Thermal Mass Stove Design/Build Workshop: La Caseta de Milmanda

La Caseta de Milmanda is a Catalan community-based eco-project located on some farmland in a beautiful, rural valley outside L'Espluga de Francolí. It provides organic, seasonal produce to local families and businesses, and also holds classes and workshops on sustainable agriculture and natural building techniques.

During 2011 and 2012, I had the pleasure of living and working at La Caseta de Milmanda, managing construction and maintenance works for their organization. And, with the help of the organizers at l'Associació Persei, in February 2012 I taught a workshop at Milmanda on How to Design and Build a Thermal Mass Stove.

During this weekend workshop, we transformed a large, old fireplace in the Milmanda farmhouse into a vastly more efficient thermal mass stove, without altering the existing structure of the fireplace and chimney. To accomplish this, we first built and installed a lightweight stove pipe system inside the old chimney. Then we built a new firebox with a white oven on top, in the old fireplace. This was connected to the stovepipe system and a variety of clay mixes were applied to absorb the heat from the exhaust gases and radiate into the house (rather than send all the heat straight up the chimney, like the old fireplace did).

The workshop was kept small, with 5 participants, because the work site was indoors and cramped. We began with the classroom element of the workshop, introducing the basic principles of the thermal mass stove and the transference of heat. We reviewed the development of these stoves through history, and introduced some well-known models, like the Russian Stove, the German Kachelofen, and the modern masonry heater.

Then we got into the specifics of designing a thermal mass stove. We learned how to properly scale the stove in relation to the space it is being built to heat. We learned how to calculate the size of the space, the insulation value of the walls, and the rate of heat loss during design conditions. The stove must meet the necessary heat rating to ensure it can maintain a comfortable temperature inside, even during the coldest months of the year.

After the classroom element of the workshop, we got down to business. The firebox, white oven and internal heat riser of the stove were built using firebricks and refractory mortar. We avoided using large lintels in the design by instead building a series of arches using a simple wooden form.

We built a stovepipe system that could be installed inside the old chimney, without modifying the existing structure. To do this, we built 2 support columns off to the sides in the old fireplace, using salvaged bricks and a cob mortar. These support columns would help to bear the weight of the new stove components. We then surrounded the entire stove and stovepipe system with various clay-based mixes. We made an insulative mix using expanded clay to slow down the rate of heat loss in the firebox, white oven and heat riser, and a denser mix was used to direct the conduction of heat towards the face of the stove and into the room.

Following the workshop, a group of volunteers and I put the finishing touches on the stove. We installed a stone facade over the stove, to help it blend in with the original stonework of the fireplace and chimney. After this, we installed new metal doors for the firebox and white oven, built for us by a local metalworker.

Below you can see the photos of the finished thermal mass stove. The photos also show that in preparation for the workshop, I had replaced the old wooden mantel shelf (the old one had some fire damage). I had also installed a bit of ductwork in the chimney to take cold air from the downstairs pantry, heat it with the new thermal mass stove, and send it upstairs to heat the bedrooms. I did this outside of the workshop because they were optional extras, and they also required modifying the existing structure – something I had promised during the workshop that we wouldn't be doing.

I would like to send a hearty thank you to everyone that made this workshop possible. My time working with l'Associació Persei and La Caseta de Milmanda was a true pleasure, and we met a really great group of students and volunteers along the way. I hope Milmanda's new thermal mass stove will be cooking Maria's delicious Catalan casseroles for many years to come!

- Ben

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