top of page
OWB, Rt 7, Worcester NY, Oct. 2021 ja photo.jpg

Outdoor Wood Boilers

E. Outdoor Wood Boilers

Outdoor wood boilers (OWBs) are whole home water-heating devices that are located in small shed-like shells set away from the home. These boilers provide both hot water and space heating through pipes running from the outbuilding heater into the home. These units are popular due to their ability to heat the entire home affordably without having to bring fuel into the home or basement. Unfortunately, due to the low combustion rate of these units caused by the water jacket design, most outdoor wood boilers are very polluting and inefficient. Outdoor wood boilers produce large plumes of opaque smoke periodically as they re-heat after a cooling period. The short smoke stacks pictured here are common and cause heavy particulate matter loads close to ground level.


Figure 34: Outdoor Wood Boiler Diagram

Outdoor wood boilers have gained some popularity in rural areas due to the lower heating costs before the EPA and states began to regulate their emissions.  The EPA now offers a voluntary certification program for Phase II boilers. These Phase II OWBs are now cleaner than traditional ones, but most states still allow the installation of any OWB, no matter how polluting. Some states have banned all units and some are imposing regulations requiring a minimum stack height and set back limits from homes and property lines.

Policy Considerations
Any program that is interested in including OWBs should only consider EPA Phase II boilers and implement setback and stack height requirements on new installations. Even EPA phase II boilers may be unsuitable for suburban areas (certainly for urban areas) and should have minimum setback requirements. As of 2014, EPA emissions standards for OWBs will be mandatory, providing protections that have been lacking the past few decades. Many organizations believe that even Phase II boilers require at least 100 feet setback due to the frequency that high moisture content wood could be used; causing higher particulate emissions than testing lab reported emissions. Policy debates around outdoor wood boilers will continue for years to come as states try to deal with the legacy of these devices. Change out programs for OWBs are likely to grow and groups will be pushing for sunset laws, such as in Vermont, where non-EPA qualified OWBs in certain locations (ex: proximity to a school) must be removed by 2012. There is also some interest in retrofitting OWB’s with catalytic converters or designing a new class with catalytic converters.1


1 Houck J., Pitzman L. Catalytic Comeback?. Hearth & Home. February 2011. Pg. 30

bottom of page