Updated: January 2018

Outdoor Boilers

Outdoor wood boilers, also known as hydronic heaters, caught on 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 excessive pollution is one of the main drivers of the new 2015 EPA stove and boiler regulations.

Today’s outdoor wood boilers are regulated by the EPA and contain more sophisticated technology than units made and sold previously. Their attraction is that operators do not have to bring the wood into their homes and can use large 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.

Certified Outdoor Boilers:
Before establishing federal requirements for hydronic heaters, The EPA began a voluntary program in 2007 with the input of hydronic heater manufacturers and regulators. As of May 15, 2015, all residential hydronic heaters manufactured and sold must be EPA-certified and emit no more than 0.32 lbs per mmBTU.

Based on EPA data provided to EPA by manufacturers, certified hydronic heaters range from 39% to 90% efficient. As of December 2017, the average certified outdoor wood boiler is 66% efficient and the average pellet boilers is 84%.

State Policies Addressing Use of Outdoor Wood Boilers:
A number of states, towns and counties have placed restrictions on use of hydronic heaters due to excessive smoke generated by older, non-certified models or poor use of certified models. Some counties and towns have banned their installation and some states have passed rules that establish property line setbacks and minimum stack height requirements 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 banned use of outdoor wood boilers.

More background and resources on outdoor boilers
  1. AGH's analysis of comparisons of OWB regulations, click here
  2. Resources, links and advice from the state of Minnesota
  3. New coverage and consumer input from Popular Mechanics
  4. Central Boiler claims Outdoor Boilers Cleaner than Wood Stoves
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