Vermont DEC Air Pollution Control Division

Outdoor Wood-Fired Boilers
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Science & Testing of Outdoor Boilers

According to Connecticut officials, the emission from one (old style) outdoor wood furnace is equivalent to the emissions from 3,000 to 8,000 gas furnaces.
The science of air pollution is very complex and technical. Below is a "Technical Primer" giving the basics to help to intrepret the research and technical documents.  See the Links below for documents on the science and technology of OWBs.

Technical Primer

Particulate Matter (PM): PM is simply particles that get into the air and can get into our lungs when we breathe. Particles can be composed of sand, dust, flyash or organic chemicals that have condensed together. PM can be larger particles or very fine microscopic particles. The finest PM is called PM2.5 or fine particulate that is 2.5 microns and smaller. These fine particles are most able to penetrate deeply into lungs causing various health problems. The visible smoke from wood burning is mostly PM, the vast majority being PM2.5.

Air Toxics: All the air we breathe contains some toxins in the form of various chemicals including volatiles (like solvents), heavy metals and particulate in various forms. All wood smoke contains toxins, the type and amounts depend on how completely and efficiently the wood is burned. In general, the hotter the fire, the less toxins are produced assuming an adequate oxygen supply.

Efficiency: A much used term with many interpretations. In the wood heating world, thermal efficiency is probably the most important because it is a measure of the how much of the potential heat in the fuel is transferred into useable heat. But even this term is more complicated than this simplistic definition because it can be calculated in different ways. The term "combustion efficiency" that you may see in ads for some wood combustion devices is vague and probably meaningless. It may mean that some stated percentage of wood is burned but it has no relationship to how much useable heat is produced. Be wary of all efficiency claims.

Emissions Standards: Emissions standards are limits put on the amount of pollution emitted by an air pollution source. Standards can be expressed in different ways and are rarely understood by those outside the air pollution field.

  • Mass per unit time: Generally expressed in units such as grams per hour or pounds per hour of some pollutant such as PM. US EPA standards for indoor woodstoves are expressed in grams per hour.
  • Mass per energy input: This is the weight or mass of the pollutant per unit of energy in the fuel expressed as grams or pounds per BTU, mmBTU (million BTU), Joules or Megajoules. This type of standard does not consider thermal efficiency.
  • Mass per energy output: This type of standard considers the efficiency of the device and is expressed in units of mass per unit of useable energy produced by the device.
  • Concentration Standards: The most difficult to understand and often expressed as mass (lbs, grams or kilograms) per unit volume (cubic feet or cubic meters) or mass of exhaust gases. Chemical or toxic emission standards may be expressed in parts per million (ppm). These standards often require correction to a percentage oxygen or carbon dioxide in the exhaust gases.

Testing: "Stack testing" is done by drawing a sample of the exhaust gases out of the stack to determine the amount of contamination in the gases. The details of testing are extremely complicated and technical and there is a diversity of formal test methods for testing the various pollutants from a variety of devices. An explanation of stack testing is beyond the scope of this website but how air pollution testing is done and how the results are expressed are critical factors when trying to evaluate wood burning devices.

Ambient Monitoring: Ambient monitoring is testing the general outdoor air to determine the levels of air contamination. This type of testing, although equally technical, is done differently from stack testing. Ambient concentrations of pollutants are generally far less concentrated than the pollutants measured in the exhaust gases of stacks. Ambient monitoring may be done in an area away from specific sources of air pollution just to determine general air quality or it may be done near an air pollution source to try to determine the impacts of a specific source's emissions on ambient air quality. See Vermont's ambient monitoring website at http://www.anr.state.vt.us/air/Monitoring/index.htm for more information.

Air Pollution Modeling: Computer models can be used to estimate the impacts of pollutants emitted from sources of air pollution. Such information can be used to estimate whether or not pollutants from sources will cause pollutant levels that are a health concern or that exceed certain set standards. Detailed protocols have been established for the use of the various types of air pollution computer models. Predictions by such models are useful but are dependent on the data input to the model, such as stack height, emission rate and stack gas temperature. Weather conditions greatly influence the dispersion of pollutants and weather data can be input to many of the model programs to predict pollutant impacts under different weather conditions.

Science Links

An Assessment of Risk from Particulate Released from Outdoor Wood Boilers - Brown et al (pdf)

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European Biomass Boilers (pdf)

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Wood Burning Primer (pdf)

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Chemical Characterization of Fine Particle Emissions from Fireplace Combustion of Woods Grown in the Northeastern United States - Fine, Cass, Simoneit 2001 (pdf)

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Design, Construction & Performance of Stick-wood Fired Furnace, Hill '79 (pdf)

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Design and Operating Factors Which Affect Emissions from Residential Wood-Fired Heaters: Review and Update - Paul E. Tiegs 1995 (pdf)

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Domestic Wood Burning & PM 2.5 trace elements (pdf)

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Effect of Airflow Setting on the Organic Composition of Woodheater Emissions - Timothy Jordan & Andrew Seen  2005 (pdf)

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Emission of PCDD/F, PCB, and HCB from Combustion of Firewood and Pellets in Residential Stoves and Boilers - Hedman, Naslund, Marklund 2006 (pdf)

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Emission characteristics of modern and old-type residential boilers fired with wood logs and wood pellets - Linda S. Johansson et al 2003 (pdf)

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EPA- Emissions from Outdoor Wood-Burning Residential Hot Water Furnaces -Valenti, Clayton 2/98  (pdf)  Summary (pdf)

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EPA -Residential Wood Combustion Technology Review Vol 1. Report - Houck, Tiegs 1998 (pdf)

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In-Field Fine Particulate Monitoring of an Outdoor Wood Boiler:
Public Health Concerns
- Johnson, NESCAUM 06 (pdf)  Full report (pdf)

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Low emissions from wood burning in an ecolabelled residential boiler - Maria Olsson et al. 2005 (pdf)

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Model development for spatial variation of PM2.5 emissions from residential wood burning - Yong Q. Tiana et al. 2004 (pdf)

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Modelling PM10 concentrations and carrying capacity associated with woodheater emissions in Launceston, Tasmania - Luhar, Galbally, Keywood 2006 (pdf)

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NESCAUM Source Characterization of Outdoor Wood Furnaces (pdf) Sept. 2008

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NESCAUM Source Characterization of Outdoor Wood Furnaces - Appendix (pdf) Sept. 2008

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NESCAUM Report on Outdoor Wood Furnaces pdf 3/06

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NY DEC Modelling Assessment (pdf) &
EPA Review of NY DEC Modelling Assessment (pdf)

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OMNI Test Labs- Publications

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OWB Emissions Modeling - What's It All About? (ANR) (pdf)

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 Air Pollution Dispersion Modeling for Outdoor Wood Boilers in a Complex Terrain Setting (ANR) (pdf)

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PCDD/F, PCB, HxCBz, PAH, and PM Emission Factors for Fireplace and Woodstove Combustion in the San Francisco Bay Region - Brian K. Gullet et al  2003 (pdf)

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Polycyclic aromatic hydrocarbon size distributions in aerosols from appliances of residential wood combustion as determined by direct thermal desorption-GC/MS - Michael D. Hays et al 2003 (pdf)

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Smoke Gets in Your Lungs: State of NY, Office of the Attorney General (pdf)

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What is Particulate Matter? American Lung Association (pdf)

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 Woodsmoke Health Effects: A Review (pdf)

European Data

Technology Developments of boilers for domestic heating with wood fuels - Austria, April 2007 (pdf)

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Heating Boiler for chipped wood - Austria 2002 (pdf)

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Health Relevance of Aerosols from Biomass Combustion in Comparison to Diesel Soot Indicated by Cytotoxicity Tests - Switzerland (pdf)

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Testing Standards and Emission Requirements for small boilers in Europe - (pdf)

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Outline of European Standard EN303 - (pdf)

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Round Robin Test of a Wood Stove - GB (pdf)

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Swan labelled boilers for solid bio-fuels - (pdf)

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Swan labelled boilers for solid bio-fuels - 2008 (pdf)

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Wood Energy in Switzerland 1999 - (pdf)

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Review of European Small Boiler Regulation and Technology - (ppt)

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Vermont Department of Environmental Conservation
Air Quality & Climate Division

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