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 interpret 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, fly ash, 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 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 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.