Guide to Designing Retained Heat Cookers

The guides information is best viewed from this PDF file:


The Hot Box

Frequently Asked Questions

How does the HOTBOX work?

The HOTBOX uses the principle of insulated cooking. If you can keep the heat that is used to cook food, no replacement heat is necessary to complete the cooking process.

What type of dishes can you make in the HOTBOX?

Absolutely any food or dish that you would simmer in its liquid once you have brought it to the boil. All beans, legumes, grains and pulses; all soups, stews and casseroles; custard and yoghurt; steamed veggies; basmati, white, brown and sushi rice; mieliepap, samp and even pasta.

What are all the different uses for the HOTBOX?

The HOTBOX has a wide variety of uses. It is used to cook foods, to transport and continue cooking your food, as a warming oven and also used next to the braai keeping meat and all braai foods piping hot, as a plate-warmer keeping plates perfectly hot in the dining room, the garden or on the beach, it is a cooler box which is ideal for keeping drinks ice cold and storing ice, an incubation chamber for the making of foods like yoghurt and breads, a foot cushion or ideal camera rest when doing wildlife photography from your vehicle. (Really!)

Do I need to heat the HOTBOX?

Keep the HOTBOX away from open flame or fire. The heat required to cook the food is generated conventionally with electricity, gas, fire or paraffin. The HOTBOX is never heated in any way.

How long do foods cook for in the HOTBOX?

The cooking time for different foods varies from 20min (whole rolled oats) to 12 hours (oxtail). Foods mostly cook for more or less the same length of time or just slightly longer.

What environmental impact would the regular use of the HOTBOX make?

If you used the HOTBOX only 5 times per week your household would save 119kg of CO2 per year. If 500 000 households did that it amounts to more than 60 000 tons of CO2 per year. At least 70-80% of cooking time is saved and therefore the use of valuable resources such as electricity, gas, wood, paraffin, money and time is drastically reduced.

Does it really cook my food?

Yes! It really cooks your food. To truly benefit from the HOTBOX a subtle shift in thinking is required but once you see the incredible benefits you could never look back. It cooks your food and keeps it piping hot. The more you incorporate it into your daily life, the more you will find you use it.

What if my food isn’t cooked?

A degree of experimentation is necessary to get exact cooking times for different meals. It is important to use pots with tight-fitting lids and check that you’re not opening the lid unnecessarily. Hard and dense foods that have to be soaked such as chickpeas may need to be re-boiled and placed back into the HOTBOX for the last few hours. Alternatively just boil food on the stove for a little longer before transferring to ensure that all the food is at boiling temperature and not just the water.

How long does food stay hot for?

Food stays hot for up to 8 hours and remains warm for a few more hours. After 8 hours, unopened, the temperature of the food in the HOTBOX is approximately 56 degrees Celsius.

What do I use it for when I’m braaing or on holiday?

Keep braai meat and veggies hot as it comes off the fire. Cook and keep meals hot whilst traveling or hiking. Ideal when traveling in confined spaces such as caravans or yachts because you can reduce the amount of cooking gas needed by up to 50% which frees up your space.

Is there any safety measures involved in the use of a HOTBOX?

The HOTBOX must never be heated or held close to open flame or fire due to the flammable nature of the polystyrene balls. For health reasons don’t put a partially-eaten pot of lukewarm food back into the HOTBOX without first heating it, since HOTBOXES are not only excellent cookers but also ideal incubation chambers for yoghurt and other bacteria-rich food.

Why is it a healthier way of cooking?

Once the food has been transferred to the HOTBOX, the heat drops quite rapidly from boiling point to approximately 88 deg Celsius. This heat is then maintained and very gradually drops by an average of 4-5 deg per hour. It is a known fact that high heat destroys the live enzymes in your food and therefore cooking at a lower temperature preserves nutrients. HOTBOX cooking can never over boil or burn your food and food definitely retains more juiciness and flavor.

What type of pot do I use in the HOTBOX?

The pots that you usually use at home. A nice tip is to line the bottom HOTBOX cushion with an old dish cloth to protect the base of the HOTBOX from dirty or stained pots.

How do I wash the HOTBOX?

Hand wash or machine wash on a gentle/delicate spin cycle with cold water. Wash at max 30deg Celsius. Dry thoroughly in the sun – shake during drying to move polystyrene balls and to dry equally.

Do not dry clean or iron. Machine washing is the sole responsibility of the consumer. Fabric has not been pre-washed.


Brown and White Rice:

  1. Put 1 cup of rice and 2 cups of cold water in a pot.
  2. Add salt to taste.
  3. Place lid on pot and bring to the boil.
  4. Simmer for 1 minute.
  5. Remove from the heat and place in the HOTBOX for 30 minutes (white rice) or 45-60 minutes (brown rice), or until all the liquid is absorbed.
  6. Rice remains perfect in the HOTBOX for hours as it does not dry out or overcook

Lamb or Beef Stew:

  1. Fry onions, garlic and spices in oil.
  2. Fry your cubes or knuckles of meat until brown.
  3. Add selection of chopped vegetables, tinned tomato and stock.
  4. Ensure that the food is covered by the liquid.
  5. Bring food to the boil and cook for 15-20 minutes.
  6. Transfer to the HOTBOX – bigger and tougher pieces of meat require up to 12 hours of cooking in the HOTBOX.
  7. Return to stove and thicken your stew with Bisto or cornflour just before serving (optional)
  8. Serve directly from the HOTBOX with rice or pasta and a green salad.

Creamy Chicken & Corn Soup (a little time consuming but delicious)

  1. Place a whole chicken in a pot and fill with water, barely covering the chicken.
  2. Add celery sticks, whole garlic cloves, stock powder, bay leaves, salt and pepper to the water.
  3. Bring to the boil for a few minutes and transfer to the HOTBOX for approximately 2 hrs.
  4. In a separate pot melt approx 100-150 grams of butter until it sizzles.(the more butter you use the richer your soup will be)
  5. Add a variety of chopped veggies (such as cabbage, carrots, broccoli, leeks, onion, beans and courgettes) to the butter.
  6. Stir it with a wooden spoon to coat the veggies in the butter.
  7. Turn the heat down as low as possible and place the lid on tightly.  “Sweat” the veggies in the pot until soft, stirring every once in a while.  The sweating process takes about 30-40 minutes.
  8. Once the chicken is cooked drain off the water/stock into a jug or suitable container (You will use this lovely chicken stock to make your white sauce)
  9. Make a regular béchamel/white sauce with a small amount of milk and use the chicken stock for the rest of the sauce.
  10. Debone your chicken – the meat will be very soft and tender – and cut chicken into small bits.
  11. Add the chicken, “sweated” veggies and fresh or frozen corn to the white sauce.
  12. Add a dollop of cream or Greek yoghurt to the soup and season according to your taste.
  13. Garnish with ground black pepper and a small bunch of fresh coriander.

Traditional South African Mielie pap:

  1. Bring 2 ½ cups of water to the boil
  2. Stir 1 ½ cups of mielie meel and a pinch of salt into the boiling water.
  3. Stir thoroughly whilst boiling until all the water has been absorbed.
  4. Transfer to the HOTBOX and leave for approximately 30 minutes.
  5. Serve directly from the hotbox.

Samp and Beans

  1. Place I cup of samp & beans in a bowl, cover with water and soak overnight.  Rinse and drain.
  2. Bring samp & beans to the boil in 3 cups of salted water and simmer for approximately 20 minutes on the stove.
  3. Bring it back to a rapid boil and then transfer to the HOTBOX for approximately 4-5 hours or until soft and all the water is absorbed.
  4. Add butter, freshly ground black pepper, seasoned salt and crumbled feta cheese and enjoy as a light meal or accompaniment to a meal.

Thermos RPC-6000 6L Thermal Cooker

Thermos Thermal Cooker RPC-6000W 2x3L Thermo Pot

RPC-6000 thermal cooker with two 3L inner pots

Thermos also offers the RPC-6000 in a two 3L inner pot configuration which adds to the versitility of this thermal cooker. You are able to cook two recipes at the same time for example, rice and beans, rice and curry etc. or fill them both up with the same recipe for double the amount.  I’ve also used it with just a single 3L pot to cook smaller portions and filled up the empty space inside the outer pot with a small blanket, towell or rag to help retain the heat better.

Thermos RPC-4500/CC4500 4.5L Thermal Cooker

Product Name: THERMOS® 4.5L Thermal Cooker

Model #: RPC-4500/CC4500

The THERMOS® Thermal Cooker also known as the Shuttle Chef or cook and carry system, is a unique two piece cookware set that is enhanced by vacuum technology. THERMOS makes the Shuttle Chef line of thermal cookers and also has sold one of the models under the THERMOS Nissan brand in the USA. The Thermos Nissan CC4500 is identical to the Thermos RPC-4500 except that the color of the Nissan model had a black lid and bottom with stainless steel center.


  • TherMax® double wall vacuum insulation for maximum temperature retention and lightweight transport
  • Practically unbreakable stainless steel interior and exterior
  • Energy efficient clad stainless steel inner pot for cooking
  • Cool to the touch with hot foods
  • Non-slip, scratch-resistant base
  • Bail handle for easy portability
  • Excellent for catered events, potluck dinners and tailgate parties
  • Can be used anywhere and any time. Cook even while you are traveling, boating, camping, etc.
  • Capacity: 4.5L / 4.7 Quart (4-6 servings)

Model: RPC-4500/CC4500


Assemble all the ingredients in the inner pot, put it on the stove and bring to a boil. Then remove the inner pot from the stove and place it inside the outer pot which serves as a vacuum insulated container to keep the contents hot.  There is no need to plug in any power cord.  The food will continue its “thermal cooking process” using the retained heat.  After the required time (e.g. rice 30 min; chicken stock 2 hrs; beef brisket 3.5 hrs), just open the outer pot, and a nutritional and flavorful meal is hot and ready. The Thermal Cooker has excellent heat retention capacity; the food inside the pot can retain a temperature of about 160 degree Fahrenheit even after 6 hours.


Safe: It is not a pressure cooker, there are no power cords, no switches or electrical hazards to worry about.

Energy Saving: After the food has been boiled for a short time, the cooker needs no external energy while thermal cooking. Food stays warm automatically after it’s ready.

Convenience: The thermal cooking process requires no further supervision or monitoring. Food can be cooked while you are traveling. You can cook with the pot anywhere, anytime and it’s safe to use indoors or out.

Economical: Decreases fuel costs, economizes time and energy.

Healthy: Entraps flavor, minerals and vitamins; generate less odor, grease and smoke in the kitchen.

User friendly: Never over cooks and cleans up easily.

Durable: Unlike foam insulation used in other brands, Thermos’s vacuum insulated outer pot is a technology that foam insulation can’t begin to touch. Thermos produces the most effective insulated container and is engineered to last.

How does a Thermal Cooker work?

Thermal cooker is a patented product of Thermos®. It is an epoch-making cooking concept, consisting of an inner pot and outer vacuum insulated container. The inner pot is a three layers structure: two layers of stainless steel with a layer of carbon steel of high heat conductivity. It is able to conduct and absorb heat quickly. The outer container is vacuum insulated. It prevents heat loss and is able to keep warm and keep cool efficiently. The cooking process is easy and safe.

If you compare a thermal cooker vs a slow cooker you get the same functionality but without the power requirements. A thermal cooker is like a non-electric crock pot, you apply all the heat up front to the food while simmering on the stove and afterwards the recipe finishes cooking in it’s own heat.

Does a thermal cooker work? Absolutely! With two vacuum thermal cookers you could prepare your whole days meals in the morning. One would be ready to eat at lunch and the other still hot and ready to eat for dinner.


  • Unique two piece cookware enhanced by our vacuum technology.  It provides the ability to cook, excellent temperature and flavor retention, and allows for easy transport in one self contained unit.
  • Inner cook pot is made of high quality practically unbreakable NISSAN stainless steel.
  • Vacuum insulated steel outer pot effectively preserve food temperature
  • Each Thermal cooker comes with an exquisite cookbook.
  • Portable models like the RPC-4500/CC4500: feature a locking bail handle for secure and easy transport
  • Available in 4.5L (4.7 Quart) capacity
  • Thermos Thermal Cooker can be used as a:
    • Slow cooker
    • Portable oven
    • Rice Cooker
    • Bain-Marie (double broiler)
    • Cooler or Ice Bucket
  • 5 years limited warranty.


Fireless Cookers – recipe books

Here is a list and links to a number of books written in the late 1800’s, early 1900’s on Fireless cooking.  They contain some great info on the method from recipes to how to make them and how to use some that were being sold at that time. These Fireless Recipe books are directly applicable for use as recipes in a thermal cooker, wonder box, wonder oven, hot box, hay box, or other retained heat cooking methods. The following are available to read and download online but are also available in reprinted books for purchase or to download on your Kindle.

The Fireless Cooker ( 1908 )–_The_fireless_cooker

Book of Caloric fireless cook stove recipes; a compilation of more than three hundred superior recipes of all kinds, meats, game, poultry, fish, cereals, vegetables … etc., especially adapted to the new Caloric fireless cookstove ([c1908])

The Fireless Cook Book : A Manual of the Construction and Use of Appliances for Cooking by Retained Heat : with 250 recipes (1913, c1909) by Margaret J. Mitchell

The Duplex cook book, containing full instructions for cooking with the Duplex fireless stove ([191-?])

Haybox how to and description

Haybox how to and description


Haybox cooking (also called retained-heat cooking) is an age-old method that can be used to conserve energy not only during times of crisis, but anytime. Depending on the food item and amount cooked, the use of a haybox or insulated cooker saves between 20% and 80% of the energy normally needed to cook a food. The longer an item usually takes on a stovetop, the more fuel is saved. For example, with a haybox, five pots of long-cooking dry beans will use the same amount of fuel to cook to completion as just one pot cooked without a haybox.

The principle of retained-heat cooking is simple. In conventional cooking, any heat applied to the pot after it reaches boiling temperature is merely replacing heat lost to the air by the pot. In haybox cooking, food is brought to a boil, simmered for a few minutes depending on the particle size (5 minutes for rice or other grains, 15 minutes for large dry beans or whole potatoes), then put into the haybox 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. The hayboxed food normally cooks within one to two times the normal stovetop cooking time. It can be left in the haybox until ready to serve, and stays hot for hours. “Timing” is much less important than in stovetop cooking: stick a pot of rice, beans, or stew in at lunch time, and it will be ready when you are, and steaming hot, at dinner time.

The haybox itself is any kind of insulated container that can withstand cooking temperatures and fits relatively snugly around the pot. Hayboxes have been made using hay, straw, wool, feathers, cotton, rice hulls, cardboard, aluminum foil, newspaper, fiberglass, fur, rigid foam, and/or other suitable materials as insulation. The insulation is placed between the rigid walls of a box, within a double bag of material, or lining a hole in the ground. “Instant hayboxes” have been created by wrapping a sleeping bag, blankets, and/or pillows around a pot. The most effective insulating materials create many separate pockets of air, which slow down the movement of heat. 2 to 4 inches of thickness (depending on the material) are necessary for good insulation. Some materials, such as aluminum foil or mylar, actually reflect heat back toward the pot. Important characteristics of any insulating material incorporated into a haybox include:

  • It must withstand cooking temperatures (up to 212 degrees F or 100 degrees C) without melting.
  • It does not release toxic fumes (any kind of foam insulation needs to be covered with aluminum foil or mylar) or dangerous fibers (fiberglass also needs to be covered).
  • It can be fashioned to be as snug-fitting as possible around the pot. A little pot in a big box will not cook as effectively; it’s better to wrap pillows, towels, or blankets around it to fill up the space.
  • It can be made to form a relatively tight seal, so that heat does not escape from the cooking cavity. Since hot air rises, a container designed to open at the base rather than the top will retain more heat.
  • It is dry, and can be kept dry, since wet materials don’t insulate as well. An inner layer of aluminum foil or mylar helps keep cooking moisture from entering the walls of the box. Mylar, which can be salvaged from used food storage containers, balloons, etc., tends to be a more durable inner layer than aluminum foil.

Cooking containers, too, should have tight-fitting lids, to prevent the escape of heat and moisture.

Since water is not lost in haybox cooking the way it is during extended stovetop simmering, the amount of water used to cook grains and beans is normally reduced by one-quarter. Instead of adding 2 cups of water per cup of dry rice, try adding 1 1/2. Also, the larger the amount cooked, the more effective haybox cooking is, since a full pot has more mass and therefore more heat storage capacity than a half-full pot. Haybox cooking is ideally suited for a family or large group, or anytime there’s a reason to cook in quantity. If you’re cooking alone, try cooking full pots of food using a haybox, then reheating small portions for individual meals–this too can conserve fuel.

Retained-heat cooking has many other advantages in addition to energy and water conservation. As mentioned, it makes “timing” less critical, since it keeps meals hot until serving time. Once the initial boil-and-short-simmer stage is past, it also eliminates the danger of burning the food on the bottom of the pot (the sad fate of too many pots of grains, beans, or other foods left simmering too long without stirring on the stove). Hayboxed food can actually be better for you, and tastier, than food prepared exclusively on a stovetop, because most of the cooking takes place in the 180 degrees F to 212 degrees F range, rather than at a constant 212 degrees F (lower temperatures preserve more flavor and nutrients, as they also do in crockpot cooking and solar cooking).

If you want to prepare multiple items for a meal but have only a limited number of flame sources, hayboxes can also greatly facilitate the logistics of food preparation. For example, you can bring your beans to a boil, simmer them 15 minutes, put them in a haybox; then bring your rice to a boil, simmer it 5 minutes, put it in another haybox; then prepare your vegetable stir-fry or soup, etc. At the end, you’ll have a uniformly hot, unburnt, multi-dish meal, all off a single flame, probably consuming less total fuel than you would have used simply to cook the longest-cooking item alone without a haybox. You’ll also have used one-quarter less of your drinkable water supply in preparing the food.

Presoaking and draining beans always makes them easier to cook, as well as to digest. A few particularly long-cooking foods, such as garbanzo beans, may need reboiling part-way through the cooking process. For health reasons, meat dishes should always be reboiled before serving.

Hayboxes are second only to solar cookers (which, however, are dependent on sunshine) in their potential to conserve resources. They’re easy to build, easy to use, and have many other advantages. Y2K or no Y2K, they deserve a place in every home.

instructables – haybox cooker

Haybox or retained heat cooking is simply cooking a liquid based food like a soup or stew in it’s own heat. During WWII cooking oil was rationed for the war effort this method became popular as a way to conserve cooking fuel. They used hay in a box because the air spaces in the hay trapped in heat and allowed the soup or stew to cook in it’s own heat. Anything like hay, shredded news paper, rice hulls, cotton balls, corn husks etc will work as long as it packs loose and creates air spaces.

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