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Air Quality Program

Wood-fired Hydronic Heaters

A photograph of 
		an outdoor boiler showing lots and lots of thick white smoke.

Outdoor wood-fired hydronic heaters, also called outdoor wood-fired boilers, use wood to heat water (or water and anti-freeze) which is piped to buildings to provide heat and hot water. Unfortunately, some of them can emit more than fifty times as much smoke per hour as a Washington approved wood stove. The air pollutants in this smoke are bad for your health and that of your family and neighbors. Some wood-fired hydronic heaters can also be located indoors. These indoor hydronic heaters are subject to the same emission regulations as the outdoor devices.

Are any outdoor wood-fired hydronic heaters legal in Washington?

Not at this time. EPA has a voluntary program for manufacturers of wood-fired hydronic heaters. Devices accepted into that program meet stricter guidelines and pollute less than older models. However, test results for this program have been achieved using a hardwood (oak) test fuel, which may under-report the amount of smoke emitted compared to other wood-burning devices.

Manufacturers who wish to sell wood-fired hydronic heaters in Washington State must have the device tested using EPA Method 28 WHH or ASTM E2618 but using Douglas fir test fuel; and submit test results to Ecology showing the device emits no more than 4.5 grams of fine particles per hour. This rate limit applies to both the annual average emissions and the highest individual test run emissions.

Are any indoor residential wood-fired hydronic heaters legal in Washington?

Yes. Several manufacturers have met the conditions outlined above and are approved for sale here.

What causes outdoor wood-fired hydronic heaters to smoke?

Some models use inadequate combustion technology that cuts the air supply to the firebox when a set temperature is reached. This causes large amounts of smoke. Most of the smoke emitted is fine condensed organic material that does not burn under cool, low-oxygen conditions. Burning wood that is too wet can also cause poor combustion and heavy smoke. Our State law says wood to be burned must have less than 20 percent moisture content.

How can smoke from burning wood be harmful?

Wood smoke from wood stoves, wood-fired hydronic heaters, and other similar devices is harmful because it contains very fine particles. These particles are so small they can be carried deep into the lungs where they cause significant health problems such as 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 children, the elderly, and people with heart and lung problems. It is estimated that fine particle air pollution costs the citizens of Washington hundreds of millions of dollars each year in health care costs and lost productivity due to illness.

If I want to burn wood, what should I do?

  • Buy a wood burning device that is certified by EPA, and that is as clean and efficient as possible.
  • Consider your neighbors. Burning wood in dense residential neighborhoods may never be a good idea, but it is especially bad during temperature “inversions,” when the air is still and cold.
  • 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.

Learn more about the health effects of wood smoke pollution:

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