Updated: May 2014

Outdoor Boilers

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. Outdoor wood boilers gained popularity in colder, rural areas where fuel is abundant, particularly in the Great Lake states where most of them are built. They began as a very basic, unregulated technology that was advertised as being an affordable and efficient way to heat your home. Their attraction is that operators do not have to bring the wood into their homes, and can use large, unsplit pieces of wood. Many homeowners also use these boilers to heat their domestic hot water, which means they operate year round at levels far below what they are designed for, adding to smoke problems.

Unregulated Outdoor Boilers:
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. The typical unregulated outdoor wood boilers is likely to be in the 35 55% efficient range.

Certified Outdoor Boilers: The EPA began a voluntary program in 2007 instead of developing national regulations. Under the voluntary program, companies that meet cleaner emission standards on units can advertise as "EPA Phase II Qualified." As of May 15, 2015, these new boilers will be EPA certified.

Based on EPA data, qualified outdoor boilers range from 39% to 78% efficient. The average qualified outdoor wood boiler is 63% efficient and the average pellet boilers is 74%.

A number of states, towns and counties have outright or effectively banned outdoor boilers, in part because even the qualified/certified ones can emit excessive smoke if not used properly. In addition, many states have passed regulations that establish property line setbacks and minimum stack heights to ameliorate problems and prevent installation in densely inhabited areas. These states include Indiana, Maine, Maryland, Massachusetts, New Hampshire, New York, Oregon, Pennsylvania, Utah and Vermont. Washington State, Oregon and much of California have effectively bans outdoor wood boilers.

For more information on AGH's comparisons of OWB regulations, click here.

More background and resources on outdoor boilers
  1. Resources, links and advice from the state of Minnesota
  2. New coverage and consumer input from Popular Mechanics
  3. Central Boiler claims Outdoor Boilers Cleaner than Wood Stoves
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