Masonry Heaters – using ancient technology to create clean wood heat
Masonry heaters are very efficient wood burning devices that provide radiant heat, similar to the way a wood stove works. While they have been used for centuries in the cold regions of Europe, the technology has only recently gained popularity in North America in the past few decades. At heart, it is a very simple technology that uses basic materials such as clay, sand, and rock to burn wood in a clean and efficient manner. The Cold Climate Housing Research Center uses a masonry heater to meet nearly 10% of its annual heating load.
A masonry heater burns a load of wood rapidly (for about two hours) and stores the heat in its large thermal mass, which can be made of stone, brick, ceramic, or another solid medium (often around 4 tons). Because wood combusts quickly at very high temperatures, it burns clean and produces no creosote and few emissions; it is approximately 10 times cleaner than an EPA-certified woodstove. Unlike a traditional wood stove, which vents exhaust immediately through a stack pipe, exhaust gas in a masonry heater travels through a carefully designed flue that snakes through the heater, transferring energy into the thermal mass as it goes. The thermal mass heats up to between 120-160 degrees, acting as a giant radiant panel by releasing heat slowly into the room over 16-24 hours and producing a cozy, even temperature.
Masonry heaters have a combustion efficiency of up to 85%, depending on the model, compared to efficiencies in the high 70s for pellet stoves, low 70s for catalytic stoves and low 60s for non-catalytic stoves certified by the EPA, according to EPA’s list of certified appliances.
Masonry heaters can be designed to heat an entire home or specific zones. An average size energy efficient home might require a heater with about 20,000 BTU/hour to meet whole-house heating needs. In this case, you would burn about 3,000 pounds of seasoned wood per month (one full cord of dense hardwood or 1.5-2 cords of softwood, depending on the species). Even in a less efficient house, masonry heaters can offset a big chunk of space heating demand without using a lot of firewood.
A readily available fuel source in much of Alaska, wood can be considered a renewable resource if harvested sustainably. Because masonry heaters extract the most energy possible from a piece of wood, they use less wood overall, a big benefit where firewood is a limited resource. Using fully cured wood (20 percent moisture content or lower) is cleaner and more efficient because less energy is lost in the process of vaporizing the excess moisture stored in the wood.
Additional uses
Masonry heaters can be designed to include a coil for hydronic heating or domestic hot water. Water running through the coil is heated and can then be distributed throughout the house, transferred to a thermal storage tank, or used to heat domestic hot water via heat exchanger. Masonry heaters can also be designed to include integral bake ovens or heated benches.
Building a masonry heater is a major project and a long-term investment, costing several times as much as a high-end wood stove. The brick core is carefully engineered for performance and typically requires a skilled mason to construct the heater to ensure it drafts properly and can withstand the expansion and contraction of thousands of firing cycles. Since masonry heaters tend to be quite large, the best time to install one is during the construction of the home, where it can be easily incorporated into the home’s design and heating requirements. Although the initial cost may be higher, the tradeoff is that a masonry heater can last a lifetime or more with minimal maintenance.
More information
The Masonry Heater Association of North America publishes plans for different masonry heaters and offers information on building codes, manufacturers, and certified masonry heater builders in Alaska.