Heat Retainers - Thermos Flasks and Fireless Cookers

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Retaining heat is a good way to delink the time of food preparation and the time of food consumption. This is especially helpful to complement a fuel that is not available on demand like solar energy. It allows having hot food even after sunset, when sunlight is not available to cook.
Heat retainers cannot substitute a stove, but they can be used to complement any stove and any fuel. In order to retain heat for cooking, heat needs to be generated first. By combining the heat retaining devices with other energy-efficient technologies (pressure cookers, solar stoves, improved cook stoves) or practices, it can contribute to reducing the fuel consumption considerably.

Depending on the material used, heat retainers can be as well used for cooling.

Thermos Flasks

Thermos flasks are an inexpensive way to maintain the heat of water, beverages or liquid foods like soup and porridge over time. In places, where food preparation is based on pouring hot water over food items such as in Tibet, thermos flasks save fuel and enhance cooking comfort. Alongside solar cookers, families own up to five thermos flasks and store the water heated by the solar cooker during the day. The hot water is used for preparing both morning and evening meals (mainly soup and porridge) enabling the solar cooker to satisfy almost all household energy needs. However, this represents a rather specific case, which may not be transferable to many other countries.[1] Yet, thermos flasks are currently underutilized and could probably play a more important role. When promoting the use of thermos flasks, features concerning price, durability, cleaning, size and material (toxic) have to be also considered.

Fireless Cookers

A women’s initiative making fireless cookers in Kisumu, Kenya

In some parts of the world, the concept of the fireless cooker has been known about for centuries. It became popular in the western world in the years between the 1890s and the 1930s. Initially, the heat retention cooker was mainly used to make food more portable for use by people on the move such as fishermen, hunters and soldiers. However, during the first decades of the twentieth century, the fireless cooker also became a permanent fixture of many American and European households, an appliance often found next to the cooking stove. Nowadays, it is mainly promoted in developing countries.[2]

Other names are: heat retention box, heat retention bag, heat retention cookers, hay box, hot bag, food warmer.

Principle of Fireless Cookers

The fuel consumption during the cooking process is not constant. A lot of fuel is required to heat up the content of a cooking pot to boil. However, once it is boiling, it only takes little energy to keep it hot and maintain the temperature at boiling point.

Cooking with heat retention boxes or bags means that food is brought to a boil, simmered for a few minutes depending on the particle size and then put into the fireless cooker to continue cooking. Since the insulated cooker prevents most of the heat in the food from escaping into the environment, no additional energy is needed to complete the cooking process.[3] This is why this is called 'fireless cooking'.

Fireless Cooker Sketch Bernd Müller.jpg
Fireless Cooker Sketch

Construction and functionality of the heat retention box (fireless cooker, hay box, hot bag etc.) depend on the insulative properties of the material used. The underlying principle of insulation is that isolated air is a poor heat conductor. The more insulated pockets of air you can create between the cooking pot and the ambient, the more heat will be retained inside the pot. Insulative materials are available almost everywhere available. For example organic residues such as hay, sawdust, cotton waste, or dried leaves, e.g. from banana plants can be used. Other materials are waste paper, polystyrene beads, or vermiculite. An insulated lid or cushion closes the box or bag.[4]

Depending on the type of stove used for boiling, food cooked, and traditional cooking practices, using heat retention bags can reduce fuelwood consumption to a great extent. According to a test of The Partnership for Clean Indoor Air (PCIA) the energy savings of fireless cookers combined with different stove types amount to an average fuel reduction of 50%.[5] Some people report that they can save up to 80% of their fuel, depending on the food and the efficiency of the insulation.

Especially for food that does not require any stirring, e.g. (sweet) potatoes, meat, cassava or rice etc., fireless cookers are very suitable. Food can be kept warm for up to 6 hours, e.g. for people who return home late from the field or the market and want their children to find hot food when they come back home from school.

Advantages of Fireless Cookers

  • Slow cooking retains many more of the food’s nutrients and vitamins than if prepared on a constantly hot fire.
  • Food can be kept warm for a long time.
  • Fuel savings.
  • Food cannot burn.
  • Women are less exposed to smoke.
  • Food can be left unattended in the hot bag, leaving women more time for other activities.
  • Women can reduce the frequency of cooking by preparing enough food for two meals and putting half of it in the hay box to keep warm.
  • Depending on the material used, heat retainers can be as well used for cooling, too.
  • Heat retention bags and boxes can be made with almost no costs or purchased for little money.[3][6]

Disadvantages of Fireless Cookers

  • The technology is not applicable to all types of food; it is limited to dishes that are cooked slowly in liquid (beans, rice, meat etc.) and do not require stirring.
  • If you open the lid of the pot and stir the content, the temperature goes down quickly and might go below the point where the food still cooks.
  • Change of taste: if beans are prepared in a fireless cooker, they don't acquire the strong smoky taste as if they were cooked on a fire for long hours. They might be healthier, but they taste different and are sometimes rejected for that reason.
  • It is not advisable to keep the food warm longer than six hours. Otherwise it might promote the growth of microorganisms in the food which puts the health at risk.
  • In case the pot is used on open fire, people might refuse to put the black pot into the box or bag, in order not to soil it
  • Cooking with a heat retention bag or box requires changing accustomed cooking processes. In order to show people the advantages of the heat retention box or bag cooking demonstrations, cooking books, etc. might be required.[7]

Examples of Using Fireless Cookers

Diversity of fireless cookers, Kenya

Heat retention boxes/bags have been successfully introduced for taking care of sick people in Malawi. Patients often cannot eat one big meal but have to eat or drink small portions of food or tea many times throughout the day and night. Without a fireless cooker, this would require frequent food preparation. In a retained heat cooker, food can be kept warm near the bed of a sick person who can take hot food or tea at a time of his convenience for a period of 3-4 hours. This has particular relevance for taking care of HIV/AIDS patients.[8]

Promoting Heat Retention Boxes or Bags?

As a project, if deciding between the introduction of a heat retention box or a bag, the following points could be helpful:

Heat retention box
Heat retention bag
Easier to keep clean Material easily available and cheap, but difficult to wash or clean
Not adaptable to different pot sizes Adoptable to different pot sizes
Higher transport costs
Less space required for transportation, lower transportation costs
Higher production costs (material, labor)
Lower production costs (material, labour)
Source for income generation Source for income generation
Cooled items can be stored cool in the box. The bag can also be used for cooling
Usable as seat, in case box is robust Few space required for packing away

GIZ TJK Volkmer first heat retention box 2011.jpg

GIZ TJK Mueller heat retention bag 2011.jpg

First model of a heat retention box made from cardboard box, polystyrene, and reflective insulative foil in Tajikistan
Picture: Heike Volkmer
Heat retention bag with trivet promoted in Tajikistan
Picture: Friederike Müller

Further Information

Manuals, reports and other articles:

Useful websites:


This article was originally published by GIZ HERA. It is basically based on experiences, lessons learned and information gathered by GIZ cook stove projects. You can find more information about the authors and experts of the original “Cooking Energy Compendium” in the Imprint.

  1. GTZ HERA (2007): Here Comes the Sun. Options for Using Solar Cookers in Developing Countries.
  2. We Insulate Our Houses, Why Not Our Cooking Pots? In: Low-Tech Magazine, July 1, 2014. http://www.lowtechmagazine.com/2014/07/cooking-pot-insulation-key-to-sustainable-cooking.html
  3. 3.0 3.1 Heat retention cooking. http://solarcooking.wikia.com/wiki/Heat-retention_cooking Cite error: Invalid <ref> tag; name "Heat retention cooking. http://solarcooking.wikia.com/wiki/Heat-retention_cooking" defined multiple times with different content
  4. GTZ (2010): Retained heat cooker. energypedia.info/images/0/00/GTZ_4a_FS_fireless_cooker_2010.pdf
  5. Partnership for Clean Indoor Air (PCIA) (2012): Test Results of Cook Stove Performance. http://my.ewb-usa.org/theme/library/myewb-usa/project-resources/technical/TestResultsCookstovePerformance.pdf
  6. Including experiences made by Christa Roth in Malawi.
  7. Based on experiences made by Christa Roth in Malawi.
  8. Christ Roth (2006): The role of biomass energy conservation for HIV mitigation

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