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 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.

We could turn to nuclear power, of course, but it takes years to build the plants, aside from any other problems. Solar, wind and tidal power do not supply the concentrated power that oil, gas, and coal do – that’s the reason they haven’t been cost-effective before now.

If you want to tell me that someone will invent something – well, I have no doubt someone will. But will it be a something that will solve all our problems, be affordable, be implementable around the world in a short time, and not have any side effects worse than the original problem? Because that would be a first.

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. It is an old, and still valid, method of preserving forests, but trees like hazel still take a decade or more to return.

Compared to that, the second problem with fireplaces seems minor: they are spectacularly inefficient. Old buildings in Ireland will have the fireplaces stuffed with newspaper the whole way up, and there is still a draught. 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.

“Shelter magazines illustrate the scene often, in a spirit of nostalgia,” Lyle notes. “The open fire, the colonial family gathered around, the pot hung from a crane over the coals. Undeniably it’s an attractive scene. But by the tens of thousands Americans bricked up their handsome fireplaces as iron stoves became readily available. The fireplace used vast amounts of wood. It gave too little heat. It must also have been a trial for the woman who had to use it for cooking.

With the affluence of the oil age we have uncovered the old fireplaces in colonial homes. But today they are objects of aesthetic delight, not work spaces or devices for serious heating. And some of them, once again, are being bricked up in favor of stoves.”

Beyond that, there is the threat of fire spreading outside the fireplace. Believe it or not, kitchen fires were the leading cause of death for women in 18th century Britain, and presumably one of the leading causes of death for women in most places in most eras. We forget how flammable most cities were until recently, and how easily and frequently fires swept through cities until nothing remained.

Iron stoves were superior to fireplaces, but metal heats up quickly and loses heat quickly, so the fire must be frequently restarted or restocked if it is to keep heating the room. And, as their surface becomes very hot, they create a risk of catching the house on fire.

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 smoke and heat rise inside insulated ducts which do not conduct heat quickly, interior temperatures rise very high and hydrocarbon gases ignite as they do in a catalytic converter. 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 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 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 a recent article on the subject by Low-Tech Magazine.