Rocket Mass Heater Design/Build Workshop: Restaurant Selmellà

In the country of Catalunya, in the mountains of Alt Camp, located just outside the village of Pont d'Armentera is a family-owned restaurant called Restaurant Selmellà. From the ancient mountaintop estate of Cal Figueres where the restaurant is located, guests enjoy dazzling views of the rugged mountain landscape, the ruins of Castle Selmellà, and the Tarragona Valley far below. On a clear day, they can even see the Mediterranean shimmering far off on the horizon.

Restaurant Selmellà offers home-cooked meals made from local, organic, seasonal produce. And, with the help of the environmentalist organization l'Associació Persei, they also now run an education center that offers seminars and workshops related to sustainable living, organic gardening and natural building techniques.

In November 2011, I had the privilege of teaching the first hands-on workshop held at Restaurant Selmellà: How to Design and Build a Rocket Mass Heater.

The rocket mass heater is a super-efficient, clean-burning, wood-fired masonry heater. It can be designed to heat a home, make hot water, and/or for cooking. Its simple design means no professional stove-builders are needed - homeowners can design and build one on their own. The flexibility of its design means it can be built from a variety of materials, including locally sourced clay and straw, and salvaged materials like old bricks and barrels. This means it can be built at very low cost and with minimal environmental impact.

It was a full house, with 12 workshop participants from across Europe: Catalunya, Spain, and some from as far as Scotland and Holland. We started the workshop in the classroom, with an introduction to the basic principles of combustion, heat transference, and the development of the rocket mass heater since its conception in the 1970's. We outlined the varying components of the stove, the relationships between these components, and how to scale them in relation to each other to form a functional stove.

After the classroom element of the workshop we got to work building a rocket mass heater, in the bar and lounge of the restaurant. The core of the stove was built using firebricks and refractory mortar, and it featured a window of high-temperature glass peeking into the combustion chamber - a rare detail in these stoves that interested many of the participants!
 

 
We built a high-mass bench to radiate the heat from the exhaust gases into the room, using old salvaged bricks, a cob mortar (made from clay-rich Catalan soil, sand and straw), and old salvaged sections of stovepipe. The bench was backfilled with a mix of clay, sand and high-mass aggregate (we used stones and bits of broken bricks). Behind and underneath the stove and bench we insulated with a mix using expanded clay, an ecological mineral insulation that can withstand high temperatures.

It was a big project, but everyone was hard-working and eager to learn. Restaurant Selmellà provided home-cooked meals and lodging for everyone during their 3-night stay. After the weekend workshop was done, I stayed on a few days more. With the help of a few volunteers, we put the finishing touches on the stove, gave it a nice gypsum plaster render, and left Restaurant Selmellà with a beautiful new rocket mass heater.

Scroll down for more details of the building process!

The high-mass bench contains 2 stovepipes of 12cm in diameter and a third stovepipe of 10cm. In total, this produces a cross sectional area of 304cm². In the photos above, we are building the bench using old salvaged bricks and a cob mortar. The bench is then backfilled with a mix of clay, sand and a high-mass aggregate of stones and pieces of bricks. This way the mix closest to the exhaust pipes has a very high density, which enables it to quickly absorb the heat of the exhaust gases. Later we covered this high-mass mix with a layer of cob.

The core of the stove was built with firebricks and refractory mortar. The combustion chamber and the internal chimney (the "heat riser") both have a cross sectional area of 297.5cm² (17 x 17.5cm). Factoring a window of high-temperature glass into the design was a pretty complicated step - the firebrick layout of the combustion chamber had to be asymmetrical, and buttresses were built around the core to make sure everything was stable.

Next the core of the stove was covered with different mixes: the heat riser was surrounded with a cylinder of hardware cloth and filled with a mix of clay, sand and expanded clay, an ecological mineral insulation. This mix was also used to insulate around the combustion chamber.

Using old salvaged bricks, cement blocks, and a clay-based mortar, we built a platform to support the barrel with grooves to channel the exhaust gases into a manifold, which hooked up to the pipes running through the bench. Next, cob was used to strengthen the design and to give the stove a soft, rounded shape.

Lastly, we gave the rocket mass heater and bench a final render of gypsum plaster for a durable, clean, striking finish.

Here you can view photos of the finished rocket mass heater. You can see the top of a smaller barrel, which we converted for use as the stove door. Next to the stove is a built-in cubby for drying out firewood. We also put a tile mosaic on the stove, inspired by the restaurant's logo - Thanks to Mira for this fantastic detail!

 

The stove was built to complement the old fireplace in the room, so we built into the bench a place for storing firewood for use in the fireplace. The exhaust pipes from the rocket mass heater share the old chimney with the fireplace. After the workshop, we also installed a damper in the chimney that the rocket mass heater can bypass, so that the old fireplace can be shut down to conserve the heat in the room while the rocket mass heater is still burning.

I want to say a big thank you to all the excellent people who participated in this workshop, and to all the volunteers who helped us to finish the project. We met some really interesting people, and we all learned quite a lot! I also want to thank the organizers at Restaurant Selmellà and l'Associació Persei for making this workshop possible. I hope we will all meet again this winter - possibly warming up next to the beautiful new rocket mass heater at Restaurant Selmellà!

- Ben

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