Wood boilers. Wood boilers don’t provide direct heating, but instead heat water for distribution through a hydronic heating system. There are generally two types—outdoor single-stage models (one of the most polluting ways to burn wood—avoid these) or high-efficiency two-stage gasification models that are usually installed in the basement.
Two-stage gasification units use oxygen sensors and electronic controls for an almost fully automated burn with a calibrated fuel/air mix. First, wood is heated in an oxygen-poor chamber to draw out combustible gases. These gases are then burned at a high temperature in a separate, second chamber. The heat is stored in water, generally in large tanks up to 1,000 gallons, and circulated through the house either through radiators or hydronic tubing in the floor. Boilers provide domestic hot water and have the potential to be used in conjunction with solar water heating systems. Econoburn is a U.S. manufacturer of two-stage boilers. HS Tarm is a highly regarded European model that also is available in North America.
Wood boilers, combined with the water storage, can take up a sizable chunk of space—50 square feet or more. Plan on spending close to $10,000 (or more) for their purchase and installation (and potentially much more if you also need to have a hydronic distribution system installed). Wood boiler advantages include their efficiency and low emissions, single-loading (one burn per day), and the production of household hot water. Their main disadvantages are their required size and cost, and that they remove the fire as a centerpiece of the home.
High-efficiency masonry heaters. These site-assembled cordwood-burning stoves use many of the same principles of advanced wood-heater technology—a well-insulated firebox, preheated secondary air intake, and proper firebox shape—to create high-temperature fires capable of achieving the same or better efficiencies of wood or pellet heaters. Built with high-heat bricks, which make up the inner portions of the heater, and sometimes faced with soapstone, these high-mass heaters absorb heat from the burning flue gases and then gradually release it. In passive solar homes, depending on their placement, these heaters can also collect solar gain.
When wood burns in the firebox, the burning flue gases are forced into an upper combustion chamber. From this location, the fire gases are directed downward, into side channels. By forcing the gases to follow a long path to the chimney (known as “contraflow”), the heat can be absorbed by the masonry. Masonry heaters are engineered to generate quick, hot fires in their fireboxes. When the fire is out, the damper is shut, stopping the draft that would otherwise cool the heater. A hot fire means improved combustion efficiency, and masonry heaters boast some of the highest efficiencies among wood-heating appliances.
A masonry heater, as defined by the Masonry Heater Association of North America, must have a mass of at least 1,760 pounds; tight-fitting doors that are closed during the burn cycle; and an overall wall thickness not exceeding 10 inches. The gas path through the internal heat-exchange channels must include at least one 180° change in flow direction (usually downward), before entering the chimney. The length of the shortest single path from the firebox exit to the chimney entrance must be at least twice as long as the firebox’s longest dimension.
Unlike a steel or cast-iron heater, in which the fire may need to be regularly stoked to maintain comfortable room temperatures, masonry heaters are typically only fired once in an 18- to 24-hour period. For example, a “large” (6,292-pound) Tulikivi reaches its peak approximately six hours after its fire is started; it has released half of its heat output 18 hours later, and about 75% 31 hours later.
Masonry heaters work best in a home with an open floor plan, and their placement needs to be well-considered. Since these heaters tip the scales at several tons, it’s recommended that they have an extensive footing. Because of this, they may be difficult to retrofit into an existing structure without considerable expense.
Although most masonry stoves are individually crafted and high-dollar items ($10,000 or so), they are often works of art that become a home’s centerpiece. Lower-cost alternatives are prefabricated masonry stoves or masonry stove kits, such as Tulikivi and Temp-Cast, that can be assembled by someone with masonry skills.
Stephen Hren is the author of, most recently, Tales from the Sustainable Underground: A Wild Journey with People Who Care More About the Planet than the Law.
Wood heater recommendations • forgreenheat.org
Wood & pellet heater reviews • wiseheat.com
Chimney Safety Institute of America • csia.org • Installer search engine