February 22, 2017

DIY Thermal Cooker

Camping cooler? No. DIY Thermal Cooker in action.

When it is easy to use a seemingly endless supply of energy, we tend to use more than we need. When using energy efficiently is more difficult or time consuming, the tendency is to use it more thoughtfully and frugally. That is what thermal cookery is all about.

Also known as "retained heat" cooking, this ageless method has been around since humans discovered that insulating a pot of hot food with banana leaves, or buried in the ground, is a great way to use energy more efficiently.

A thermal cooker is essentially a slow cooker without a power cord. Of course one can buy all manner of fancy thermal cookers, but this method hasn't been used for thousands of years because it is complicated or expensive.

The thermal cooker is a perfect DIY experiment in simple living, and one that Linda and I investigated this week in the NBA Simple Cooking Research Laboratory, or, our "kitchen". Armed with white lab coats and oversized magnifying glasses, we started with sorting/rinsing 3 cups of pinto beans, destined to become one of our staples, refried beans.

Bringing dried pinto beans to a boil and 10 minute simmer on the electric cooker.

On the stove top we brought the water-covered beans to a boil, then simmered covered on medium heat for 10 minutes.

After 10 minutes we took the bean pot off the stove and immediately put it into our DIY Thermal Cooker, a camping cooler stuffed with a large blanket. You can also use straw, sawdust (shouldn't that be "treedust"?), balled up newspaper, old pillows, an old sleeping bag, or any other insulating materials.

The blanket was stuffed firmly around the pot, lid closed, then left to continue cooking overnight. The next morning the still warm beans were done and ready for flavouring and mashing to make refried beans.

The vessel used for the cooker can be about anything as well. It can be built in a cardboard box, laundry basket, hole in the ground, or a heat retention bag sewn specifically for this purpose.

Example of heat retention bag for thermal cooking.

Why waste energy when there is an easy way to use it more efficiently? Perhaps because that more efficient method may not be as convenient. Unfortunately, convenience always entails unintended consequences, like excessive waste. But the worst of it is that convenience-at-all-costs leads to a less beautiful, self-sufficient, mindful life.

Rather than convenience, I like to think about what the most efficient and life-affirming way of doing things might be, even if that means something I do is "harder" or more difficult. Imagine life if we never did things that are hard to do. Just because something is harder, doesn't mean it isn't beneficial.

Conducting experiments in living more simply, and trying out new things, is not inconvenient, it is challenging, rewarding and a great deal of fun. The DIY thermal cooker was a good example of that principle. We are adopting this energy efficient method of cookery in our kitchen. I mean, laboratory.

See more on thermal cookers HERE.
"Cooking, ultimately, is about heat, how heat enters the food and what happens to the food when it enters." 
- Cooking For Engineers


  1. Love this. Amazon sells Wonderbags for over $60!!! But, there are plenty of websites that give you directions to make your own. I like yours; just using things you've got on hand like an old cooler and a blanket. Old fashioned ingenuity and a strong desire to save the earth. That always works!

    1. Deva,

      We try to use what we already have as much as possible. It is amazing what can be done with only a few things.

  2. Anonymous2/22/2017

    Great idea Gregg! And yet I have never thought of it.
    Funny how many times rethinking things from first principles leads to a better solution than what we've been doing our whole lives... Why are slow cookers using electricity when this works just as well???
    Thanks, Clara (Australia)

    1. Clara,

      Some things just can't be improved upon. We have forgotten this in our rush towards "new and improved" and thinking that new technology always moves us forward. It does not.

  3. I'm astonished you can cook beans like this! I'm going to give it a go. Did you soak the beans overnight first?

    I imagine this wold be a great way to cook oat groats overnight for those cold Winter mornings. In a similar vein, I have a book about traditional living in japan that describes a free way of heating water. People would put tall, ceramic jars of water outside in the sun and in the evening have hot water for bathing and washing clothes. My guess is that having to go out to work is what precipitated the demise of many of these brilliant ways of living, as they are much easier to implement when you are at home (eg the solar cooker).


    1. Madeleine,

      We usually soak our beans, but did not for our thermal cooker experiment. We wanted to see what could be done with just the cooker on its own. But soaking beans is a nice no-energy way of pre-preparing dry beans for shorter cooking times.

      It would be a great way to have oat groats warm and ready for breakfast. We will be trying this.

      So true about having to go off-site to work - such a lifestyle requires the support of convenience appliances, endless energy, and often, processed foods. Does not seem like the healthiest way to live, and certainly is not the most efficient.

      I prefer my work to be the meaningful stuff I do at home, rather than mindless work to make money so you can buy things to compensate for not being at home more. It is a vicious circle. Go to work, have less time, need more things.

      A solar cooker combined with a thermal cooker is a match made in energy efficiency heaven. Perfection.

  4. Anonymous2/23/2017

    I think this works because of the second law of thermodynamics, heat will travel to the coldest place. The thermal cooker slows the heat from leaving.
    Love the beans. It's funny when you practice simple living, beans become part of your life. I'm eating a lot of black beans at the moment.
    Keep up the good work.

    1. Alex,

      Dry beans are frugal and nutritious, not to mention very tasty. I sometimes make my refried beans from black beans. Very different from pintos, and very good eating.

      I love a bit of physics with my meals.

    2. Anonymous2/02/2018

      "A bit of physics with my meals" ha ha yep. I always cook my rice thermally, I just bring the rice to a boil and wrap the saucepan tightly in two large towels and put a weight on top, is ready in the morning. My beans don't turn out quite soft enough this way so I'll try a chilly bin and see if that works better. Great post!

    3. Anon,

      I have been experimenting wrapping pots with towels on my counter top. It is not as efficient as the chilly bin, but works well, like you said, for rice. I also cook oatmeal this way. Towels are also an excellent way to keep food warm for a long time after conventional cooking, instead of simmering on the stove.

      We are on electricity Time of Day Metering. In the winter months (peak season) power is more expensive between 7 am and noon, then 4pm to 11 pm. During winter we do a lot of cooking thermally.

      We cook our evening meal before 4, then keep it warm in the cooker till we are ready to eat. The chilly bin keeps food hot for many hours, so it is perfect to take advantage of the cheaper power between noon and 4 pm, then open the cooker and sit down to a steamy cooked meal.

  5. https://en.m.wikipedia.org/wiki/Tandoor
    Variations as per need

  6. In Holland we call it the 'haybox-method'. Blankets and hay in wooden box. Mostly used for the potatoes on farms. Read about it in old cooking books. Like your camping cooler solution. Got one should try it out, thanks for sharing.

  7. Anonymous3/06/2021

    My grandma was using the haybox method for soups / stews up to the 1990s. She said it was used a lot during WW2.


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