Outdoor wood boilers, also known as hydronic heaters, are a controversial technology that has spawned thousands of complaints and lawsuits across the country due to excessive smoke. Recently, cleaner outdoor boilers have reached the market, but they can still be problematic in populated areas.
Outdoor wood boilers gained popularity in rural areas where fuel is abundant. They began as a very basic, unregulated technology that was advertised as being an affordable and efficient way to heat your home.
The EPA began a voluntary program in 2007 instead of developing national regulations. Under the voluntary program, companies that meet certain emissions and efficiency standards for at least some of their units can advertise as “EPA Phase II Qualified.” Based on independent third party testing, these boilers are estimated to be 55 to 65% efficient, while the unqualified ones are 35 to 55% efficient. The list of hydronic heaters that qualify for the EPA's Outdoor Wood-Fired Hydronic Heater program is here. States that have passed regulations to allow only the cleaner, Phase II units to be installed are: Indiana, Maine, Maryland, Massachusetts, New Hampshire, New York, Oregon, Pennsylvania and Vermont. Washington State effectively bans outdoor wood boilers.
The EPA is now in the process of making mandatory regulations, which will likely take effect in 2014 thus requiring all states to only allow the installation of the cleaner, more efficient units.
According to the State of Washington Department of Ecology, "most OWBs employ very primitive combustion technology" and "are designed to burn wood at lower combustion temperatures and generally have shorter stacks, which emit smoke closer to homes and neighborhoods." Lower combustion temperatures result in less complete combustion, causing increased emissions of particulate matter. OWBs often run on idle periodically to avoid overheating the water, and produce large plumes of opaque smoke as they reheat after a cooling period.
For more information on AGH's comparisons of OWB regulations, click here.