Outdoor Wood Furnace and Boiler Pollution

By Uni Blake

[Feb. 14, 2006]

Emissions from outdoor wood burning stoves drift across property lines raising health concerns for neighbors who inadvertently breathe the smoke filled air. In a bid to avoid rising fuel costs home owners are turning to wood for their domestic heating needs. Along with this increase in wood burning, there is also an increase in wood burning related pollution. Wood smoke contains dust and soot (particulates) and other hazardous air pollutants that include some carcinogens. This pollution has both short-term and long-term negative human health effects. To exuberate the problem further outdoor wood boilers/furnaces presently do not have performance or safety standards to curtail this pollution. It is with this knowledge that outdoor wood boilers/furnaces (OWBs) are getting heat from the public and attention from regulators alike.

Outdoor wood boilers also known as outdoor water stoves and outdoor wood furnaces are generally composed of a wood burning firebox surrounded by a water jacket or reservoir vented by a chimney stack. The water jacket is enclosed in an insulated water-resistant enclosure that resembles a small shed. The combustion of the wood in the firebox heats the water in the surrounding jacket. The heated water is then pumped via insulated underground pipes into the residence. Once in the residence, the heated water heats the home via radiators or a heat exchanger duct system. The OWBs can also supply residential hot water.

When compared with other heating units (indoor woodstove, natural gas or fuel oil), there are some advantages associated with using an OWB. The OWB removes the risk of fire and indoor pollution by taking the heat source outside. The large size of the firebox allows for larger wood loads requiring less attention and longer burn times. The wood fed into the firebox does not require to be split, saving time and money.

OWBs require an immense amount of work and are inconvenient for some to use, loading your burner outside in the cold of winter, daily monitoring and adjusting, finding an ample and consistent source of wood, and a large dry storage area is required for the wood. Unlike the indoor woodstove, during a power outage you do not have heat. The OWBs are expensive, costing between $3,000 to $10,000 to purchase and install the unit. Another disadvantage of the OWBs is the amount of wood smoke released from the combustion. This is the source of most complaints against the stoves.

The design of the fireboxes encourages a slow burning cooler fire, which is supposed to maximize the amount of heat transfer. However, slower cooler fires are inefficient and create more smoke and creosote. The unit's excessive smoke can also be attributed to the incomplete combustion of the wood, wood choice (soft versus hardwood) and the moisture content in the wood. OWB owners sometimes burn trash, household garbage, tires, cardboard and garden refuse, which leads to elevated smoke level and unpleasant odors.

How does the OWB pollution affect people?

Wood smoke is a complex mixture of chemicals and particulates. It contains carbon monoxide and other organic gases, particulate matter, chemicals, and some inorganic gases. Some of these compounds are toxic, aldehydes and phenols, and some are known carcinogens, benzoprene and cresols. The particulate matter is mostly made up of elemental carbon, soot and inorganic ash.

Exposure to carbon monoxide (CO) gas at low levels to individuals with compromised heart and circulatory conditions can lead to hospitalization. CO is also known to lower birth weights and increase mortality of newborns. Particulate matter in wood smoke that is less than 10 microns in diameter finds its way into the alveoli in the lung. Once in the alveoli the particulate matter can cause structural and chemical changes which interfere with oxygen uptake. The toxic compounds and carcinogens also enter into the bloodstream via the alveoli. These chemicals act as irritants and interfere with normal lung function leading to lung inflammation and pulmonary edema.

How to curtail smoke from OWBs

Owners and Manufacturers

The proper use of OWBs can reduce emissions significantly. Some OWB owners are modifying their furnaces retroactively to address the emission dilemma. Manufacturers of OWBs are also working with owners and regulators to solve the emission problem. Some of the recommendations include:

  • Installing insulated coolant reservoirs to store heat during low demand heat periods. This helps to maintain optimal combustion conditions.
  • Installing fans at the bottom of the smoke stack to inject fresh air into the stack to increase upward velocity of smoke for higher and greater dispersion.
  • Installing a taller chimney for higher smoke dispersion.
  • Limiting operations of these boilers to wintertime when windows are more likely closed and few outdoor activities.
  • Choosing and burning wood with the correct moisture content and size.


The Clean Air Act of 1970 charges the USEPA to identify and set standards for all air pollutants. In 1988 the USEPA established limits on emissions for a limited number of indoor residential wood heaters. OWBs are relatively new, therefore there are no regulations in place. As a result, New York, Connecticut, Maryland, Massachusetts, Michigan, New Jersey, Vermont are some of the states that have petitioned the USEPA seeking regulation at the federal level of the outdoor furnaces. In 2005 USEPA initiated a review of the petition and results from the review are expected in spring 2006.

In response to the National Research Council Report on Air Quality Management in the United States, an Air Quality Management (AQM) Work Group was established. The AQM Work Group met in October and on their agenda was the High Priority issue of Residential Wood Smoke. The following were some recommendations made by the AQM Work Group:

  • EPA needs to gather more information on the nature and magnitude of the impact of the OWB emissions on air quality.
  • EPA needs to further develop the Residential Wood Smoke Reduction Initiative.
  • Develop an ASTM test method for measuring OWB emissions.
  • The agency needs to decide whether to take a regulatory or non-regulatory approach to monitoring OWB emissions.

In October 2005 the US Government's Department of Energy (DOE) announced a national "Easy Ways to Save Energy" campaign. This campaign was initiated in light of the rising energy cost and focuses on the Government and public alike. It calls for the government and public to turn to renewable energy sources and calls for a need to increase the efficient use of our available energy sources. Wood burning as a fuel source will therefore continue to be on the rise. The solution to the wood burning dilemma depends on the ability of the public, the regulators and the manufacturers of the stoves to come up with agreeable regulations that promote public health and stay in line with the DOE's energy campaign.

Bibliography is below

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  • "Home Page" Wood Heat Organization Inc September 1999. December 2005 http://www.woodheat.org
  • United Sates. USEPA. Emissions from Outdoor Wood-Burning Residential Hot Water Furnaces. February 1998. December 2005 http://www.epa.gov/ttn/atw/burn/woodburn1.pdf
  • United Sates. American Lung Association. Facts about Particulate Matter Air Pollution April 2000. December 2005 http://www.alaw.org/pdfs/particulate_matter_air_ pollution.pdf
  • United Sates. Air Quality Management Group. Status Summary of Phase 1 Recommendations November 2005. December 2005 http://www.epa.gov/air/caaac/pdfs/aqm-11-2005- recommendations.pdf
  • United Sates. Air Quality Management Group. Final Report to the Clean Air Group Advisory Committee December 2004. December 2005 http://www.aaae.org/government/300_ Environmental_Affairs/_pdf/report12-13-04.pdf

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