Retained Heat Cooking: The Secret to Stretching Your Fuel Supplies

The power is out and you have no idea how long until it will be restored. How will you cook your food in an emergency with limited fuel? Retained heat cooking can help you stretch your stored fuels to outlast the crisis.

All cooking methods have one thing in common: fuel is required. If you run out of fuel, you no longer have a executable plan. Retained heat cooking, also known as thermal cooking or haybox cooking, is immensely valuable to an emergency cooking plan. By maintaining cooking temperatures (via insulation) to slowly cook food, the amount of fuel required is drastically reduced.

The success of your overall emergency cooking plan is ensured, because it no longer hinges on large amounts of fuel being stored. Retained heat cooking increases fuel efficiency by extending cooking temperatures for 8-10 hours. Surely, this is a method you need to learn more about!

Let’s explore the following points of interest as we study retained heat cooking together:

  1. Historical Evidence: It Works!
  2. The Importance of Fuel Conservation
  3. Modern Approaches
  4. Best Practices
  5. Application Ideas
  6. Getting Started

Historical Evidence: It Works!

Throughout history, people have cooked using pits in the ground, hay and even blankets; each an example of cooking by trapping heat which is precisely the idea of retained heat cooking. As early as Medieval times, instructions have been found to explain the process of placing a hot pot with food inside into another larger pot, box or hole in the ground and insulate it with moss, dry leaves, hay or other materials to extend heat times and save firewood.

The earliest known specific device invented for this purpose was by an inventor named Karl Von Drais who designed a wood-saving cooker he called a “hay chest” in the first part of the 19th century.

World War I brought special challenges and fuel shortages to the point that in cities such as Vienna and Frankfurt, hayboxes became part of a kitchen’s standard equipment as new homes were built.

If you weren’t so lucky as to live in a house with a built-in haybox, they were also made available and sold as portable units. Hayboxes made it possible to get by on fuel rations of the time. Hayboxes once again became popular in World War II when saving fuel was very important.

The Importance of Fuel Conservation

We can definitely do all within our power to store fuel (and we should). The problem, however, is even if we do our best, it wouldn’t be wise to put all our eggs in one basket. All options (excluding solar cooking) require fuel to cook and even with solar, a back-up plan is needed since time of day, seasons and weather all affect solar cooking.

So, if all options (excluding solar cooking) require fuel to cook, we are left to meticulously calculate how much fuel is needed for the food we have stored. Are you up for the challenge of calculating the exact amount of fuel you’ll need for every meal you plan to make? Added to that, variables, as unpredictable as wind, affect the amount of fuel needed. We simple can’t expect to design a perfect plan.

In my own explorations on the dilemma of fuel storage, I’ve crunched the numbers to determine how much fuel would be needed to cook the food in our own family’s long-term meal plan (minus the gift of retained heat cooking) and found those amounts to be astronomical and completely out of reach as I consider keeping stocks of fuel current and ready to use.

My conclusion has become that we have no other choice but to lessen our dependence on fuel stores. We lessen our dependence by extending the life of the fuel we do have stored. Experience has taught me that retained heat cooking is the best option we have.

We can’t risk running out of fuel. This, in fact, would completely unravel the plans we have to feed our families. Retained heat cooking gives greater value to the fuel you already have stored and allows for a margin of forgivable error. Without this leeway, there’s potential you could end up with all the supplies in the world to cook, but would be unable to do so.

Modern Approaches

Moving to modern day applications of retained heat cooking, everyone knows a Thermos holds heat keeping ‘hot things hot’, ‘warm things warm’ or ‘cold things cold’. It works by insulating whatever substance is put into it and maintaining that temperature. Wouldn’t it be great if we could translate that same idea to a larger scale, something big enough to cook a family meal?

There are two mechanisms which are used today: vacuum and foam.


Vacuum insulation technology (a Thermos is a good example, but think of it on a larger scale) is available in a variety of containers. The size of container you purchase determines the amount you can cook. Vacuum technology is the mechanism by which insulating occurs.


Foam based retained heat units present themselves as bean bag pillows or different variations of bags stuffed with foam. The foam comes in shredded foam or small foam beads. These foam cookers work as the container of food nests into the bag of foam, and with that pressure the foam is pressed together to become a barrier surrounding the container and insulating against temperature change.

The pictured examples provide a better idea of these two types of retained heat cookers.

As with any tool, there are advantages and disadvantages to both styles of cookers. Generally, vacuum (container) style cookers are a bit easier to tote around. They also come equipped with the containers they require. Foam units, on the other hand, are more flexible in accommodating different sized pots for cooking.

The style I find most useful, the Wonder Oven, comes with two bean bag type pillows and is contained in an 18-20-gallon plastic tote. I even have the flexibility of using skillets I already own as containers for casseroles and layered meals.

As for price, vacuum units are initially more expensive ($150-$200 range including shipping costs), however, this accounts for the fact that the cooking container is included. Foam units are initially less expensive ($50-$100 range including shipping), but you’re paying for the insulation method only. Most of the time, your current cooking pots and containers can easily be re-purposed for use, but depending on your goals, you may need to purchase additional containers.

Best Practices

As with any style of cooking, retained heat cooking requires practice to become comfortable doing. Advice from a veteran can be helpful. The easiest way to think of a retained heat cooker is as a powerless Crock Pot that requires you to add food at a cooking temperature (boiling is recommended) to work.

In my practice with retained heat cooking, I’ve almost exclusively worked with foam units because I prefer the freedom it allows me to use my own cookware. There are many experts in vacuum units, but I’m sharing my own experience so you can keep it in mind as we proceed to discuss some best practices. These are the important pieces you want to remember when cooking with foam retained heat units.

First, it’s important to realize the power you hold when you achieve a boiling temperature. The Wonder Oven style cookers I use lose heat at a rate of 6 degrees per hour. That’s a pretty standard amount of heat loss for foam units.

Vacuum units lose heat at a rate of between 8-10 degrees per hour (as taken from temperature tests done by Cindy Miller of Thermal With that in mind, you may have 8-10½ hours of “safe” cooking time if you begin at a boiling temperature and insulate well.

The truth is, you really have all the flexibility of slow cooking with a Crock Pot (but without the electric bills) when you cook using retained heat!

Let’s walk through some tips for optimal results.

  • First off, large amounts of food retain heat better than small amounts. That makes sense, right? The more food you have (brought to a hot temperature), the more heat there is to continue keeping your overall container hot.
  • Second, pots filled to a greater capacity retain heat better than those only partially filled. Air space is your enemy when it comes to heat retention. It’s much better to have your pot filled with hot food rather than air space (which doesn’t contribute anything to keeping the pot hot and allows heat to dissipate)/.
  • Third, the cooking pot itself is important to keep in mind. Two notable mentions here: thin metal is preferred to thick metal and a tight-fitting lid is important. There’s generally no need to go out specifically shopping for a “thin-metaled pot”. Usually, what you’re already using will be just fine. The point here is to avoid using any especially thick metaled or ceramic pots. They require an increased amount of fuel to heat and there’s no reason to do that.
  • Next, it’s important to heat the lid of the pot to be extremely hot just as the pot itself. I like to visualize to those I teach that the pot is becoming its own oven when you’re preparing to cook with retained heat. The temperature should be the same all throughout; container and lid.
  • Lastly, when using a Wonder Oven, it is a good idea to just check carefully for escaping heat as you position the bean bag pillows. Re-position pillows as necessary to insulate well.

Application ideas

There are many benefits of effectively insulating against temperature change. I’ll share with you just a few of the delicious recipes I’ve learned to cook in my Wonder Oven. I consistently achieve wonderful results and have included many of these recipes in our family’s emergency meal plan. For step-by-step explanations of how to cook using this technique, visit me here at My Food Storage Cookbook.

I’ve made the following and more…

  • Bread
  • Muffins, brownies and cakes
  • Skillet casseroles
  • Skillet Frittata
  • Slow cooked meats
  • Yogurt
  • Tamales
  • Overnight oatmeal

Another application, beyond cooking, is being able to keep hot water hot even after using it to cook.

In some of the above recipes mentioned, boiling water is placed within a larger pot and the food being cooked is put inside a smaller pot. This smaller pot is placed inside the larger one with boiling water surrounding it to cook. (Please refer to the illustration for a better explanation.)

After the initial cook time is finished, you are left with a significant amount of very hot water. This water can continue to be kept hot by putting it again inside the Wonder Oven. By doing this, you’re essentially piggybacking on the hot water you have already heated and eliminating the need to reheat water other needs. It’s a double savings!

Beyond insulating hot temperatures, my Wonder Oven is useful to me in that it’s able to keep cold things cold. In a power outage, I could leave something that needs to be chilled out overnight (medications for example) and then hold that cool temperature steady throughout the daytime hours using this tool.

Another idea to this effect (which we especially enjoy as we prepare to bring lunch along to sporting events or picnics) is wrapping refrigerated ingredients up in one of the pillows to hold the cool temperature of meats and cheese until it’s time for lunch! I secure the pillow using pins or bungee cords so it stays wrapped.

Likewise, I use it to keep items cool when bringing ice cream and other cold items home from the store during the summer. I keep a Wonder Oven in the back of my vehicle and the problem is solved! No need to rush home so the ice cream won’t melt. I can take my time and our food stays nice and cold.

Getting Started

It’s easy to get started cooking with retained heat and enjoying the other benefits of these tools! Most often, as people ask me where to begin, I suggest to start by just discovering for themselves that it works!

Boil a full pot of water, put it into the cooker of your choice and see it work for yourself. There’s no need to start big, just start! Once you catch the bug and have a little experience, you’ll be motivated to try more.

A good second recipe to try would be Overnight Oatmeal, or a similar type of recipe. This recipe simply requires bringing all ingredients to a boil and then setting it to cook overnight. The next morning you wake up to a delicious steel cut oatmeal breakfast, with no additional preparation required. It’s ready when you are!


Well, there you have it. Retained heat cooking is an indispensable skill you need in your emergency plans. It saves fuel by allowing the flexibility to cook many things which otherwise would be too expensive and fuel prohibitive to make.

You are able to rest a little easier knowing you can extend the supply of the fuel you have stored because the fuel you have will work more effectively and last longer. It simply makes sense on many different levels and besides that… it’s fun!

Guest Post by Megan Smith – Visit her blog at My Food Storage Cookbook to learn more about retained heat cooking and delicious recipes!

Photos included in this post are used by written permission and are copyrighted by or belong to Megan Smith.


Kylene Jones is a blogger, content creator, published author, motivational speaker, homesteader, prepper, mother, and grandmother. She practices self-reliance, provident living, and emergency preparedness in her everyday life. She loves working with her husband, Jonathan, and is committed to helping our community be prepared to thrive during the challenges that lie in our future.

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