Build your own rocket mass heater

When we built our wood-frame classroom last year, one concern was how best to heat it. We are completely reliant on firewood here, and a regular wood stove was going to make a big dent in our precious woodpile. Then I saw a rocket mass heater in Permaculture magazine, and it seemed like the perfect solution. For those who haven’t come across one before, the rocket mass heater (RMH) is a well-proven, though not widely used, way of burning wood very efficiently, and then capturing all the heat produced, into a mass, typically a bench or bed passing the flue horizontally through it.

First, it burns small fuel, which we don’t normally use in our range kitchen. Second, it burns very efficiently and therefore cleanly, so less fuel is required. We were a little skeptical about the claims made for the RMHs, especially the cleanliness of the exhaust and the temperatures that could be achieved inside the burner, but we were intrigued and decided to try building one. Something that requires a 20 foot horizontal flue, a heat retention bank and a 50 gallon steel drum needs to be designed as part of a building, and so it was here, the base of the heater was built at the same time as the building foundation. itself.

Although the basic design of the RMH is relatively simple, the devil is in the details, and after reading as much as possible on the subject, we obtained a copy of Ianto Evans’ excellent book. Rocket mass heaters, and followed the dimensions as closely as possible; some of the ratios within an RMH must be within fairly small tolerances to function effectively. It was mainly these problems that the builders seemed to be suffering from because they had changed some element of the design. Some of the materials (brick, clay, and a steel barrel) were fairly easy to come by for free; the 8inc spiral duct, firebrick and insulated chimney had to be purchased which brought the total cost to approx £200. We normally have great success finding materials for nothing but the firebrick and the large quantity of Fumes needed for the dough eluded us this time.

Initially, all the parts were laid out to get an idea of scale, and then the foundation for the fire was built. We used a layer of glass bottles as an experimental insulation layer, with sand and then paving slabs, perfectly leveled.


Once the base is prepared, we put in the flue: we used an 8-inch coiled duct on the bench, which has an inspection cap every time it changes direction to allow for easier cleaning later. So (above) on the back, you can see two openings: the one on the left will be a cleanout with a cap, the next one is where the vertical triple wall flue exits and exits through the roof. The middle one is where the burner exhaust enters the flue, and the one on the right is yet another cleanout.



Here we start designing the firebox: the refractory brick could be replaced with old red brick and will be bonded together with a mortar made of pure clay and very fine sand in a 1:1 ratio. Having tried (and failed) to clean the enough clay soil to make a soft clay mortar, I just went out and bought some clean clay, which was thirty pounds well spent.


The refractory brick is built to form the opening through which the fuel will be fed; the firebox and the base of what will become a vertical chimney inside the outer steel drum.


The ducts are now securely attached and sealed, and here you can see the burner built up to the top of the fuel feed (right) and the bottom of the riser (left). Building with fire bricks and very soft clay mortar means that the joints are very small, about 1/16th of an inch to keep as much heat as possible and the fire super efficient. The next stage is to build a masonry around the base of the refractory brick to take a metal cage, inside which we can put vermiculite insulation. The goal is to insulate the entire firebox as much as possible to contain the heat and increase the efficiency of the fire.



Here is the rather sooty inner drum built around the brick chimney, the outer drum has been removed for inspection and adjustment.


A continuación, se coloca el tambor de acero exterior, dejando un espacio de aproximadamente 2 pulgadas entre él y la chimenea interior, a los lados y en la parte superior. Aquí hemos sellado los huecos alrededor de la base del tambor y comenzamos a machacar en el banco. Antes de agregar una carga de mazorcas al banco, selle temporalmente alrededor del tambor y encienda un fuego para probar el tiro en el fuego.


The clay mix is a bit hit and miss, but it’s about 1 part clay to 4 parts sand. For the areas around the burner, we have limited ourselves to using clean clay purchased from a ceramic supplier. For everything else, it will be the locally dug clay, about 1 grit to 3-4 clay. Apparently there is no straw in the bits immediately adjacent to the pipe, although it’s a bit late for that at one or two points.


We mixed and deposited a couple of tons of cob in the bank, and it seemed that something had gone terribly wrong, as the excellent current we had seen seemed to have faded, but persistence paid off, and when the cob dried up, and the fire put less effort into driving out moisture and more into heating the dough, things got better and we’re happy (and relieved) to report that everything now works.

Última mazorca en el calentador de masa cohete

Last cob in the rocket mass heater

It needs to be used little and often to keep the heat in the dough, but once loaded, it keeps a large space comfortably warm, with people and pets fighting for a spot on the warm bench. Now we’re getting to know our heater and its little weaknesses: turning it on from the cold or on a calm day can be a challenge, and until it warms up, it occasionally coughs again in the room, but perseveres a bit and it goes away. very well. It’s definitely one of those things that visitors love, although the 50-gallon drum isn’t for everyone, and we’ve had plenty of suggestions to ‘improve’ its appearance. Other than that, it’s hugely successful and incredibly successful.


Having collected all of this and dropped it on top of the insulated flue, it was time for crunch time. Rocket mass heaters are not known for their easy lighting, as until they are heated, there is a tendency for smoke to blow back the fuel feed. Aside from some initial smoke, it came out great, especially considering you’re trying to dry out a load of clay mortar on your way. The keg got nice and hot, while the insulated flue stayed cool, and the exhaust was actually quite a bit of smoke and mostly hot steam, just as I’ve seen in countless videos. I don’t know why, but I’m still amazed that it actually works.

In this short video you can hear the distinctive ‘rocket’ sound that gives these stoves their name.

This article originally appeared on

Steve is co-owner of Lackan Cottage Farm, a small permaculture property in County Down, Ireland, where he and his family live as lightly as possible on the land and offer the opportunity for visitors from around the world to volunteer. and learn. Visit for more information.

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