Rated outputs for wood stoves are estimations, and actual output will be dependent upon combustion air supply, chimney draft, and the species and dryness of firewood. Typical residential models range from 50,000-200,000 Btu/ hr. Burn efficiency is also difficult to accurately establish, but manufacturer claims range from 65-80%.
Pellet stoves have more accurate ratings, as pellets have relatively uniform density and moisture properties. Typical residential models range from 30,000-160,000 Btu/hr. Manufacturers claim burn efficiencies ranging from 70-90%.
A freestanding unit is placed, usually centrally, in the home. The unit has a burn chamber where a fire is lit.
Combustion air is introduced through a manually or mechanically controlled inlet. Ideally, this combustion air is supplied through a sealed inlet directly from outdoors. Masonry heaters do not use dampers on the air inlet, as the fire is burned at the highest possible temperature, requiring a significant column of air.
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Exhaust gasses do not exit directly into a straight chimney, as with woodstoves. Instead, a masonry chimney with a non-linear path receives the gasses. Once the combustion chamber and chimney have become heated, the entire chimney pathway acts as a secondary burn chamber. At full temperature, practically 100% of the gasses are combusted within the chimney. These units are not limited in their burn temperatures by any metal components, and can burn at about 2,200-3,300°F (1200-1800°C).
Heat is absorbed into the masonry of the combustion chamber and chimney, and radiated to an outer sheathing of masonry, separated from the core by an air space. In this way, the interior of the home is not exposed to very high temperatures. Instead, the majority of the stored heat is slowly released to the room at comfortable temperatures. The outer sheathing can be made from a wide range of materials, from stone or brick to clay or tile. Spent gasses leave the building via a straight run of metal or masonry chimney.
An integral part of the mechanical system for most new homes is active ventilation. Any home designed to achieve a high degree of energy efficiency is built airtight and will benefit from active ventilation to exhaust stale, humid, and/ or contaminated air and replace it with fresh air from outside (see “Key Concepts for the Air Control Layer,” in Decoration 6). Many code jurisdictions require active ventilation systems.