Masonry heater
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A masonry heater (or masonry stove) is a device for warming a home (or any interior space) that captures the heat from periodic burning of fuels (primarily wood), and then radiates that heat over a long period at a fairly constant temperature. The technology probably descends from the Roman hypocaust, a system for heating the floors and walls of buildings (especially baths) using the smoke and exhaust of a single fire. In Eastern and Northern Europe these steinofens evolved in many different forms and names, such as a Russian Stove/Fireplace, a Finnish Stove/Fireplace (also known as a "contra-flow" fireplace or heater), and the Swedish Stove (also called a "Tile Stove").
As the name suggests, this is made of masonry, rather than steel, and can be free-standing, but is often built on top of a foundation as part of a building's structure. It consists of a firebox and an extended flue (or flues) that provide additional surface area to absorb the maximum amount of heat from the hot exhaust gases before they exit into the chimney. Because the firebox is masonry, not metal, fires can burn much hotter than in a metal stove. When not being fired, the connection from the masonry heater to the chimney is then damped or closed to prevent the captured heat from escaping up the chimney, but now radiates out from the heated masonry block.
Because masonry takes longer to heat up than metal, it will also radiate this heat over a longer period of time, and typically at a lower temperature, than a metal stove which is hot only when there is a fire burning inside the stove. In eastern europe, builders have incorporated seats and even beds into the masonry stove because the external surfaces never really get hot enough to burn skin, so are very safe to be around.
Metal stoves are sometimes installed with a very long metal flue connection in an attempt to make the same heat available to the space being heated. This is an imperfect solution because that heat is only available when the stove is being fired, and creosote will often build up in that long flue, as it will anywhere smoke and other products of incomplete combustion come in contact with a much cooler surface.
These heaters are primarily fired by wood, and those fires are meant to run hot and quick (never damped down, as is often the case with standard wood stoves), but not necessarily continuously. This method of heating may have been a reaction to the dwindling resource of wood before the advent of coal and other mineral energy sources. Open hearth fireplaces were an important source of light, as well as heat, and with an "unlimited" supply of wood to fire them, there is no incentive to increase the efficiency of their heat output, which is rather poor. However, once firewood became a scare resource, fireplace builders began to enclose the firebox to capture as much heat as possible. Because masonry heaters burn hot and fast, they can accept most fuels, although the faster burning the better (so wood should ideally be no more than three inches in diameter), and in some areas of Central and Eastern Europe are effectively fired on grass, straw, and hay.
Heat stress is the major concern in the construction of masonry heaters given the differential of temperatures that the masonry can be exposed to when being fired, which can cause uneven expansion and cracks in the structure. Various designs, such as the contra-flow, attempt to minimize this stress on the masonry.
Masonry heaters take a long time (some take several days) to get up to temperature, and so are not useful to "take the chill off" a cool evening or morning. They work best during long periods of cold weather because their heat is so constant, but can overheat a house in during spring and fall warm spells.
[edit] Resources:
The Book of Masonry Stoves: Rediscovering an Old Way of Warming by David Lyle (Chelsea Green Publishing Co., 1984)
