Retained heat (haybox) cooking: introduction

What is retained heat cooking?

If removed from a cooker, food in a cooking pot will continue to cook until the pot cools down. Placing the pot in an insulated box to prevent cooling means that the food will continue to cook without the use of further fuel. This is known as retained heat cooking, aka thermal cooking, haybox cooking, sit boiling or fireless cooking. Traditionally the insulation was straw or hay, hence the name haybox. In the middle ages, earthenware pots were used in a hole in the ground insulated with moss, leaves or hay; and in the 1940s the Girl Guides were taught how to make a haybox using a large biscuit tin and straw. Nowadays we have much better insulating materials and you can make a very efficient haybox. It is one of many different low-impact cooking methods.

The first steps in making a haybox for retained heat cooking

Making a retained heat cooker from insulation board: cutting the board with a heated knife.

The energy savings can be dramatic. For example, potatoes or rice are normally brought to the boil and then simmered for 15-20 minutes. If they’re brought to the boil and then the pot is placed in a haybox, that’s 15-20 minutes’ worth of fuel saved. Modern cookers based on the haybox principle are commercially available from the usual outlets. Known as thermal cookers, they use an outer vacuum flask with a removable inner stainless steel cooking pot. They work in the same way as the traditional haybox, but are smarter-looking and more expensive.

The next steps in making a haybox for retained heat cooking

Pieces of insulation board cut to accommodate handles and lid.

What are the benefits of retained heat cooking?

  • a haybox cooker can save up to 80% of the energy used to achieve the same results by simmering, so there are both economic and environmental benefits – i.e. cash saved and much lower pollution / CO2 emissions
  • food cooked slowly at a lower temperature tastes nicer and is better for you, as more of the vitamins are retained
A pot of food with lid in place ready for retained heat cooking

Adding a pot of food with lid.

  • retained heat cooking times are not critical; leaving the pot in the haybox ‘too long’ doesn’t harm the food (unlike boiling which turns vegetables to mush), so preparing several dishes to be ready at the same time is easy; this is particularly useful if you only have a single-burner cooker
  • once the initial boiling is over, the pot can’t boil over and will not burn the food – so it can be left unattended, freeing up time; hence The Idle Hour Cookbook, published in 1927 by Chambers Manufacturing, extolling the virtues of their ‘Fireless Gas Range’
  • the amount of water used to cook rice, potatoes or beans can be reduced a little because there is little or no evaporation with retained heat cooking compared to normal simmering
A home-made haybox ready to use for retained heat cooking

Home-made haybox in use.

What can I do?

Make a haybox cooker

  1. choose a pot: the lid has to fit well, and the size is important, as the pot needs to be full for it to work properly – so select a pot that matches the number of portions you normally cook
  2. acquire some insulation board; this can be bought from builders’ merchants or found (skips may be a good source)
  3. first cut a square that is bigger than your pot for the base. Cutting can be done with a saw or knife (wear a mask to protect against dust). Some types of board cut more easily if the knife has been heated, but make sure you are in a well-ventilated area
  4. cut several more squares the same size until you have a pile that is a little taller than the pot
  5. mark and cut a circle from middle of one of the squares so the pot fits through (the board must fit closely around the pot)
  6. continue cutting until you have a pile of pieces with a solid top and bottom and a hollow centre
  7. you can put the whole thing into a box, but insulation board is quite solid so you don’t actually need a box – you can just tape or fix the boards together somehow

You could also, of course, use a box full of hay, if hay is available where you are.

The end of result of retained heat cooking in a DIY haybox

Finished meal.

Cooking with a haybox

  1. bring the pot to the boil on the hob in the normal way
  2. simmer for a couple of minutes to make sure that all the contents and lid are up to boiling temperature
  3. remove from heat and transfer to your haybox
  4. place a brick or similar weight on top to make sure there are no gaps between the layers of insulation board
  5. allow your food to cook for around twice as long as simmering on the hob – but there is no harm in leaving them longer; for example, you can leave potatoes or rice for almost an hour, and they will be fine
  6. don’t keep opening the haybox to check on your meal – it won’t burn; opening it lets some heat out which is not replaced
  7. NB: make sure your food has boiled thoroughly before putting it into the haybox; there is a risk of bacterial growth if not
A real haybox with actual hay also works a treat for retained heat cooking

A real haybox – i.e. a box with hay in it – used regularly by friends of ours; works a treat.

Estimated cooking times

As a rough rule of thumb, for retained heat cooking you can at least double the normal cooking time. However, it’s difficult to specify precise times as a little longer during the boiling stage would mean a lot less time in the haybox. The size of the food also makes a difference; whole potatoes need more time than if cut into quarters, for example. If you find the food is not cooked through you can do a second short boiling at the end. After a little practice you will soon learn what works for you and your haybox. See the bottom of this page for recommended cooking times.

Commercial thermal cookers are also a solution for retained heat cooking

Commercial thermal cookers.

Using a thermos flask

For cooking small amounts (say a single portion of rice or pasta), a quality thermos flask can be used in a similar way. The principle is the same as the haybox, but as the flask itself can’t be heated, follow these steps: pre-heat the flask with boiling water; bring rice/pasta to the boil in a saucepan; empty the flask; carefully pour (use a jam funnel if you have one) the contents of the saucepan into the flask; seal and leave to cook. Don’t hold the flask in your hand when pouring, in case you spill the boiling water. Don’t forget that rice and pasta expand when they cook so don’t fill the flask to the brim, and be careful when opening in case the flask has become pressurised.

A 1920s German example of a fireless cooker using the retained heat cooking method

‘Fireless cooker’ from 1920s Germany.

 

Thanks to Steve Williams for information and pics.


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