Masonry Ovens

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 get chilly in the winter, and deserts get very cold at night. We can keep warm by huddling together or wearing heavier clothes, but sooner or later we have to start burning something.

We currently get much of our heat from fossil fuels – coal, oil, gas – or from electricity that comes from burning fossil fuels. In a few decades we will see many more people in the world and far less fossil fuels, and we will have to warm ourselves in some other way. We could turn to building wind farms, solar arrays or nuclear plants, turning their power into electricity and then into heat, but that would be a long and complex process. It would be much simpler and cost-effective for people to use the oldest method of heating, fire.

Using fire, though, presents a few problems. For one thing, we destroyed most of the world’s forests when we only numbered in the millions, or hundreds of millions. Now there are seven thousand million of us in a world with a fraction of the forest we used to have, and what remains – the great rainforests of the world, for example – are needed as the home of much of the life on Earth.

We could coppice trees (cut them off at the base) or pollard them (cut them at man-height) and let them grow back. It is an old, and still valid, method of preserving forests, but trees like hazel still take a decade or more to return.

Also, traditional fireplaces were spectacularly inefficient: A fireplace and chimney send only 10 percent of its heat to the room, and the other 90 percent goes out into the sky. Old buildings in Ireland will have the fireplaces stuffed with newspaper the whole way up, and there is still a draught.

There is, however, a little-remembered method that was used in Central and Eastern Europe until the beginning of the fossil fuel era – the masonry oven, also called a Russian stove or tile 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 smoke and heat rise inside insulated ducts which do not conduct heat quickly, interior temperatures rise very high, and makers of masonry stoves claim their products are 85-90 percent efficient.

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 but the thermal mass – 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 where children or elderly could sleep.

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 sticks. The fast-burning straw creates little soot to build up and block the flue, so their users say they require little cleaning.

Masonry ovens, like thatched roofs, bale-building and cob, is an old method recently revived when more people began to realize its advantages. If it takes off, 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 an article on the subject by Low-Tech Magazine.

Photo by Wikicommons.