The Basics of Rocket Mass Heaters

Rocket Mass Heaters

A rocket mass heater (RMH) works by efficiently burning wood fuel at a high temperature through the use of air turbulence. The high heat produced in the insulated burn chamber travels through the stove pipe that is embedded in a thermal mass component. The thermal mass has the capability of storing a large quantity of heat and releasing it slowly by radiation and conduction. The comfort one gets from this form of heat is another of the key aspects of RMH units.

To understand how an RMH works, it’s easiest to walk you through a diagram first.

RMH diagram by Paul Wheaton
Paul Wheaton’s awesome diagram.

Ok, with this awesome diagram we will now be able to understand the basics of the rocket mass heater and rocket stove.
Let’s name the components in our diagram a bit now.
Where the sticks go is called the feed tube.
Where the fire happens is the burn chamber.
Where the heat rises (red spirals) is fittingly called the heat riser.
Where this heat then cycles back down inside the barrel is the heat exchange.
Where the exhaust travels through the thermal mass is the exit/exhaust flue, or I like to call it the pipe-run.

Feed Tube:
This is where your fuel goes, usually small diameter sticks and branches, though split wood is also good. The fuel is fed vertically (the horizontal feeds aren’t looked at in this article) allowing the fuel to burn slowly from the bottom instead of all at once.

Burn Chamber:
This is where the fire happens. It is important that this chamber is well insulated for a couple reasons. First is that this helps with burn efficiency, which is an important aspect of each RMH unit. Second, we want the heat to make it to the thermal mass for storage and slow release.

Heat Riser:
The heat from the fire rushes up the internal riser like a rocket – giving these units their name. The top of the barrel gets VERY hot from this, as does the rest of the barrel. Again, it is important that this be insulated for proper functioning.

Insulated heat riser and open manifold. Barrel goes around this.
Insulated heat riser and open manifold. Barrel goes around this.

Heat Exchange:
The hot gases that have rushed up the riser then get drawn back down the barrel into a manifold before travelling through the pipe run and heating the thermal mass.

Though the manifold is not in the diagram, it is also an important aspect. This is where the main unit connects to the thermal mass and pipe run. It is the transition point.

This is the length of pipe that is embedded in the thermal mass. Depending on the dimension of your pipe, you will want to have a run from 20-30 feet before exiting the building.

Thermal Mass:
The thermal mass for RMH units is typically built of cob (clay, sand, & straw). While the hot gases travel through the pipe run, the thermal mass draws the heat out of the gases, storing it in its bulk. This mass is often in the form of a bench or bed, providing the user a nice warm place to rest.

Wonderful, now we know what all the sections are, and what happens in each section. It is important to mention that the dimensions and proportions are crucial to proper functioning, so do your research before you build!
Here are a few other things to take into consideration.

The pros and cons of rocket stoves:

  • Rocket mass heaters require a lot less wood to create a lot more heat.
  • The high efficiency of their burn provides a great source of heat that is also a lot cleaner than most other wood fired heating devises.
  • Rocket stoves are also fairly inexpensive and really fast to build.
  • If you’re a bit of a pyro, then you will love these.
  • The heat stored in the thermal mass is available for use for quite some time.
  • They can be made into any shape, and as creatively as you can invent.

    RMH from 2014 Workshop
    RMH from 2014 Workshop
  • Though they are relatively easy to build, the dimensions and proportions are crucial to get right.
  • The heat exchange of an RMH should be left accessible for it to be cleaned out once in a while. For some, this can be a fairly messy ordeal. But by “once in a while” I mean maybe once a year or two years.
  • The partly exposed barrel is not an aesthetic for everyone, though it can be covered with cob to beautify it. Covering your barrel with cob likely means that you will rely on your smaller cleanouts to keep it free of debris. I suggest waiting a couple years to enclose the barrel so you can monitor the amount of debris that accumulates in a 2-4 year period and assess if covering it is going to be worth it. I’ve also taken a grinder to my barrels, the effect is very nice.
  • The unit requires some regular attention while the fire is going, making it harder to just load and walk away. To get around this, you may want to consider a batch-box style RMH, which is a little more technical to build.

Where best to use a rocket stove:

Pipe-run in cob tub
Pipe-run in cob tub

I recommend sticking to using an RMH to heat a cob bench or lower wall section. Heating your floor is also another good application for a rocket stove, especially if radiant heat from water is not a viable option for you. Saunas are another successful application of the RMH unit, with a few simple modifications.
I have experimented with putting the pipe-run through the body of a cob Japanese style soaking tub as well.



For further reading and research, Paul Wheaton’s site and Ernie & Erica’s site are two of the best. If you really want to dive into these units with the best of the best, you may be interested in the online forum dedicated fully to rocket mass heaters and their variations.

If you are interested in taking one of our workshops on RMH units, check out our workshop listings for upcoming opportunities.

You can also buy the book written by Ianto Evans!

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