Masonry Stoves

Almost no one enjoys the cold, yet most people in the world live where it is cold for part of the year – even subtropical or Mediterranean climates can get chilly in the winter, and burning deserts can get cold at night. Right now, we keep warm through burning fossil fuels, or from electricity – most of which comes from burning fossil fuels. In the future, however, we can expect billions more people in the world, and far less fossil fuels.

The simplest method of keeping warm, of course, is the oldest one -- fire. Restoring fireplaces to our homes and offices, however, would present a few problems. First, we destroyed most of the world’s forests when we only numbered in the millions, or hundreds of millions, and now there are seven thousand million of us. We could coppice trees (cut them off at the base) or pollard them (cut them at man-height) and let them grow back, and willow grows man-high shoots in a year, but they are suitable only for kindling. Also, fireplaces are spectacularly inefficient; according to author David Lyle, a fireplace and chimney send only 10 percent of its heat to the room, and the other 90 percent goes out into the sky.

There is, however, a little-remembered method that was used in Central and Eastern Europe from the Middle Ages until the beginning of the fossil fuel era – the masonry stove. It relies on a simple concept: it is a hearth surrounded by a thermal mass like cob, brick or tile, which heats up with the fire and slowly releases heat throughout the day. Instead of having a single vertical flue that takes the heat directly into the sky, masonry ovens have a flue that winds around several times before heading outside -- the smoke is typically cold by the time it reaches the outside. All the heat is transferred into the mass, and thence into the room.

Since the initial fire burns fast and hot, it does not generate a great deal of soot to build up inside, and does not need to be cleaned -- although cleaning one would be a much more dificult task, with the internal bends and turns. Fires in masonry ovens do not need to be tended and kept going, as it is not the fire itself that keeps the house warm; thus most oven owners simply set one fire in the morning, and then let the heat radiate through the day. As they release the heat slowly, so they tend to be warm but not hot to the touch – some old Russian ovens were made with spaces on top for people to sleep where it was warm.

Perhaps most importantly, since the ovens need only a brief and quickly-burning fire, they do not require chopped wood for fuel, but can use faster-growing and more common material like straw or the aforementiond willow shoots. The fast-burning fuel would create little soot to build up and block the flue, so their users say they rarely, if ever, require cleaning.

Masonry ovens, like thatched roofs, bale-building and cob, is an old method recently revived when more people began to realize its advantages. Barring any unforeseen complications, millions of people could build sustainable heating systems out of nothing more than clay and stone, and heat themselves with material that is renewable and almost free.

For more information check out David Lyle’s excellent Book of Masonry Stoves, or a recent article on the subject by Low-Tech Magazine. 

Article originally published in 2009. Photo of a German masonry heater, courtesy of Wikicommons.