Washington State Department of Ecology

BY PROGRAM

FIND INFO ABOUT

Air Quality Program

Air Quality > Outdoor Wood-Fired Boilers

Outdoor Wood-Fired BoilersA photograph of an outdoor boiler showing lots and lots of thick white smoke.

With the rising cost of heating oil, more Washingtonians are looking to wood as a source of heat and hot water. Not all wood heat is the same. While indoor wood stoves have been tested and certified by EPA for emissions since 1990, outdoor wood boilers (OWBs) are not. OWBs cause dense smoke and are equipped with very short smoke stacks so the smoke does not disperse well. This smoke endangers the health of you, your family and neighbors as well as the environment.

We want you to have the facts before making an investment. Because of the way they are manufactured, OWBs are not allowed in the state of Washington. This site is designed to help you get the facts. If you have any questions about OWBs, please contact us.

What are Outdoor Wood-fired Boilers (OWBs)?

OWBs are wood-fired water heaters that are located outdoors or are separated from the space being heated. The fires in the large fire boxes heat water that is circulated into the home through underground pipes. The energy may be used to heat houses, shops, domestic hot water, greenhouses, swimming pools and spas. Indoor installed boilers are a variation of an OWB. They are in the same legal category as OWBs, and subject to the same regulations.

Why Is There Controversy?

OWBs tend to cause dense smoke that impacts neighbors by creating a nuisance and health problems. Most OWBs come equipped with very short stacks. The smoke from these low stacks disperses poorly. These units are intended to be operated year around, so smoke is emitted constantly. In addition, because of the way they are manufactured, OWBs are prohibited in the state of Washington.

What Causes OWBs To Smoke?

Most OWBs employ very primitive combustion technology. When the water circulating through the furnace reaches an upper set point (usually around 180°F) the air supply to the fire is cut-off, cooling the fire so the water will not overheat. The furnace operates in this "idle" mode until the water temperature hits a lower set point and the air supply is re-established. Under some conditions, the OWB may be in idle mode far longer than in operating mode. This type of operating causes very poor combustion and heavy foul smoke. Most of the smoke emitted is fine condensed organic material that does not burn under cool, oxygen starved conditions. In addition, many owners burn green wood full of moisture which also causes poor combustion. Wood from the outdoor winter wood pile may be very cold when loaded into the OWB causing an even colder fire.

Are OWBs Worse Than Indoor Woodstoves?

Yes. Newly manufactured indoor woodstoves are required to meet strict Washington State particle emissions standards, 2.5 grams per hour for catalytic stoves and 4.5 g/hr for noncatalytic stoves. Certification tests are conducted in EPA approved laboratories. In contrast, tests done by the Northeast States for Coordinated Air Use Management (NESCAUM) found that the average fine particle emissions (a particularly harmful pollutant) from one OWB are equivalent to the emissions from 22 EPA certified wood stoves, 205 oil furnaces, or as many as 8,000 natural gas furnaces. One OWB can emit as much fine particle matter as four heavy duty diesel trucks on a grams per hour basis. The smallest OWB has the potential to emit almost one and one-half tons of particulate matter every year. Although older style indoor wood stoves emit more than new certified stoves, they are still several times less polluting than OWBs. Due to the poor combustion conditions, it is also probable that OWBs emit proportionately more benzene, polycyclic aromatic hydrocarbons, formaldehyde and other toxic partial combustion products which have been linked to asthma, heart attacks and cancer.

Is Natural Wood Smoke Harmful?

Yes, all wood smoke is harmful. Fine particles from smoke emissions can be carried deep into the lungs and can be responsible for significant health problems, including asthma, lung diseases, heart diseases and death.  These particles can also increase the risks of certain types of cancer.  Breathing smoke is especially harmful for sensitive populations, including children, the elderly, and people with heart and lung ailments.  It is estimated that fine particle air pollution costs citizens of Washington State hundreds of millions of dollars each year in health care costs and lost productivity due to illness.

I Want To Burn Wood-What Should I Do?

  • First, consider your neighbors. Burning wood in dense residential neighborhoods may never be a good idea.
  • Have an energy expert inspect your home.
  • You may find that more insulation or other energy saving improvements may be a better investment than an expensive wood heating device.
  • If you have an older non-certified wood stove, consider purchasing a cleaner, more efficient natural gas, propane, pellet or EPA certified woodstove.

Law and Policy

The Revised Code of Washington mandates that no uncertified solid fuel burning device shall be installed in new or existing buildings. It also requires that an adequate source of heat other than wood stoves most be installed in all new and substantially remodeled residential and commercial construction that fall within urban growth areas or areas designated by EPA as being in nonattainment for particle pollution.

A solid fuel burning device is any device that burns wood, coal, or any other non-gaseous or non-liquid solid fuels for aesthetic or space-heating purposes in a private residence or commercial establishment, which has a heat input less than one million British thermal units (Btu) per hour. Virtually all devices used to heat a home would produce less than 1 million Btu per hour.

Therefore, by state definition, an outdoor wood-fired boiler is a solid fuel burning device. While EPA regulations still exempt certain solid fuel burning devices from testing and/or certification requirements, Washington regulations require that all solid fuel burning devices be tested and certified to comply with the more stringent Washington particle emission limits before it may be offered for sale in Washington. Any retailer, wholesaler or distributor offering for sale or selling a non-compliant device may be subject to formal enforcement action.

Because wood-fired boilers are not manufactured in a manner that would allow it to be tested to determine if it meets Ecology’s standards, they are prohibited in the state of Washington.

Complete text of the Washington state solid fuel burning device rule can be found at http://www.ecy.wa.gov/pubs/wac173433.pdf.

Indoor installed boilers are a variation of an OWB. They are in the same legal category as OWBs, and subject to the same regulations.

If you have any questions, please call Tom Todd at (360) 407-7528.

Effects on Health and the Environment

While all smoke is harmful, outdoor wood boilers generate more particle pollution ("soot") than indoor wood stoves. The units are designed to burn wood at lower combustion temperatures and generally have shorter stacks which emit smoke at house level. Wood smoke releases fine particles, carbon monoxide, and other toxic pollutants.

Breathing air containing wood smoke can;

  • reduce lung function, especially in children;
  • increase severity of existing lung diseases such as asthma, emphysema, pneumonia and bronchitis;
  • aggravate heart disease;
  • increase susceptibility to lower respiratory diseases;
  • irritate eyes, lungs, throat and sinuses;
  • trigger headaches and allergies.

Long term exposure to wood smoke may lead to:

  • chronic obstructive lung disease;
  • chronic bronchitis;
  • increased risk of cancer and genetic mutations (based on animal studies);
  • cardiovascular disease.

For more information, please visit

How to control wood smoke pollution:

Other State Information

Other Air Quality Web Sites

PDF forms and other type documents may require one of these free readers.

Contact Us

Copyright © Washington State Department of Ecology. See http://www.ecy.wa.gov/copyright.html.

  • Access Washington, Official State Government Web site