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Low-tech Solutions

Sunbathing in the living room: oven stoves and heat walls

Oven stoves are greener, more efficient, healthier, safer and cosier than all modern heating systems. Why are they gone and how do we get them back?

An oven stove is a very efficient and robust oven that radiates heat all day. In the US it was introduced only 20 years …

Oven stoves are greener, more efficient, healthier, safer and cosier than all modern heating systems. Why are they gone and how do we get them back?

An oven stove is a very efficient and robust oven that radiates heat all day. In the US it was introduced only 20 years ago, but in Europe the technology is almost one thousand years old. Especially in Russia, Scandinavia and Central Europe the oven stove has a long and rich tradition.

In the 18th century, several European governments financed research to improve the technology, as a way to overcome an acute shortage of firewood: ecotech before the term existed. However, its further development and distribution was thwarted by the arrival of coal, gas and oil. Oven stoves are large, heavy and slow, but they offer so many advantages that they – again - deserve to be subsidized by the government.

Most people think that the metal stove was the successor of the campfire and the fireplace, and if you only look at the US, that is true. In the New World, there was never a shortage of firewood and therefore no incentive to improve the inefficient fireplace. But in Europe and Northern-Asia, there was an important and succesful link between the fireplace and the metal stove.

It is known as the Russian, Austrian, Finnish, Swedish or German stove, or as “kakelugn” (in Sweden), “pechka” (in Russia), “kachelöfen” or “steinöfen” (in Germany and Austria), and as “tulikivi” (in Finland).

More general terms are tile oven, brick oven, ceramic stove, tiled stove, soapstone heater or masonry heater. The technology is more closely related to a traditional oven than to a metal stove – therefore the German term “kachelöfen” (oven stove) describes it best as an umbrella term.

Oven stoves already appear on drawings and paintings in the 1300s (see illustrations above and below). They are in fact the first real heating appliances in history. Earlier, the Romans invented the hypocaust, a forerunner of central heating, but that knowledge was largely lost when their civilization collapsed.

Oven stoves are traditionally fuelled on wood, but today they can also be equipped to work on gas, or alternately on both fuels. They can even cooperate with a central heating system. An oven stove can take any form or dimensions: it can be almost invisible, built into a wall or underneath the stairs, or it can be an impressive work of art standing in the middle of the living room.

Stone versus metal

The most essential feature of an oven stove is that it is made out of some kind of stone or brick, while all our modern heating appliances are made of steel. Metal heats up fast, but it also cools down just as quickly. Therefore, a metal heating appliance has to be fuelled almost continuously.

Stone requires more time to heat up, but once it has, it holds the heat much longer. An oven stove is only fired for a short time, from a quarter of an hour to one or two hours and only once or twice per day. An average oven stove then radiates heat for at least 12 hours.

Smoke channels

main part of the heating appliance consists of a labyrinth of smoke channels and smoke rooms. Their aim is to hold the warm gasses inside the oven as long as possible, so that the stone can absorb the heat before it leaves the chimney.

The energetic output of an oven stove is 80 to 90 percent, compared to 40 to 50 percent for metal stoves or central heating appliances, and only 10 to 15 percent for a fireplace – where most heat escapes via the chimney. One of the most striking features of a (wood fuelled) oven stove is the stokehold, which looks ridiculously small compared to the stove itself.

Thanks to the high output, a modest masonry heater or tile stove (heating a room of 60 square meters) only needs 6 cubic meters of wood per year: one tree. If you have even a small garden, you can easily fuel your oven stove by means of your own cuttings – thin wood is very well suited for tile stoves, although it needs to be dry enough.

Radiant heat

All our contemporary heating appliances warm a house or a room mainly by means of convection: they heat up the air. An oven stove does it by means of radiant heat: infrared radiation, comparable to the heat of the sun. In a room that is heated by an oven stove, a thermometer can hardly measure anything.

The effect is comparable to that of a skier who enjoys a schnapps while sunbathing, in spite of the freezing temperatures. Radiant heat does not (only) warm up the air, but particularly also the body of the skier directly.

An oven stove acts in the same way as the sun: it does not so much heat the air, but the floor, the walls, the furniture and the people in the room. These objects in their turn radiate that absorbed heat to their environment – similar to a city radiating heat after a long hot summer day, when the walls and the pavement slowly release back the heat from the sun.

This might sound a bit weird for 21st century computer wizards, but until 150 years ago heating was by definition radiant heating. Heating by convection is a very recent invention, and it has more drawbacks than advantages.

Hot air balloon

Convection causes constant air movement in the house, because hot air is pushed upwards (the principle of a hot air balloon) and cold air is being sucked in (all convectors need a constant supply of air). Warm air rises to the ceiling, while the people that it should warm find themselves on the floor. This is not very efficient. Moreover, it is always too hot close to the radiator or the stove and always too cold at the other side of the room.

Secondly, convection is not healthy. The dust in the house starts floating around, which irritates the respiratory system. In combination with the drying effect of warm air and by the scorching of dust by the metal surfaces of radiators and stoves, this leads to an unpleasant, alkaline air climate, which can cause headaches.

This problem is mostly solved by air humidifiers or electric water heaters, which blow a mix of steam and air into the room. Unfortunately, this creates the ideal conditions for house dust mite and for nasty fungii. You can not open a window to ventilate, because then the warm air escapes quickly.

Sleeping on the stove

An oven stove has none of these disadvantages. Because it hardly warms up air, there is no dust circulation. Because the surface of the oven does not become as hot as the surface of a metal stove or a radiator, there is no scorching of dust.

And because the air does not rise, the heat is distributed evenly across the room, instead of rising to the ceiling (or via an open staircase to the upper floors). This means you can open a window upstairs to ventilate the house, without losing energy.

Because the exterior of an oven stove does not become too hot, there is no danger of burning yourself. This quality is sometimes used to integrate a bench or a couch into the stove, a luxury that no other heating appliance can offer. In Russia it used to be a habit to install beds atop of an oven stove.

Thanks to their warm (but not too warm) exterior, oven stoves offer more possibilities. They are very well suited to keep pots and pans warm, or to dry laundry – metal stoves or radiators are mostly too hot for that. A ceramic stove can be equipped with hot plates and an oven, so that it can be used as a cooking appliance, too.

Complete combustion

Oven stoves are an alternative for all modern heating appliances, but compared to wood stoves they have another, important advantage. More and more people choose wood as an energy source for heating, so that they are not dependent on the unpredictable energy prices of oil and gas.

In one sense this is not a bad thing, since unlike gas and oil, wood is a renewable and CO2-neutral fuel (the CO2 that is produced by the burning of wood was taken out of the atmosphere by the tree during the years before). The problem is that wood stoves are not very efficient, and extremely polluting.

Wood can be burned without too much air pollution, but then the temperature has to be high enough: 1100 to 1200 degrees Celsius. In that case, 99 percent of the wood is converted to CO2 and water vapour, almost without smoke. A metal wood stove, however, only reaches a temperature of 650 to 700 degrees, with an incomplete wood combustion as a result.

Wood consists for two thirds out of combustible gases and for one third out of combustible material. In the case of an incomplete combustion, these gases escape as smoke via the chimney. In regions where many people use wood heating, the air quality worsens dramatically (an incomplete wood combustion is more polluting than the burning of oil or gas).

Overheating

In a wood stove the fire is quelled by diminishing the supply of air, if not the room would overheat by the fast release of warmth by the metal appliance. Because an oven stove does not immediately release the heat of the fire, but stores it temporarily in the masonry mass, wood can be burned at a very high temperature without overheating the room.

An oven stove is always stoked at full power, even if a lower temperature is desired: in that case you simply stoke a smaller portion of wood, or you stoke less often.

A complete combustion is not only advantageous for air quality and efficiency, it is also safer. In the case of an incomplete combustion, the chimney gets ever more densely set with creosote, which can lead to a chimney fire when one day the stove is fuelled at full power (the reason why a chimney has to be cleaned regularly).

Instant heat

stoves have some disadvantages, too, although none of them are insuperable. Probably the largest drawback is the fact that a tile stove or masonry heater does not deliver the instant heat that we got used to.

If you turn on the gas stove, you are almost immediately rewarded with heat. But an oven stove takes a couple of hours before it starts radiating heat. That is not such a problem during long, cold periods, because once the cycle has been started and the stove is stoked shortly each morning, the house is always warm.

It is less obvious during periods with large temperature fluctuations, or when you travel a lot. You can make the best of that, by dressing warm for instance, or in a less low-tech way by installing an extra heating appliance – like a gas stove – in the same room.

This second heating generator may also be a much smaller tile stove, or a ceramic oven (a small tile stove meant solely for cooking). The small one can then be used to heat up the room relatively fast while the large one is warming up. A tile stove equipped with two stokeholds, a big one for the heating and a smaller one for cooking, offers the same possibility.

Overheating

A masonry heater neither delivers the fine temperature tuning of other heating appliances. If a wood or gas stove is overheating the room, you simply turn it lower and the temperature goes down fast. With a tile stove, this is impossible to do.

If you burn too much wood or gas in the morning, this decision can not be reversed during the course of the day. There is no option left but to open the windows, and that is not so efficient anymore. Heating a room to the right temperature thus requires some practice and dedication, especially in a climate which is not like it is in Russia or Finland.

A third disadvantage of the tile stove is a consequence of radiant heat. An oven stove only heats the room where it is positioned. Opening the door will not warm up the room next to it, because it is largely lying in the “shadow” of the radiant heat.

This is again comparable to a warm sun on a winter day: if you step out of the sun into the shade, the radiant heat is gone and all you feel is the air temperature. A gas or wood stove does not perform too well on this either, but a central heating system seems to be the indisputable winner. However, a tile stove does not exclude a system of central heating.

Heat walls

Heat walls or heat floors are another way of applying radiant heat. Here, warm water is not led through plate steel radiators as in a conventional central heating system, but through plastic hoses which are integrated into the walls (or into fake walls). Because of its porous surface a stone wall can not heat up air, it radiates like a tile stove.

Heat walls can be combined with an oven stove. The stove is connected to a boiler system, which distributes the warm water via hoses in the walls or the floor throughout the house. In this way one tile stove (assuming it is powerful enough) can heat up all rooms in a house, something which is otherwise not possible. In the same way a tile stove can deliver warm water for a household.

Heat walls can also be combined with an existing central heating appliance. This is already a step in the right direction, because the energy consumption goes down (the water does not have to be as warm as in the case of a traditional central heating system, because the heat is radiated over a larger surface area) and you get a better interior air climate.

As a third possibility an oven stove can also be connected to a network of metal radiators (as in this church), but in that case you lose the advantage of radiant heat. Steel plate radiators warm up the air.

Heavy, bulky and expensive

Another, quite fundamental drawback is that oven stoves are rather big and heavy. A modest tile stove amounts to at least 800 kilograms, and big ones (especially in the US and the former Soviet Union) can weigh up to 5 tonnes, or more. Smaller models exist, but they lose some advantages compared to their larger brothers (they have to be stoked more often, and do not always reach a complete wood combustion, or the same energy efficient output).

masonry heater needs to be large and heavy, there is nothing one can do about that. Metal heating appliances have the indisputable advantage of being compact.

Masonry stoves also cost 2 to 3 times as much as other heating systems. Since the energy consumption costs are lower, and a well-built oven stove lasts a lifetime, this investment pays itself back after a while. However, you have to be able to afford the purchase.

In Finland, a major producer of soapstone heaters, the purchase of an oven stove is subsidized by the government, with the consequence that 90 percent of new houses has them inside.

Oven stoves are expensive because they are products of craftsmanship. They are not really suited for mass production (although efforts are being made), and are therefore not of much interest to the heating industry. They prefer to sell, for example, pellet stoves.

Pellet stoves

A pellet stove is the only wood stove that reaches a complete combustion like an oven stove does. Sophisticated technology is used to accomplish this – which is also the weak point of this heating system.

pellet stove needs electricity to power all the high-tech (conveyor belt, ventilation, temperature regulation, and so on), which means that this electricity use should be taken into account when calculating the environmental score. Furthermore, a pellet stove stops delivering heat when there is a breakdown.

A pellet stove can only be fed with pellets, small rods made of wood, which are processed in a factory. Pruned wood or sawed wood are of no use in a pellet stove. Producing these pellets requires a lot of energy.

Another consequence is that once again you are dependent on the unpredictable prices of an energy supplier. Also, a pellet stove mainly heats up the air and hardly emits any radiant heat, so there is no health advantage compared to a wood stove, a gas stove or a central heating system. Thanks to all the high-tech inside, however, a pellet stove costs as much as an oven stove.

Fake oven stoves

Now that oven stoves are gaining some popularity again, the manufacturers of metal heating appliances try to get a slice of the pie. A few have started offering them according to traditional methods, sending masons to your house to build whatever oven stove you desire.

Most manufacturers, however, now use radiant heat as a greenwash promotion strategy. There are now stoves on the market that look like an oven stove, only they are not. They are normal metal stoves, wrapped in tiles. They betray themselves by grates to let in cool air, something an oven stove does not need (see picture below).

Of course the tiles get hot when the stove is running, but they cool down just as fast when it is turned off. These appliances are promoted as an ideal combination of both technologies, but they are simply convection stoves. They hardly emit any radiant heat, they scorch dust and they have a lower performance. Yet, they sell.

Another example is this German wood stove, pictured below. It is enthusiastically being promoted as a modern variant of an oven stove, and it will appeal to steampunks and survivalists, yet this is just a plain wood stove, not very efficient and very polluting.

Burning the woods

The oven stove is the only technology that allows a clean burning of wood without the need for another energy input, and thus it is the only technology that promises an environmentally friendly alternative for the dwindling resources of gas and oil.

The crucial question is whether the earth can produce enough wood to keep a significant amount of people warm. Today’s energy crisis is not the first in human history. Starting in the 15th century, some countries in Europe already faced a serious shortage of firewood and this became an acute problem in the 17th and 18th century, which only got “resolved” by the arrival of coal.

It seems impossible that the present European population could again be warmed by wood, because there are now many more people than in those times. But there is some hope that the potential is larger than expected.

The massive deforestation in the Middle Ages was the consequence of the fireplace, which needs ten times as much fuel as an oven stove (wood was also the main construction material). Even though the tile stove is almost 10 centuries old, it only broke through on a large scale in the 19th century, and even then only in some European countries.

The potential of the technology was never fully developed because of the start of the industrial revolution, and the abundance of coal. Research has yet to reveal how large the potential of wood as a fuel can be if we would use oven stoves, but it is surely greater than if we would return to wood stoves.

© Kris De Decker (edited by Vincent Grosjean). Thanks to De Twaalf Ambachten

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