How much fuel does retained-heat cooking save?

Aprovecho Research Center reports savings of about 50 percent of cooking fuel and time on average in tests across 18 stoves, and a range of 20 to 80 percent depending on the food and the amount cooked. A 2014 lab test of an insulated cooking bag measured savings of 36 to 48 percent for rice, potatoes, and vegetables, and 22 to 26 percent for a chicken curry. Long-simmering foods such as dried beans and stews stand to gain the most because most of their normal cooking time is simmering.

Is it safe to cook food in a cooler or haybox?

It can be, if the food goes in at a full rolling boil and stays hot. USDA says hot food should be kept at or above 140°F, and bacteria grow fastest between 40°F and 140°F. Check the food with a thermometer before serving. If it has dropped below 140°F and you cannot be sure it has been there less than two hours, throw it out.

Can I cook dried kidney beans with retained heat?

Only after they have been fully cooked by boiling. The FDA says kidney beans should be soaked for at least five hours, drained, and boiled in fresh water for at least 30 minutes, and warns that slow cookers do not get hot enough to destroy the natural toxin in kidney beans. An insulated cooker never returns to a boil, so do the full 30-minute boil on the stove first.

Can I put a boiling pot in a picnic cooler?

Many households do, but cooler liners are plastic and are not built as cookware. Set the pot on a folded towel or wooden trivet, wrap it, and check what your cooler's maker says about hot contents. Never pour boiling liquid straight into the cooler.

Home Self-Reliance Food Cooking Retained-Heat Cooking

Self-Reliance · Food · Cooking

Retained-Heat Cooking

Bring the pot to a boil, then tuck it into a box of insulation and let it finish on its own. The stove does the hard part for a few minutes. The insulation does the rest for free.

How to do it The two safety rules

By Dan Whelan, founder of New World Survival

Researched from government agencies, university extension services, and other published sources. See our sources

How it works

Stop paying for the simmer

Once a pot reaches a boil, most of the fuel a stove burns afterward goes to replacing heat that leaks out of the pot. A pot of beans that simmers for two hours is mostly paying for that leak. Retained-heat cooking plugs it: bring the food to a full boil, move the covered pot into a well-insulated container, and the heat already in the food keeps cooking it with no more fuel^[3].

The method goes by a lot of names. A haybox was the traditional version, a wooden box packed with hay. People also call it fireless cooking, thermal cooking, or wonder-box cooking, and a wide-mouth thermos is a single-serving version of the same idea. The U.S. Environmental Protection Agency published a design guide for these cookers with HELPS International, and in its tests a good homemade cooker held five liters of boiling water above 80°C (176°F) after three hours^[1].

It suits food cooked slowly in liquid that does not need stirring: rice and other grains, dried beans, soups, stews, and tougher cuts of meat^[11]. It is no help for anything that needs browning, frying, or a crisp crust. Food also comes out a little different. With no steam escaping, you need slightly less water^[12], and nothing scorches on the bottom.

What it saves

Roughly half the fuel, sometimes more

Aprovecho Research Center, a nonprofit that tests cookstoves, found that adding a retained-heat cooker saved about 50 percent of the fuel and time spent cooking across tests with 18 different stoves^[2]. Its range is 20 to 80 percent, depending on the food and how much of it you cook^[3]. That matters most when fuel is limited: a camp stove's propane, the last bag of charcoal, or the battery in a solar generator running an induction plate.

Measured savings by food

One lab test of an insulated cooking bag, comparing ordinary cooking with boiling first and finishing in the bag^[4]. Orange bars are a bottled-gas burner, light bars an electric induction cooktop.

Potatoes

Gas: 48%

Electric: 41%

Mixed vegetables

Gas: 45%

Electric: 45%

Rice

Gas: 36%

Electric: 42%

Chicken curry

Gas: 26%

Electric: 22%

Fuel time versus free time

Cooking times from the EPA design guide^[1]. Orange is time on the stove burning fuel. Light is time in the insulated box burning nothing.

Dried beans · 1 hr 30 min on the stove, then 3 hr in the box

Beef stew · 1 hr 10 min on the stove, then 50 min in the box

Chicken stew · 20 min on the stove, then 35 min in the box

Vegetable stew · 10 min on the stove, then 30 min in the box

Rice · 8 min on the stove, then 25 min in the box

Pasta · 5 min on the stove, then 20 min in the box

Two honest caveats. The savings test covered rice, potatoes, vegetables, and chicken; nobody we found has published a measured fuel number for dried beans, so we show their time split instead of inventing a percentage. And the box times above came from one cooker design in Guatemala. Yours will run faster or slower with your pot, your insulation, and your altitude, which is one more reason the thermometer below is not optional.

Two rules that make this safe

1. It goes in at a full boil and stays hot until you eat it.

USDA says bacteria grow fastest between 40°F and 140°F, doubling in as little as 20 minutes, and that hot food should be kept at or above 140°F. Food should not sit in that range for more than two hours, or one hour above 90°F^[5]. An insulated cooker is slowly losing heat the whole time, so the risk is a pot that drifts down into that range over a long afternoon or an overnight hold. In the lab test above, a chicken curry left four hours in the bag came out at 60.5°C, about 141°F, just one degree above the line^[4].

So start hot and check before you serve. Bring everything to a rolling boil all the way through, and keep large pieces of meat boiling 10 to 15 minutes longer before they go in the box^[1]. Poultry needs to reach 165°F inside, ground meat 160°F^[6]. Before eating, push a food thermometer into the middle of the pot. At 140°F or above, it is ready to serve. Below that, if you cannot be sure it has been there less than two hours, throw it out.

2. Dried kidney beans get a full 30-minute boil before the box.

Raw kidney beans carry a natural toxin that boiling destroys and gentle heat does not. The FDA says to soak them at least five hours, pour off the water, and boil them in fresh water for at least 30 minutes. It warns that slow cookers do not get hot enough, and that several outbreaks have come from beans cooked in slow cookers or casseroles that reached only about 75°C (167°F) or less^[7]. An insulated cooker never gets back to a boil, so treat it like a slow cooker here: do the full 30-minute boil on the stove first, then let the box finish softening them.

Red kidney beans carry the most toxin, white kidney beans (cannellini) about a third as much, and the FDA notes the same toxin turns up in many other bean species at lower levels^[7]. We give every dried bean the same 30-minute boil before it goes in the box. It costs a few minutes of fuel and removes the guesswork.

How to do it

Five steps, one thermometer

Choose the pot

A tight-fitting lid matters most. Pick a pot that fills the insulated space with little air around it^[1]. A fuller pot also holds its heat longer than a nearly empty one^[4].

Boil it through

Not just bubbling at the edges: boiling all the way through. Large pieces boil 10 to 15 minutes more^[1]. Dried beans get their full 30 minutes.

Move fast

Lid on, straight into the box. Fill every gap with towels, blankets, or pillows, and cover the top^[1]. Dead air space is where the heat goes.

Leave it closed

Every peek costs heat. If you do open it, put the pot back on the stove and bring it to a boil again before it goes back in^[1].

Check, then serve

Thermometer in the center of the pot. At or above 140°F, eat^[5]. If it needs more cooking, it goes back to a boil, not back in the box.

If the boil happens on a camp stove, grill, or other fuel-burning cooker during an outage, that part stays outdoors. The CDC warns never to use gas, propane, or charcoal devices inside a home, garage, or carport, or near doors, windows, or vents^[9]. The insulated box can come inside once the pot is off the flame.

Use what you have

Three ways to hold the heat

You do not need to buy anything to try this. In the EPA's tests, a wood basket lined with a wool blanket and a cardboard box lined with foam board both held boiling water between 80 and 85°C (176 to 185°F) after three hours, not far behind a purpose-built unit that stayed above 87°C, though the guide notes neither homemade design holds up long in regular use^[1].

The picnic cooler

The most common household version, and the only one big enough for a full stockpot. A cooler holds heat in as well as it holds cold out.

Cooler liners are plastic, though, and they are not made to be cookware. Set the pot on a folded towel or a wooden trivet, wrap it, and pack towels around it. Check what your cooler's maker says about hot contents^[10]. Never pour boiling liquid straight into the cooler itself.

The box or basket

The classic haybox: any sturdy box or basket with a nest of insulation thick enough to surround the pot on every side, including the top.

Keep the insulation dry. Wet insulation stops working, and the EPA guide warns that soiled insulation becomes a health hazard and should be replaced often^[1]. Washable blankets or a removable cloth liner make that easy.

The thermos

A wide-mouth vacuum food jar handles one portion of oats, rice, or soup. It cannot go on a burner, so preheat it with boiling water, bring the food to a boil in a pan, empty the jar, and fill it^[12].

Small amounts lose heat faster than large ones^[4], so check the temperature before eating, the same as a big pot. Open the lid carefully in case pressure has built up^[12].

If you own an older Thermos Stainless King jar, check it first. In April 2026 the CPSC announced a recall of models SK3000 and SK3020 made before July 2023, whose stoppers can blow out when the jar is opened after food has sat inside for a long time. The model number is on the bottom, and Thermos offers a free replacement stopper^[13].

Pairing it with the sun

A solar cooker by day, the box by evening

A solar cooker works only while the sun is out. An insulated box needs something to heat the pot in the first place. Together they cover each other: the sun brings the pot up to temperature during the day, and the box holds it hot into the evening, with no fuel burned at all.

The catch is that many solar cookers never bring food to a true boil. Box and panel designs often run well under boiling on an ordinary day, while a parabolic dish can get much hotter (our solar cooking guide covers what each type reaches). USDA's food safety specialists have questioned whether solar cookers get food hot enough, fast enough, and advise using a thermometer to confirm safe temperatures^[8].

So the same two rules apply. A pot moves from the solar cooker to the box only once it has reached a full boil, confirmed with a thermometer. Dried beans still get their 30-minute boil on the stove first, because sunlight alone may not get them hot enough to destroy the toxin.

What goes wrong

First-try mistakes

Boxing it before it boils

A pot that is only steaming starts its slow cool-down already close to the danger zone. Wait for a rolling boil, all the way through.

Peeking

Opening the box to see how it is going lets out the heat that was doing the cooking. Set a timer and trust it, then check with the thermometer at the end.

Forgetting it overnight

Overnight beans are a classic use, but a long hold is exactly when food drifts below 140°F. If it reads low in the morning and you cannot say when it got there, it goes out.

Where this fits

Next in the kitchen

Retained heat is one of the cheapest ways to make a fuel supply last during a long outage, which is why it sits alongside solar cooking in the cooking module of our 90-Day Household Preparedness Guide.

Sources

All sources accessed November 8, 2026.