Retained-heat cooking works because food at a full rolling boil holds enough heat to keep cooking for hours if it is sealed in thick insulation, so the fire is needed only for the first 5 to 15 minutes. The method is safe only while the food stays hot: it must begin at a hard boil and stay above 60 degrees Celsius until eaten, because the temperature range from 4 to 60 degrees is where bacteria multiply.
Part of Retained Heat Cooking
A pot of boiling food is a store of heat. Insulation slows how fast the store drains, and the drain decides how long the food stays hot enough to cook and to be safe.
The Pot Is The Battery, The Insulation Is The Seal
A pot of boiling water holds heat in proportion to its mass: about 4.186 kJ for each kilogram for each degree. The surroundings take that heat away by conduction, convection, radiation and escaping steam. Insulation slows the first three; a tight lid slows the last. The method needs three things in order:
- Start hot. A full rolling boil, not steaming. See Why Boiling Temperature Is Enough.
- Lose heat slowly. Thick, dry, gap-free insulation. See Thermal Insulation Basics and Heat Loss Pathways.
- Stay above 60 degrees. See Minimum Safe Temperatures.
What It Saves
The overview compares dried beans: open fire 3 to 4 hours of burning and 4 to 5 kg of wood; rocket stove 2 to 3 hours and 1 to 2 kg; rocket stove plus haybox 10 to 15 minutes of burning and 0.2 to 0.3 kg. The overview gives the overall saving as 60 to 80 percent in one place and 80 to 90 percent in another, so treat it as large and measure your own.
Common Mistakes
- Treating a haybox as a warming box. It is a cooker only if the food starts at a full boil.
- Skimping on pot fill. A pot filled less than two-thirds holds less heat.
- Letting it run all night. See Time Limits.
Safety
Warm is not hot enough
Food held between 4 and 60 degrees Celsius is in the bacterial growth range, and the haybox spends the last part of every long hold there. Begin at a hard boil, keep to the time limit and bring the food back to a full boil before eating if you are unsure. Do not put a hot cooking pot against anything that burns or melts.
What’s Next
- Learn next: Thermal Insulation Basics — what insulation does
- Combine with: Rocket Stove Design — the fire that brings the pot to the boil