During our 30-Day Grid Down Cooking Challenge, we frequently used retained heat or thermal cooking to save fuel. This ancient method of fuel conservation can be beneficial to use every day, as well as when during hard times.
How does retained heat or thermal cooking work? In retained heat cooking, food is brought to a boil in a pot and then transferred to an insulated container to continue cooking the food. The retained heat cooker maintains temperature long enough to complete the cooking cycle.
Retained heat, thermal cooking, and fireless cooking are all different names to capture the same concept. In this post, we will share with you the basics principles of retained heat cooking. Knowledge is power. This little gem of knowledge may just come in quite handy when times get tough.
Thermal or Retained Heat Cooking Basic Principles
It is possible to maintain temperature for up to 8 hours, or more, if you do this correctly. Here are some basic principles that will help you be successful.
- Food must be brought to a complete boil in a cooking vessel and all of the pieces of food should be heated completely through.
- The cooking vessel must be covered with a tight-fitting lid.
- At least 4 inches of insulation should surround the cooking pot on all sides. The more insulation, the better.
- Air is the enemy when it comes to retained heat cooking. Retained cooking works best when the pot is filled to about 80 percent capacity. When cooking smaller amounts, use a smaller cooking vessel. Do not allow any air gaps between the insulation and the cooking vessel.
- No peaking! Trust the process. If you open the retained heat cooker to check on the food, you will lose much of the retained heat and significantly reduce the length of time that it can stay warm.
Alternative Names for Thermal Cookers
It is possible that you may hear thermal or retained-heat cookers referred to by any of the following names:
- Thermal Cooker
- Hay Box
- Straw Box
- Insulation Cooker
- Retained-Heat Cooker
- Fireless Cooker
- Wonder Oven
- Wonder Box
- Wonder Bag
- Wonder Cooker
- No-Power Slow Cooker
Each of these retained cookers has its own unique twist on the concept but they all use insulation to cook food without any additional fuel. We will use many of these names as we explore this incredible method of cooking.
Insulated Cookers Keep Food Hot and Cold
Another advantage of thermal cookers is that they can also keep cold foods cold for an extended period. The focus of this post is on emergency cooking so we will concentrate on cooking and retaining heat.
What is a Thermal Cooker?
A thermal cooker is a specially designed unit that consists of both a cooking pot and a vacuum-sealed insulative container. The two parts are designed to work together to enhance retained heat cooking. A few thermal cookers, such as Saratoga Jacks come with a large pot, small pot, and a steamer pan.
I have a Thermos Cook and Carry (now the Shuttle Chef) and it is my go-to retained heat cooker if the pot is the right size for the meal. Soups, chili, oatmeal, rice, and cooked cereal are great candidates for a thermal cooker.
Thermal cookers take up less space because of the vacuum-sealed container. That makes them incredibly convenient. I have found that they don’t actually perform quite as well as the homemade versions. However, if I just wrap the container in a baby blanket or towel it significantly improves the performance.
A good thermal cooker will last for many years. Mine is about 15 years old and is still performing very well. Don’t buy cheap knock offs. We have tested some. They don’t perform well. Spend the money once and enjoy it for many years.
You can find our recommendations for thermal cookers on our Emergency Cooking Recommended Product Page.
What is a Wonder Oven?
A Wonder Oven is basically a set of Styrofoam bead-filled pillows that are used to insulate a hot pot of food during the retained heat cooking process. The best versions include those that conform to the shape of the pot nicely which improves the effectiveness of the insulation.
I use the Wonder Oven and pressure cooker in combination to decrease cooking time and save fuel. It is perfect for making yogurt or a big pot of beans.
The Wonder Oven is a bit bulky. However, the one advantage that it has over a thermal cooker is that you can use almost any size or shape of the pan between those pillows. It is quite versatile and highly efficient.
I usually place my Wonder Oven inside of a laundry basket or tote so that I can easily move the hot pot. Other designs such as the Wonder Bag or Wonder Box don’t need that additional support.
Food Flask, Vacuum Bottle, or Steel Food Jar as Retained Heat Cookers
Start with a high-quality, stainless steel, wide-mouth vacuum bottle, steel food jar or food flask for optimal results. It is best to preheat the bottle by filling it with boiling water.
Bring the food to a rolling boil. Dump the hot water out of the vacuum bottle and immediately fill with the hot food. Close the lid tightly right away.
Steel-cut oats, oatmeal, cracked wheat, quinoa, and pasta are examples of foods that you can just add to the preheated thermos along with boiling water.
Place the thermos on its side and let it work its magic. Wrap in a small towel or blanket to increase insulation. The quality varies significantly and rating for heat retention can be anywhere from 5-12 hours.
Some of the best foods for thermos cooking include; rice, hot cereals, pasta, and soups. My kids’ favorite school lunch was Ramen noodles. I would just break up a package of noodles and fill the thermos with boiling water. They were the envy of all their friends at lunchtime.
There are some amazing thermos and food jars on the market now. Retained heat cooking in a thermos is a great way to have a hot lunch wherever you are. A few of my favorite thermos type bottles for cooking are listed below.
- Maxso Vacuum Mug – Rated for 12 hours hot or 24 hours cold. This set has a 24-ounce and a 12-ounce thermos. The lids can be used as a bowl.
- Energify Vacuum Insulated Food Jar – Rated for 12 hours hot or 24 hours cold. They have attached a handy handle to the top of this thermos to make it easy to carry.
- Stainless Steel King Vacuum Insulated Food Jar – Rated for 9 hours hot or 14 hours cold. This food jar is a basic 16-ounce thermos that comes with a foldable spoon.
- Stackable Stainless Steel Thermal Compartment Lunch Box – 3 stackable containers – Rated for 4 hours hot – I love that this one because it has 3 separate little containers. Perfect for single servings.
Homemade Retained Heat Cookers
It is easy to create your own retained heat cooker out of items that you have in your home. Just remember the basic principles of thermal cooking. Make sure you have at least 4 inches of insulation on each side, no air gaps, and use washable materials when possible.
Ice Chest Retained Heat Cooker
A simple thermal cooker can be made out of an ice chest. You can use blankets or towels for insulation. Line the bottom of the ice chest with the insulation. Place the cooking vessel on top of the bottom insulation.
Place insulation snuggly around the sides of the cooking vessel. Then cover the top, close the lid, and start cooking.
We did notice that after we used the ice chest for thermal cooking for a while that ours started to split apart. It was just an inexpensive ice chest and you may have better luck with yours.
Bathtub Retained Heat Cooker
A lady came up to us after one of our classes. She shared with us that her mother had used retained heat cooking all the time when she was growing up. She would just take the hot pot and place it on a folded blanket inside of the bathtub. Then she would tuck in the cooking vessel in snuggly with additional blankets.
She said that she thought that everyone cooked their dinner in the bathtub until after she grew up. This simple method obviously worked very well for her family.
Cardboard Box Thermal Cooker
As a mom with a lot of kids, diaper boxes are something that we always had plenty of. One day I decided to get creative and make a thermal cooker or hay box out of a diaper box. I had a bunch of rigid foam pieces and so I cut them and lined the bottom and sides of a diaper box.
Next, I took some old towels to fill in the air gaps. I had a piece of reflective material and I decided to put it underneath the pot to help with the moisture and possibly reflect the heat back into the pot. Once the pot was nestled in, I covered it with an old bean bag and placed another diaper box on the top.
Admittedly, this is an ugly thermal cooker. I hauled it to many of our classes to demonstrate the effectiveness of thermal cooking. Our record for maintaining safe cooking temperature in that box was 12 hours. That’s some great insulation!
The box began to absorb moisture and cooking odors over time. After a while, it really started to stink and I had to throw it away. Lesson learned: make your thermal cooker out of washable insulation.
Repurposed Military Insulated Canvas Thermal Cooker
You can also just keep your eye out for containers with retained heat cooking in mind. We found an insulated canvas bag from a military surplus store.
It was designed for carrying water but could make a good thermal cooker with a little added insulation inside to get rid of the air gaps. It is sturdy and portable. It also collapses down a bit smaller (not completely) when not in use.
What Foods Work Best for Thermal or Retained Heat Cooking?
Foods that require boiling are ideal candidates for retained heat cooking. Soups, stews, chili, curries, and grains such as wheat, rice, oats, quinoa, barley, and millet all cook up quite nicely. It is possible to bake breads and muffins using retained heat but it takes a bit more work.
Remember everything must be heated through before placing it in a thermal cooker. A roast or large cut of meat will not cook in a thermal cooker.
Some foods can’t be boiled such as bread or a meatloaf. Place the food inside of a covered glass or metal container. Then place the covered container inside of a pot of water and bring to a boil. Boil until heated through. A pint jar with bread dough will need to boil for about 10 minutes.
The covered pot with the container inside is then transferred to the insulated cooker. The final product will not look like it was baked in an oven but it will be fully cooked.
Retained Heat Cooking Times
Cooking with retained will increase the overall cook time of foods but will reduce fuel consumption. On average, food takes about 3-4 times as long to cook in a retained heat cooker. A stew that normally simmers for 1 hour might take 3-4 hours.
Before placing the boiling food into the retained heat cooker, it needs to be boiled until all of the pieces of food are heated through. Boiling time will vary depending on the food and the size of the pieces.
Just because the food is fully cooked at that time doesn’t mean that you have to take it out. You can leave foods in the retained heat cooker for up to 8 hours, sometimes even longer. That is part of the magic of retained heat cooking. You can walk away and the food is safe, hot and ready to eat when you return.
Use a Pressure Cooker for Optimal Results
One incredibly helpful tip that I have discovered is that when you combine the efficiency of a pressure cooker along with thermal heat cooking you maximize your fuel savings.
We took a challenge to cook a meal using the least amount of fuel possible. We used a rocket stove and some sticks to bring a pot of soaked, dry beans up to pressure in a pressure cooker. Then we transferred the pressure cooker into a Wonder Oven to finish cooking.
The results were delicious! Combining a pressure cooker along with thermal cooking can help you cook your food with very little fuel.
Retained Heat Cooking Uses Less Water
Retained heat cooking traps the moisture so that you do not lose any water during the cooking process. You may not need to adjust most of your recipes but I use a little bit less water when I am cooking rice.
Is Retained Heat Cooking Safe?
Retained heat cooking is safe as long as you do it correctly. Food should be above 140°F when serving. If it has fallen below this temperature, bring the food back up to a boil to kill any bacteria that may have grown and to avoid foodborne illnesses.
Food should be brought up to a complete boil before being placed in the insulative container. The pieces of food must be heated completely through. If you have large pieces of meat, potatoes, carrots, etc., make sure that you boil it long enough that everything gets nice and hot. Once it is completely hot, the retained heat cooker can finish cooking the food.
Don’t let this worry you. You will get the hang of it in no time. I have rarely ever opened my retained heat cooker and found it to be below serving temperature. Most of the time it is between 160°F and 190°F, which is well within the safe serving range.
Advantages of Retained Heat Cooking
I discovered retained heat cooking in my search to find energy-efficient methods for cooking during a power outage. As I experimented with the concept, I learned to greatly appreciate the benefits of retained heat cooking.
The benefits of retained heat cooking include;
- No tending – Will not boil over or scorch
- No evaporation – Requires less water
- Conservation – Reduces required fuel
- Safe indoors – No emissions or toxins produced
- Portable – Can take the meal along
Thermal Cooker Construction Considerations
It is possible to create a retained heat or thermal cooker out of just about anything in an emergency situation. As a general rule, you need to have at least 4 inches of insulation all around the pot.
Insulating Materials for Retained Heat Cookers
The choices for insulation to use in a thermal cooker are pretty much endless. They should not compact during use. They must be dry and resistant to both mold and mildew.
It is important to note that the insulation will absorb moisture over time, begin to absorb food odors, and will need to be washed or changed.
Possible insulation materials for retained heat cooking may include; straw, haw, leaves, corn shucks, ash, newspaper, corrugated cardboard, Styrofoam, perlite, shredded foam, foam chips, blankets, towels, and fabric strips.
Our first “hay box” or retained heat cooker was made out of a cardboard diaper box, rigid foam insulation, a reflective aluminum barrier, and a bean bag placed over the top. It worked incredibly well.
We frequently made beans in it and took it to several of our classes to demonstrate the effectiveness of “hay boxes” for emergency preparedness. After a few months, it started to stink. We had created it out of non-washable materials so we had to throw it away. We did get to keep the valuable lessons that we learned while using it.
Where Can I Purchase a Thermal Cooker?
Thermal cookers are increasing in popularity and you may be able to find them where you normally shop if you start asking around. Be mindful of the quality. I’m a bargain shopper but when it comes to a thermal cooker you need to purchase a quality device that will work well for many years to come.
Ideally, I would like to have a couple of thermal cookers. One for larger items such a pot of soup or stew and another for a pot of rice. Thermal cooking works best when the pot is filled to 80 percent of capacity.
These are a few thermal cookers that you may want to look at as you search for the right one to fit your needs.
TIGER Non-Electric Thermal Slow Cooker – Tiger makes high-quality thermal cookers in a variety of sizes. They offer a 4.5L / 4.75qts size and a 8.45qts / 8.0L cooker for feeding a crowd.
Thermos Cook and Carry – This is the thermal cooker that I have had for years. I love it and have absolutely no complaints. The technology has improved since I bought mine about 15 years or so ago. It is rated for 22 hours hot and 24 hours cold. It is a good size for our family and holds 4 3/4 quarts.
Thermos Carry Out Shuttle Chef – This is the top of the line from Thermos and has 2 separate 3-liter pots that fit inside together. I would love to add the new Shuttle Chef to my cooking arsenal. Sometimes I just really need the smaller pots.
Saratoga Jacks 5.5L Thermal Cooker Deluxe – One thing that I really like about this thermal cooker is that you can use the larger pot alone or use the smaller pot inside of the large pot to make 2 dishes, such as beans and rice.
Where Can I Purchase a Wonder Oven?
You can purchase the Wonder Oven that you saw us use from Megan Smith at My Food Storage Cookbook She also sells patterns so that you can make your own.
Where Can I Purchase a Wonder Bag?
Hungry Fan makes an insulated thermal bag. You can check the current price here.
Wonderbag sells several versions. Check out their website and learn more about how to use a Wonderbag here. They have great recipes on their site.
Discover the Magic and Convenience of Retained Heat Cooking
Don’t let the unfamiliarity of retained heat cooking discourage you from trying it. Once you get the hang of it, retained heat cooking will become a welcomed part of your everyday cooking. Retained heat or thermal cooking can simplify life and help you to create delicious meals.
Retained heat cooking may be an incredible asset during challenging times. Even today it can help you make delicious foods using less energy. Give it a try!
Thanks for being part of the solution!Jonathan and Kylene Jones