Outdoor wood-fired boilers, also called outdoor wood heaters, outdoor wood furnaces, or outdoor wood-fired hydronic heaters, are being considered as a heating option for some families. Use of the heaters has increased in recent years, prompting complaints about smoke and concern about the particle pollution the units produce. While indoor woodstoves have been tested and certified by EPA for emissions since the late 1980s, outdoor wood boilers are not. The burners cause dense smoke that endangers the health of you, your family and neighbors.
What are Outdoor Wood-Fired Boilers?
Outdoor wood-fired boilers are wood-fired water heaters that are located outdoors or are separated from the space being heated. The fires in the large fireboxes heat water that is circulated into the home through underground pipes.
Why Is There Controversy?
Most outdoor wood boilers employ very primitive combustion technology. When the water circulating through the furnace reaches an upper set point (usually around 180F) 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 outdoor wood-fired boiler 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.
Download presentation by the Washington State Department of Ecology.
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;
- chronic heart problems.
For more information about outdoor wood boilers, visit the: EPA Burn Wise Web site.