Outdoor wood boilers are a popular heating solution in some rural areas, but they’re also controversial. While the boilers are fueled by renewable resources if the wood is sustainably harvested, current designs are woefully inefficient and polluting.
What exactly is an outdoor wood boiler? Also called an outdoor furnace, water stove or outdoor hydronic heater, it looks like a metal-clad shed with a short chimney poking from the roof. It’s basically a large firebox surrounded by a water jacket. Two insulated pipes run underground to the building it heats. One pipe delivers the heated water for space heating and domestic hot water, and the other returns the cooled water to the boiler for reheating. A single boiler also can heat a second building, such as a workshop.
Unlike other heating equipment, outdoor wood boilers have been virtually unregulated. Unfortunately, they emit at least 20 times more smoke than EPA-certified woodstoves, according to the Northeast States for Coordinated Air Use Management.
In a 2005 report, the New York state Environmental Protection Bureau found that wood boilers’ efficiencies range from 28 percent to 55 percent, with an average of 43 percent. The report compared those numbers to efficiencies for EPA-certified woodstoves, which are typically 68 percent to 72 percent.
In response to the smoke pollution problem, the major outdoor wood boiler manufacturers, along with state environmental regulators, helped to write a smoke emissions test standard for outdoor boilers under the auspices of the American Society for Testing and Materials. The U.S. Environmental Protection Agency (EPA) has adopted similar testing standards and a program which includes emissions guidelines. While the EPA program is voluntary, some states will require all outdoor boilers to meet these new guidelines, or similar standards set by individual states. Vermont, for example, has already adopted their own.
New wood boilers that meet the updated guidelines are already in various stages of development, and if everything goes well, a number of cleaner outdoor wood boilers could be on the market as early as next summer.
For more information on outdoor wood boilers, visit these Web sites: