Old Fashioned Retained Heat Cooking: The Original Slow Cooker

Retained Heat "Hay Box" Cooking: The Original Slow Cooker

Hay box or retained heat cooking  is a very old fuel saving technology, dating back to medieval times. You’ll find reference to this type of cooking method in many vintage cookery books. How does it work? You start cooking your food in a pot, then you transfer the pot to an insulated box for long, slow cooking. It’s the original slow cooker (no electricity needed).

I first learned about this method when watching wartime farm (episode 2) where Ruth cooked a stew using a hay box. I was so excited because the materials are so easily accessible. Skip to 27:00 if you want to get right to the cooker.

Why Use Retained Heat Cooking?

  • Reducing your energy consumption
  • To save money (on electricity etc)
  • Emergency cooking & fuel conservation during power outages
  • Free or cheap to make
  • Portable and convenient (take it with you).

You can fabricate your own thermal ovens quite easily at home or if you would rather buy one ready made there are several great options for you.

DIY Hay-Box Cookers

Here is a selection of  hay-box inspired cookers and DIY instructions for you to browse through.

Hay Box

With a sturdy box, crate or basket and some fine straw, hay or wool, you can quickly make a hay box cooker.  Choose an insulating material such as hay, straw or wool to pack the box with. You can stuff a pillowcase with material for the top and then cover the whole thing with a heavy lid. You can find a tutorial on making a hay box  cooker here and another good one here.

DIY Wonderbag

If you’ve got sewing skills you can make your own wonderbag style of retained heat cooker.  This is a fabric bag (sort of like a bean bag, that you nestle your pot into. You can find detailed instructions at Under The Choko Tree.

Cooler with Sleeping Bags

This is a really easy and economical way to try thermal cooking since most of us have an insulated cooler and sleeping bag kicking around in our garage or storage shed. Here is one great example of using a cooler from Our Tiny Homestead.

Plastic Storage Tote with Insulated Cushions

Here’s a method that uses a plastic storage tote with insulated cushions. You could easily make your cushions using old pillow cases if you aren’t much of a sewer. They include the directions as well as some real use examples. Find it here.

Walkerland’s DIY Retained Heat Cooker

If you want to make one like the one we use, as shown in the main photo, here’s how we did it. Take a wicker basket and line it with a double folded heavy wool blanket. Line this with an old towel. When the contents in your pot are boiling, place the pot in the basket, tuck the towel all around it, then tuck the wool blanket around that. I used another basket as the lid but you could add another blanket or a piece of wood (maybe a big cutting board) instead.

Ready Made Thermal Cookers

If  making your own oven isn’t your style you can buy several styles on-line.


Wonderbag is a simple but revolutionary non-electric, portable slow cooker. Wonderbag’s clever insulation allows food that has been brought to the boil to continue slow cooking or warm while in the bag.  You can find it here.

Modern thermal cooker

There are quite a few choices for modern thermal cookers including this one that has really good reviews: Thermos Nissan CC-4500P Thermal Cookware and Carry, 4.5 Liters. Find it here.

Important Tips

  • Boil First: Make sure the food is boiling thoroughly before placing it into the hay box.
  • Safe Temperatures: Take care not to allow the food to cool too much so that it becomes a health hazard. As a general rule the food should be kept above 60°C.
  • Soak Beans First: Avoid using dried (unsoaked) pulse vegetables because they may not cook thoroughly.
  • Suggested cooking times: You should avoid peeking into the hay box cooker because you will lower the temperature and likely have to reheat the pot and start again. Here’s a link that takes the mystery out of how long it takes for things to be done.


Thermal Cooking Recipes – A website dedicated to thermal cooking recipes

Thermal Cooker – A website dedicated to thermal cooking


The Fireless Cookbook (1913) Details how to use a fireless cooker along with recipes.

Fireless Cooking, by Heidi Kirschner, Madrona Publishers. 1981.



2 thoughts on “Old Fashioned Retained Heat Cooking: The Original Slow Cooker

  1. Sherri says:

    I love the series with Ruth Mott and Harry Dodson and have purchased each series.
    In the Victorian Kitchen Garden series, they showed the same concept except they used purchased straw boxes. These had the cooking container fitted to the box, it was a wonderful series.
    My favorite though are the restoration of the kitchen garden and the Wartime Kitchen Garden.

    Thank you for your post.

  2. Tish says:

    In new Zealand I make a slow cooker called a Heylobag. It’s portable washable and is a very efficient slow cooker. You can google Heylobags Named with reference to hatbox cooking.

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