Outdoor Wood-fired Boilers

Outdoor wood-fired boilers (OWBs) are a type of furnace used to heat hot water that present a major health hazard due to their high emissions.  The simple fire boxes in OWBs burn wood inefficiently, leading to high levels of particulate matter and other pollutants being released.  The short smoke stakes ensure that OWBs directly impact breathable air quality for both their operators and near-by residents.  Many OWBs are operated year round to provide hot water, and run continuously through the winter to provide heat.  Together, these factors combine to make OWBs a problematic source of local air pollution.

OWBs burn various forms of biomass, usually wood, and are located in small out-buildings, away from the primary residence.  They consist of a fire box surrounded by a water jacket that converts the heat to hot water.  This hot water is then circulated through underground pipes to an storage tank or forced air system to heat the home.  Cooled water is then circulated back to the OWB, where it is reheated.

The problems with OWBs lie in their inefficient design.  The fire boxes tend to be simple cavities in the water jacket that fail to reach the high efficiency of newer EPA certified wood stoves.  A 2006 study by the Northeast States for Coordinate Air Use Management (NESCAUM) found that OWBs produce 22 times as much fine particulate matter than wood stoves, 205 times as much as oil furnaces, and as much as 8,000 times as much as an equivalent natural gas furnace.  These boilers have to operate almost continually in order to provide enough heat and hot water for a building.  Additional problems arise from the relatively short smoke stacks of OWBs, usually only 10 to 15 feet, compared to flues for fireplaces or indoor furnaces.  The stacks low height results in a higher amount of the particulate matter entering lungs and near-by buildings, contributing significantly to respiratory illnesses.

OWBs are regulated by the EPA, but relaxed standards on efficiency, emissions, and design means that additional measures are needed to protect public health and safety.  Municipalities can limit the impact of OWB operation by adopting stringent setback requirements and additional regulations on particulate matter to protect adjacent homeowners.  Incentives can also be offered to encourage homeowners to upgrade their OWBs to newer, more efficient models.  Interested municipalities and homeowners should examine model legislation to see what improvements can be made in their communities.  Concerned citizens can also use our reporting tool to document smoke problems in their communities.

OWB Fact Sheet


Model Legislation

The Pennsylvania Department of Environmental Protection (PA DEP) offers a model ordinance for wood-fired outdoor-boilers for municipalities across the state.  Many other states, however, offer model ordinances with stricter standards regarding setbacks and stack height for exhaust pipes.  Here are a few examples from PA DEP, MI DEQ, and WI DNR.

Click here to download a pdf of a model ordinance for wood-fired boilers from the 

PA Department of Enviornmental Protection.

Click here to download a pdf of a model ordinance for wood-fired boilers from the MI Department of Environmental Quality.

Click here to download a pdf of a model ordinance for wood-fired boilers from the WI Department of Natural Resources.

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