The do’s and don’ts of assembling a DIY thermal/ heat retention cooker.

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This is the third in a series of posts about saucepans and slow cooker alternatives (read post 1 here and post 2 here). I have been trying to work out if I can slow cook food without the help of any special equipment and without needing any power (other than an initial injection of heat by bringing the food up to the boil) by making a DIY thermal or heat retention cooker.

Yesterday I experimented with using two freezer bags and two towels to help cook veg stock. I brought a pan full of veg peelings, herbs and water to the boil on the hob, wrapped my pan (with it’s lid on) up in a towel, placed it inside a freezer bag, zipped it up and then placed the freezer bag inside a larger freezer bag. I put another towel in the second freezer bag to fill up the gap and then zipped it up. I left it there for 8 hours

When I took the stock out after 8 hours the towel around the pan was really damp, the stock was luke warm and it didn’t taste that great. I now realise that I made a couple of mistakes (and I probably shouldn’t have tasted that veg stock).
  1. I didn’t follow the instructions for making veg stock properly!  Despite very clear instructions from Jen Gale on including apple peelings in the stock mixture I didn’t add any (read more here).
  2. I used a glass saucepan and the lid wasn’t particularly tight fitting. Glass doesn’t retain the heat well enough and even a glass lid on a saucepan is not a good idea.  Plus the lid needs to be tight fitting.
I did get some things right though (I hope)
  • I filled up the pan with as much boiling water as possible which is meant to be important (I think it helps retain the heat better).
  • I tried to leave as few gaps in the outer bag as possible for air and heat to escape through
  • I filled up all the available space between the pot and the outer bag with insulating material.
I can’t experiment any further now without investing in something new (or secondhand). Apparently a stainless steel saucepan with a well fitting lid is the best pan to use and the one I have has a glass lid.
If you are thinking of making a heat retention cooker at home with stuff you already have.
  1. Make sure you have an appropriate pan – a stainless steel pan with a stainless steel tight fitting lid is best.
  2. Find a bag or a box which either leaves as little room for air or heat to escape as possible or can be sealed somehow to make sure that happens
  3. Fill the bag or box with some insulating material, something that traps air well e.g. old woolly jumpers, towels or cushions, making sure there is still space for your pan.
  4. Ideally the bag or box will be a close fit to the pan and there won’t be too much space to fill with insulation.
  5. When you come to cook the food, make sure the pan is as full as possible with boiling liquid and follow a recipe designed for a thermal cooker (at least to start with until you get the hang of things)
  6. If the food is only luke warm when you take it out, discard it and don’t taste it.

I love making do with what I already have. It doesn’t cost me any money, it is better for the environment and I get a buzz from figuring out new (to me) ways of doing things! Experimenting can take a bit of time though and there are various heat retention cookers on the market if you want to buy a ready made and tried and tested one. Wonderbags* are one example and this particular company has a really nice ethos behind it. UPDATE: I now own a Wonderbag and wrote about it here: The Wondrous Wonderbag

I am going to take a break from heat retention cooking for the time being while I figure out what to do next in the saucepan department and crack on with making some money from my home (read more here)If you liked this post please click like on Facebook and follow on Twitter – thanks so much!

4 thoughts on “The do’s and don’ts of assembling a DIY thermal/ heat retention cooker.

  • I've been cooking peppers and onions in the skillet on the grill a lot lately. I'll have to add jalapenos next time. Yum. I like cooking bacon that way too, although hot fat + open flame adds a nice element of danger. I used to get flank steak, but my husband got me to try the carne asada cut from our local market, and I prefer it. Not sure if it's thin-cut flank or skirt (I'll have to ask), but it looks like this 

  • “I filled up the pan with as much boiling water as possible which is meant to be important (I think it helps retain the heat better).”

    What it does is provide more thermal mass and thus more energy to cook.

    The heavier the pan the better for the same reason. Something like a dutch oven works well..you can use plastic wrap to moisture seal the lid.

    The more dry material you can put around and over the pot the better.

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