture of Cooking on Candles

With the box assembled, it’s time to get cooking.

For my first attempt I took the cowards way out and went for a vegetarian meal on the basis that if it didn’t work, and since it would be unattended, it wouldn’t cause me to be running to the toilet all night.

My recipe involved a litre of stock, and a lot of veg, so I peeled & chopped, put the whole lot into the stock pot, and settled on 3 x 3 hour tealights, plus 3 x 10 hour tealights, this just about worked as planned in terms of getting the temperature up quickly, and then maintaining it for 8 hours of cooking (I was at home so did the odd check on it during cooking) – at one point before the smaller tealights died out the temperature got up to just over 90 degrees C. so I can confirm that cooking meat should be fine (though I’ll not be held responsible for any undercooked meat and subsequent brown rain).

The tealights kept the whole thing at temperature until dinner time, and everything was fully cooked and tasted (as far as I was concerned) really nice. Next step will be some pulled pork.

So that’s about it – simple, effective and does the job. You could probably put some sort of cover over the box to help keep the heat in and keep the temperature up – more of a concern if you’re cooking with meat, but hopefully it’s given some food for thought.

Comments

blodefood (author)2016-09-29

I've seen a brand name oven that uses a similar method of baking with tealights. The one advantage of your system is that the tealights are spaced out instead of grouped closely together. Close grouping make the tealights prone to flash fire. The wood box, to me, is a little frightening because of fire but it does have insulating properties that the metal box construction of the brand name oven doesn't. To borrow an idea from it though, you might try adding some sort of flat stones that will retain the heat after the candles have gone out. It could give you potential for extending the cooking time as well as releasing heat into your camping abode on a cool night when the cooking is done. If the stones are removable, you could heat them in the sun or on a fire and insert them into the box at the start of cooking time. Great edible 'ible! :-)

PaulS69 (author)2016-08-31

From a safety perspective and to also increase the internal temperature, some roof slates lining the inside of the box might be useful. Obviously you'd need to drill holes for ventilation

mary.beheler (author)2016-01-28

One caution: Kidney beans (and to a lesser extent, others) contain a toxin that can be destroyed with 10 minutes of boiling soaked beans, but becomes 5 times more potent if cooked at 80 degrees C. The effects can be quite unpleasant, even deadly. It would be wise to make _sure_ beans reach a temperature that destroys the toxin.
I have seen a few posts that say slow cooking beans at 85 degrees C is safe, but I'm not sure how long they would need to stay that hot. However, soaking beans overnight, discarding the soak water, then boiling them for 10 minutes could be done even when a thermometer is not available.
The toxic agent is Phytohaemagglutnin (Kidney Bean Lectin)

http://www.foodreference.com/html/artredkidneybean...

PaulS69 (author)mary.beheler2016-08-31

In fairness, if you're going off grid you'll surely pick your meals to suit your equipment and being off grid, safe storing of meats isn't going to be practical for any length of time so it's more practical to stick to vegetarian recipes anyway. If you're making a vegechilli then canned kidney beans or just leaving them out is the way to go.

Kevanf1 (author)mary.beheler2016-01-29

Being as this Instructable is about an off grid 'slo cooker' maybe using kidney beans is not such a good idea. I'm just thinking that at higher altitudes (which if you are off grid you could find yourself in) water boils at a lower temperature. Go high enough and you certainly cannot make a decent cup of tea even though the water is boiling. I imagine that cooking kidney beans in water at a higher altitude could end up with the beans simply not being brought to a high enough temperature for the same reason. I have to admit that this is not something I have thought of before and it only sprung to mind after reading the very sensible warning from 'mary.beheler'.

rgbarnes (author)Kevanf12016-06-29

At high altitude we cook with pressure cooker to address the problems. Heating the pressure cooker on a stove or open flame until pressure is reached, than using the box to maintain the temperature would work perfectly. We have to lower the temperature anyway, even if the cooker is on stove top or other heat source.

spoesse (author)mary.beheler2016-02-04

Your method may be safer as in wont catch fire but it wont safer food poisoning wise, especially when cooking meat. You cant slow cook meat without heat.

rgbarnes (author)spoesse2016-06-29

But you can use low heat and long cooking times. See sous vide cooking.

bobwojo (author)mary.beheler2016-01-28

I can verify that this is true, My wife tossed some into her rice cooker with the rice and got very ill. Luckily she only ate a small number of the beans, prob less than 10.

Krimlar (author)mary.beheler2016-01-28

Thank you for posting this.

Unless you are trained in the medical field, or a professionally trained Chef, you would not know this.

You also need to discard the soaking water, and not cook the beans in it; also change the cooking water at least once.

Also, adding salt will keep the beans skins tough, if you want to keep the texture similar to "raw" beans for a salad, or just like that texture.

I prefer this texture in almost all my dishes.

meddler (author)2016-07-06

I wan't to try to adapt this to work with a rocket stove pipe.

Lorddrake (author)2016-07-01

This is a really interesting idea.

as far as the concern about the kidney beans .. just use the canned kidney beans .. they are fully cooked in the can as part of the canning process so you don't have to worry about toxins.

One addition I would make is a metal plate with feet in the bottom of the box. make it tall enough to clear the candles, and the pot can sit right on top of it. It will serve 2 purposes 1) it will collect most if not all of the soot from the candles so your pot stays clean. 2) it will act a dissipater to more evenly spread the heat along the bottom of the pot and reduce "hot spots"

ex811 (author)2016-01-28

Not to be too critical, but the mixing of metric and imperial measurements is a bit confusing to some of us in America. But I love your idea and this is a good write-up.

Wild-Bill (author)ex8112016-01-28

It is easy to convert from Celsius to Fahrenheit. 100C is 212F and 0C is 32F so Multiply degrees in Celsius by180 and add 32 will give you degrees in Fahrenheit. Though not highly accurate, say there are 2.5 centimetres to the inch. These conversions I do in my head without thinking about them, but then I am from Canada and we switch both Imperial and Metric all the time.

Lorddrake (author)Wild-Bill2016-07-01

1.8 not 180

Pixiedust7 (author)Wild-Bill2016-01-28

Except that it's not easy, because nobody ever remembers this!

al_packer (author)Pixiedust72016-01-28

It's real easy; I just punch in the measurement in one scale on my ancient Palm PDA and tap for the result in the other scale. That leaves lots of room in my brain for really important stuff like the latest scandal involving whatever celebrity.

bobwojo (author)Pixiedust72016-01-28

Anyone involved in Engineering or any Science will never forget the formula it is used so much. For everyone else I agree. .

ex811 (author)Pixiedust72016-01-28

Actually, I wasn't concerned about (1) the temperatures or (2) conversions so much as the mixing of 30cmx30cmx8" on the first page. Really, it's a great write up, and if you can convert °C to °F with the formula (°C x 9/5 + 32, as my physics teacher liked to promote the simplest form of a fraction) it's fine then you shouldn't have too much issue with converting inches to centimeters (2.54cm per inch).

VendicarD (author)Wild-Bill2016-01-28

Lets try

Hmmm.. Average body temp is 37'C.

37*180+32'C = 6,692'F

So Average body temp in 'F = 6,692'F

Well done.

The correct formula is Tc * 18/10 + 32 = Tf

37'C*18/10 + 32 = 98.6 'F

Note how the integer 37 translates exactly to 98.6.

This is because science - and the rest of the world use metric. And average body temperature is fixed at 37'C by the medical community.

98.6 is just a conversion from metric standard.

MichaelR258 (author)VendicarD2016-01-28

c * 18/10 +32?

Never thought of it that way before, I was taught c * 9/5 +32

bobwojo (author)MichaelR2582016-01-28

I remember is as F-32 = 1.8C

One of the many ways to manipulate the equation.

Wild-Bill (author)VendicarD2016-01-28

oops

rgbarnes (author)2016-06-29

Off Grid Sous Vide. I have been looking for an option that would allow heating a large pot of water on the gas range or open fire, then keeping it at a somewhat constant temperature for several hours. This will work, even better than using a beer cooler. Way to go, good instuctable.

LaboursOfLadies (author)2016-01-30

An alternative that that also works well is the hay box method. Start your stew/casserole on the stove, I have gas, and bring to the boil. While doing this find a wooden box, or make one, to fit the pan but with space around the sides. Stuff the box with hay or straw, and I mean stuff, leaving only enough room for the pan, then after the pan has boiled for about 10min put it in the box, stuff more hay or straw on top and cover with a lid. The stuffing keeps the pan hot and the contents slowly cook in the heat. Much safer than a lighted tealight and quicker to make. Full instructions can easily be found on line on any eco living webpage.

this is really old school and it works. my relatives grew up in the Netherlands during WW2 that is how they did much of the cooking :)

Ha Ha! "While boiling for 10 min... make a box for it!" I know u didn't mean that, but it made me laugh.

diy_bloke (author)2016-05-04

great idea. you may want to consider adding an isolated lid that completely covers the pan so to retain the heat and save on candles.

have neen using this principle to boil rice and potatoes: 5 min boil and then an hour in an isolated box (or in yr bed under the blankets). Saves a lot on energy bills (or candles)

Lee Wilkerson (author)2016-02-13

I think this is a great idea. Thank you for the write-up.

I do have one suggestion: line the inside of the box with foil to concentrate the heat into the food.

pcgirl (author)2016-01-30

Am I the only worry wart about a naked flame in a wooden box? Those candles get really hot underneath.

grumpyrich (author)pcgirl2016-02-09

HI -the box got warm, but nowhere near the flash-point of wood (which is apparently about 300 degrees C) as the box is a reasonable size. Plus the stockpot directly above flames takes in most of the heat so it's about as safe as you can get.

Though obviously a caveat about never leaving an unattended naked flame indoors should apply

guy90 (author)2016-02-05

Despite the controversy- regarding temperatures, kidney beans and whether or not this is compatible for cooking meat; I think this is a nifty little idea. I might make something similar for power cuts! Thank you for uploading

Doireann (author)2016-01-29

How well do you think this would work using a crock pot, instead of a stock pot? Do you think the flame might be too much for the crock material? I'm so looking forward to trying this out, before a big camping trip.

Not as fancy but the Boy Scouts have been doing this for years.....

http://boyslife.org/hobbies-projects/projects/975/solar-powered-cooking/

I didnt finish it but was looking at the styrene fish boxes as something similar to raise bread. Just replace the lid with glass an leave in a warm place, my van gets stinking hot in the sun im sure you could do the same in a car, dont cook the dog though. In some ways im not keen on polystyrene, its got benzene in it. 'C' is for cancer.

isa_k (author)2016-01-28

The ideas just keep rolling in!

Thank you jkimball :)

If there is the possability of 12v off the grid power "solar" then go passive solar. Reflectors etc.. tea lights (wax / parafin) works but maybe try the 100% sustainable solution for the coming apocalypce. Regards and good food!

Krimlar (author)2016-01-28

Great build, will keep it in mind for the future.

May design it with a slide-out drawer of some type to allow replacing the candles, or to add more for a brief period to allow for higher temps.

Please be sure to read the post by mary.b. , is very important.
drj-research (author)2016-01-28

hai

I', warey to scary about the "naked" flame - chemist in past life , as an off-grid cooker fine . I use tea lights to take the "edge" off a cold room - pieces of ceramic over the flame - flower pots .

A grid based slow cooker with 60 watt incandescent light - never got around to building that - same style , safe - go out for the day - and cooked when back

When 13 those many wasted years ( as employee ) ago - used light bulb to warm the bed - an arch of wire to keep material away .Mother thought it hilarious - it worked

Years later - used two light bulbs as "tootsie toaster" - to get feet warm in mid winter at low cost

Different ways of same thing

kentobin (author)2016-01-28

Any issue with just using a stove, e.g, wood, kerosene, etc., to get the pot up to temp and then using the slow cooker? As I've only used home electric slow cookers before getting the pot up to initial temperature had never been an issue.

If one could use it to keep a pot of water at a given temp for 2-4 hours you could also use it for off grid sous-vide cooking. That would be really awesome. :-)

Thanks for sharing.

grumpyrich (author)kentobin2016-01-28

Yeah - you could easily use a small stove burner / meths burner / hexamine cubes t get the temperature up quickly, then let the tea lights take the strain, or boil / almost boil the water separately first.... No rules against cheating :o)

jkimball (author)2016-01-28

Have you considered adding an insulated lid to the system? You could use it haybox style.

grumpyrich (author)jkimball2016-01-28

I'll definitely get round to adding something, it may not be 100% insulated to allow some ventilation, but will have to experiment with that a wee bit

MichaelR258 (author)jkimball2016-01-28

Just seen this & was about to say "haybox". It's how I've always made a stew.

ahoffman2 (author)2016-01-28

What about using a variant of the terra cotta heaters? make 4 small ones enclosed in an insulated box with a hole for the pot, a sliding tray on the bottom to load/ change the candles. Put one in each corner of the container and you should be able to maintain a constant box temp of around 160-180 F. Make sure the lid is insulated as well and it should reduce over all heat loss.

dufus2506 (author)2016-01-15

Consider coating the pot (just the outside of course) with soap so the wax will condense on a surface that will be easy to clean off later.

It's an old camping trick that works quite well.

jkimball (author)dufus25062016-01-28

Or just line the outside with foil- an equally old trick.

isa_k (author)dufus25062016-01-28

Thanks for the idea! However, I can't shake off the thought about what the burning/heated soap fumes would do to the meal? Also, would any soap, ie, block vs liquid soap do?

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