Masonry Magazine February 1996 Page. 19
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Masonry heaters are exceptionally clean-burning appliances known for their performance. Their efficiency is such that they can be used as the primary heating system in a home. In a well-insulated home, a single masonry heater can often heat up to 2,000 ft. Masonry heaters capture the heat from combustion and store it, releasing it as radiant heat when needed. To maximize the radiant heat output, masonry heaters are often used as room dividers, directly heating several spaces.
The radiant heat provided by a masonry heater is a more even, comfortable heat than forced air heat. Properly designed, a masonry heater typically continues to provide heat for a minimum of 8 to 12 hours after the fire goes out. This translates to one fire per day, except in extremely cold conditions when two fires per day are needed. To retain such heat, the entire mass of masonry is utilized.
Open fireplaces use air in excess of the amount needed for combustion. This excess air is heated by the fire and is lost up the chimney. The loss of heated air is what makes open fireplaces inefficient at significant heating. Up to four times as much air is lost up the chimney of an open fireplace as is exhausted by a masonry heater. Masonry heaters use the minimum amount of air necessary for complete combustion. The air-to-fuel ratio in a masonry heater is tightly controlled by using gasketed doors and limiting the air intake and size of the firebox. The result is a rapid, high-temperature burn (typically 1,500-1,700° F) which thoroughly burns both the wood solids and the combustion gases.
Creosote, common in fireplaces, is an oddity in masonry heaters, because the gases and resins which form this dangerous deposit are completely combusted prior to reaching the flue. Thus, masonry heaters are exceptionally clean-burning. Figure 7 illustrates the typical range of particulate emissions of several wood-burning appliances. This data was obtained from research which measured emissions from appliances in the home (on-site). The particulate emissions 10 microns and less in size (PM-10) are plotted in grams per kilogram of fuel.
Elements of Construction
As Figs. 2 to 5 show, the construction of masonry heaters is not limited to one design. The four types common in the United States are based on designs developed in Europe. The direction/design of the heat-exchange channels can be used to distinguish between the different masonry heaters types. Exact detail of a given unit may vary because of the custom-designed nature of the heaters.
The Russian masonry heater, also known as a Russian fireplace, contains multiple heat-exchange channels which are routed back and forth, horizontally or vertically, upon exiting the combustion chamber to a chimney located at the top of the unit. Generally, the heat exchange channels in a Russian heater are the same size throughout the unit. See Fig. 2.
The German style heater, shown in Fig. 3, features multiple heat exchange channels which gradually decrease in size as they approach the chimney. This unit contains both vertical and horizontal channels, the number of which can be increased to provide a unit with more heating capacity. The German heater is known as a Kachelofen if finished in tile (Kachel, the German word for tile) or as Grundofen is faced with brick.
The Swedish masonry heater, known as a Kakelugn, shown in Fig. 4 contains heat exchange channels which extend from the top of the firebox, down the front corners of the unit and up the rear corners to the chimney at the top of the heater. At the present time, this design is limited to manufactured units imported from Sweden.
The downward flow of the combustion gases in the Finnish design versus the upward convection of room air past the masonry heater gave rise to the name Contraflow heater. In this design, shown in Fig. 5, combustion gases typically exit a narrow throat at the top of the firebox into a secondary combustion chamber and are routed downward in channels on the sides of the firebox to a flue at the base of the heater. Multiple channels, as found in the other heater designs, are not used. This design is commonly used in the prefabricated masonry heater core assemblies.
Like fireplaces, all masonry heaters have certain common elements, regardless of design. Most masonry heaters are built with an inner core consisting of the firebox and heat exchange channels, surrounded by a second wall of masonry. The second wall is usually the outer finished wall and may be built of brick, stone or soapstone. The top of the heater is finished with a heat-resistant capping slab. Other common elements include a foundation, base, combustion chamber or firebox, heat exchange channels and a chimney. Some designs also include a bypass damper for easier start-up and a gas slot.
Foundation and Base
The foundation of a masonry heater must be designed for the additional weight of the heat-storing mass of the heater. It may be constructed of concrete, concrete masonry or brick masonry. The footing should be a minimum of 12 inches in thickness and extend at least 6 inches beyond the foundation walls. The foundation and base may include an ash pit and an outdoor air intake passage.
Combustion Chamber
The combustion chamber, or firebox, is typically 12 to 24 inches wide by 12 to 30 inches deep. The larger the firebox, the more mass required in the heater. The firebox is designed to hold a specific charge of wood, typically 25 to 40 pounds for an average fire, to a maximum for most heaters of 55 to 60 pounds, depending upon the size of the heater and space to be heated. The combustion chamber may be built of low-duty firebrick, soapstone, refractory concrete elements or a combination of these.
Because of the high temperatures associated with a masonry heater fire and the thermal shock which is encountered, low-duty firebrick is better than high-duty firebrick. Similarly, a "soft-set" fireclay mortar can be used in the firebox to allow for thermal movement. Refractory mortar made from soapstone, fireclay or calcium aluminate is used throughout the heater core. To accommodate the expected thermal expansion, the combustion chamber is surrounded by an air space, often filled with mineral wool.
Combustion air is provided to the firebox through slots in the firebox door, or sometimes through an open grate in the floor of the firebox. A tight-fitting door, which is normally shut during operation of the masonry heater, is essential to proper performance of the heater. The firebox doors may be solid metal or have special heat-resistant glazing to provide fire-viewing.
Heat Exchange Channels
Heat exchange channels route combustion gases from the exist of the combustion chamber to the flue. They may be constructed from firebrick, soapstone or refractory concrete elements. Although clay flue
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