2011 Aug. 5: American Lung Assoc. of the Upper Midwest: Regarding Residential Outdoor Wood Fired Boilers
As the price of home heating oil, natural gas and liquefied petroleum gas
increases, more households are looking towards heating alternatives. In growing numbers,
people are turning to wood-burning devices to replace oil and gas heating. Wood-fired devices
include masonry heaters, indoor wood stoves, indoor wood furnaces, pellet stoves and outdoor
wood-fired boilers (OWB). The recent increase in OWBs are of particular concern to state
environmental agencies because of the cumulative stack emissions from these appliances which
are greater than other wood burning appliances; and, unlike other wood burning devices, are
generally unregulated. Where once these OWBs were located mainly in rural areas, they are
now being used in suburban and urban neighborhoods to provide space heating, year-round
heating of hot water, and heating of swimming pools, Jacuzzis and hot tubs.
OWBs (also called outdoor hydronic heaters or outdoor wood furnaces) are free standing wood
burning appliances that heat water, which is then pumped to one or more structures to provide
heat. OWBs can also be used to provide hot water and to heat swimming pools, etc. Units are
typically the size and shape of a small storage shed, or mini-barn with a short stack on top and
are much larger in size and differ in design, operation and emissions when compared to smaller
indoor wood stoves, pellet stoves, fireplaces and barbeque pits.
OWBs operate on a cyclical operating pattern creating an on/off cycle. Manufacturers advertise
OWBs for use with wood fuel, but sometimes other types of fuel (such as wet, un-split wood,
wood waste, yard wastes, general refuse, tires and railroad ties) are used.
Due to the design and operating practices of the OWBs, some environmental
concerns can result. Unlike indoor wood stoves and fireplaces, OWBs generally operate year
round. The quantity of (black) smoke generated from OWBs is more significant than in other
wood burning appliances. The lower combustion temperatures and cyclic nature of opening and
closing dampers to regulate the unit creates fire smoldering, build-up of smoke and creosote.
Another contributing factor to this smoke is the short stack height of OWBs resulting in limited
vertical dispersion of the resulting emissions. Certain weather conditions aggravate the problem,
including cold weather inversions, when the smoke does not rise but stays close to the ground.
The large capacity of the fuel being used and the potential use of variable fuel also results in
adverse uncontrollable emissions. Lastly, the units do not have emission control equipment such
as catalytic control devices or secondary combustion chambers.
Public Health Concerns:
Wood smoke contains a complex mixture of airborne particulate
matter (PM) and gases, many of which have been known to produce acute and chronic biological
effects and deleterious physiologic responses to the exposed population.
Short and long-term exposure to airborne PM is associated with cardiopulmonary health effects,
including increased respiratory and cardiac symptoms, hospital admissions and emergency room
visits, and premature death. Other harmful health effects include aggravated asthma, decreased
lung function and chronic bronchitis. In areas where residential wood combustion was largely
used, a major source of ambient PM resulted in higher health risks
1. Over 50% of the population
falls into susceptible subgroups (children, asthmatics, persons with pre-existing respiratory
disease or cardiac problems, older adults, and healthy adults who work or exercise outdoors) that
are most affected by PM
Wood smoke includes numerous gases, including carbon monoxide, nitrogen and sulfur oxides,
volatile organic compounds (VOCs), poly aromatic hydrocarbons (PAHs) and chlorinated
dioxins. VOCs can cause upper airway irritation, headaches, and possibly cancer
American Lung Association of the Upper Midwest’s Position:
The American Lung Association of the Upper Midwest recognizes that pollution from the
combustion of wood and other biomass sources may pose a significant threat to human health,
and supports measures to transition away from using these products for residential heat
The American Lung Association of the Upper Midwest supports regulating how and when
OWBs can be used. This would include regulations for design and operation, emission
limitations and fuel usage.
The American Lung Association of the Upper Midwest calls for effective enforcement of
existing laws and regulations governing the use of OWBs, as well as the expanded regulation of
air pollution emissions from these sources.
The American Lung Association of the Upper Midwest recommends continuing research on the
health effects of burning wood and other biomass sources, and the technologies to reduce the
potential emissions associated with the combustion of these fuels.
Review of the National Ambient Air Quality Standards for Particulate Matter: Policy Assessment of Scientific and
Technical Information. EPA-452/R-05-005. U.S. EPA Office of Air Quality Planning and Standards: Research
Triangle Park, NC 2005.
Johnson PRS, Graham JJ. Fine Particulate matter national Ambient Air Quality Standards: public health impact on
populations in the northeastern United States.
Environ Health Persp 2005;113:1140-1147.