MRE Heaters: How Do They Work and How Do You Use Them?

Pretty much everyone who has ever served in the military and a great many preppers will be entirely familiar with MREs, or meals-ready-to-eat, those typically nutritious, sometimes delicious and often gut wrecking pre-cooked field rations.

Modern MRE’s contain many components, and one component that is often discarded is the FRH, or flameless ration heater.

These ingenious little packets are capable of producing a piping hot or at least reasonably warm entree and just a few minutes with absolutely no external source of heat. 

How does an MRE heater work, and how does one use it?

MRE heaters function by way of an exothermic reaction, the reaction of water or air with chemicals inside the heater that produces a significant amount of heat in. The foil pouch containing a food item is kept next to the heater which is subsequently warmed to a pleasing temperature.

How exactly this process is conducted is dependent upon the specific heater.

Most folks that I know who eat MRE’s on the regular don’t even bother with the included heater, and that is a shame since they are surprisingly effective, highly convenient and generally safe in use.

If you’ve never bothered to read through the instructions printed on the package we are here to tell you everything you need to know about MRE heaters.

What Kinds of MRE Heaters are There?

If you’re toting the newest generation of United States military-issue MREs introduced after the halfway point of the 2010s you’ll be enjoying the very latest in flameless ration heater technology.

These ration heaters require no water and simply stick on to the side of the individual meal pouch to thoroughly, quickly and evenly heat the entree with only air needed as the necessary additive to start the reaction. 

However, if your MRE’s are government issue any time prior to that point going back to the mid 1990’s, or one of the dizzying varieties of commercially available MREs on the market, your flameless ration heater will be the older variety that requires water to start the reaction, with the entrée being placed inside the heater pouch itself before being left to heat.

Both are effective enough and both are easy to use, though the newest generation employs technology that makes them even easier, safer and faster.

Using an MRE Heater Step by Step

Both styles of flameless ration heater are easy enough to use.

The prime difference between the two is that the older style that is still the most common variety to be found requires a little bit of water for activation. 

We have provided step by step instructions for each type below.

Testing How The Army Creates HEAT With WATER

Water-Reactive FRH (Older Type)

  1. First, open the entree box and remove the entree pouch. Keep the box intact as you will need it.
  1. Second, open the flap on FRH and add clear drinking water to fill the line. Be careful not to overfill.
  1. Next, insert the entree pouch into the FRH sleeve before placing the FRH sleeve back inside the entree box.
  1. Prop up the entree box at a slight angle so that water does not spill out of FRH. Use non-flammable, heat-insensitive objects for this purpose, such as a rock.
  1. Heating will take approximately 10 minutes. Take care to avoid steam emitted from FRH at this time and handle heated entree pouches with caution.
How air-activated MRE heaters work

Air-Reactive FRH (Newer Type)

  1. Locate both FRH panels inside MRE. Determine which side of the FRH panel contains adhesive. Set aside.
  1. Next, remove the MRE entree from the box if applicable. Discard the box, as you will not need it for the remainder of the operation.
  1. Being careful to remove only the tape covering the adhesive, stick one FRH panel to one side of the MRE pouch before sticking the other FRH panel to the opposite side of the pouch.
  1. Once both FRH panels are firmly adhered to, MRE pouch peel open exposed side of FRH panel to expose felt liner covering reactive elements. Heater will begin to warm rapidly.
  1. Thorough heating will take approximately 5 minutes. Take care, as FRH and pouch may become quite hot.

That’s all there is to it, pretty easy huh?

In most conditions either of these heaters are capable of producing a piping hot or at least warm MRE entree with a very little work required, and this can be a great morale booster when a hot meal is called for.

How To Use the MRE Heater

How Effective Are They?

Despite the griping and complaining you might hear about them from people in military service, both types of ration heater are generally effective, and an excellent convenience option when a fire is not practical or safe.

The older style heaters in particular suffer from being a little erratic, and especially when damaged or when quality control at the factory was having an off day they might not get hot enough to thoroughly heat an entree pouch all the way through no matter how much you stir the contents when it is done.

Most of the time, though, so long as the instructions or followed carefully they could be relied upon although you did have to give up a little bit of your drinking water in the bargain in order to operate them.

The new style functions much better with less setup and less irregularity, making them the de facto choice if you have access to them.

These evenly heat the entree pouch from both sides, ensuring that your meal is piping hot and wholesome when the heating process is finished.

Additionally not having to add any water means you’ll save resources and not have to worry over precise measuring when all you want to do is get off your feet and have some warm dinner.

Bottom line, both work, and both are totally viable for preppers.

MRE Flameless Ration Heater - Longterm test - BP Series

Are MRE Heaters Safe?

Both varieties of FRH are safe but, naturally, there are drawbacks attendant with both, particularly the older generation that requires water.

The old, water-based MRE heaters used a magnesium-iron powder among other elements that react energetically with water to produce heat, thereby heating your ration pouch.

While undoubtedly effective there are some concerns with this approach, namely the generation of hot steam (which could burn you) and far scarier the generation of hydrogen gas which can, in fact, build up in an enclosed space and produce an explosive atmosphere hazard. Yikes.

That is why these older MREs were not allowed on submarines, aircraft and in certain sensitive facilities.

If you are using these older style MRE heaters make sure you allow for appropriate ventilation, especially in confined spaces, and keep the operating heater away from open flames.

The newer style that requires only air to produce heat is far safer, not the least of which because it does not produce any explosive gasses that could build up.

Additionally, even though they might seem like a greater burn risk because they are only stuck to the exterior of the MRE pouch which is then not placed inside any box or other container the efficiency of these newer generation heaters, means that the vast majority of the heat generated goes directly into the MRE pouch and the heater itself can be handled cautiously.

You should still be careful when handling your newly heated MRE pouch, though, because it could be hot enough to burn exposed skin.


MRE heaters, new and old, rely on a simple chemical reaction in order to generate heat and thereby warm an entree pouch from your MRE.

One version relies on water to start the reaction while the newest, best version relies on air alone.

With a little bit of common sense you’ll find that both are entirely safe, convenient and capable of producing a warm meal when you don’t have the time or the opportunity to build a fire.

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