New Program to Cut Outdoor Wood Boiler Emissions


Outdoor wood boilers (OWBs) have enjoyed increased popularity in recent years as homeowners, particularly in rural areas, have looked for lower cost alternatives to oil or gas heat. At the same time, the boilers’ reputation for high emissions levels and low efficiency has caused some states and municipalities, in the absence of federal oversight, to regulate them. According to a report released in 2005 by the New York State attorney general, sales of OWBs in the state tripled between 1999 and 2005. Now the U.S. Environmental Protection Agency (EPA) has announced a voluntary program aimed at cutting the emissions of OWBs and educating consumers.

Looking like a small shed situated near a home, each OWB consists of a firebox where wood is burned to heat a surrounding reservoir. The heated water is piped underground to the home, where it is often connected to both the heating and domestic hot water systems. To operate the OWB, a user loads the firebox and manually lights the fire, which then burns untended, often for up to 24 hours. A damper, controlled by a thermostat inside the home, controls the burn rate of the firebox; when the desired temperature is reached, the damper closes, depriving the fire of oxygen and causing it to smolder until more heat is required.

Compared to indoor wood stoves, OWBs are relatively inefficient at turning fuel into heat. EPA has found conventional wood stoves to have an average efficiency of 54%, with EPA-certified stoves reaching up to 72%. According to the New York report, OWB models range in efficiency from 28% to 55%, with an average of 43%. In addition, OWBs produce a large amount of smoke, which contains particulates and causes air quality concerns.

According to Phillip Etter, an environmental analyst with the Air Pollution Control Division of the Vermont Department of Environmental Conservation, “Several states want to regulate these [boilers], but the regulatory process is long and involved.” The Northeast States for Coordinated Air Use Management (NESCAUM) hopes to streamline that process and, with support from EPA, has developed a model rule that states and cities can use to create regulations for OWBs. The rule created a two-phase emissions reduction plan that requires new OWBs to limit their emissions of particulate matter to 0.44 pounds per million Btus (lbs/MMBtu; 0.19 g/MJ) by March 2008 and 0.32 lbs/MMBtu (0.14 g/MJ) by March 2010. (By comparison, an EPA-compliant woodstove producing 50,000 Btus per hour that emits 5 grams per hour would emit 0.22 lbs/MMBtu (0.095 g/MJ.)

Etter told EBN that Vermont, Massachusetts, Maine, and New York are all looking to adopt some form of the NESCAUM rule, and some Midwestern states, including Michigan and Wisconsin, are also looking at regulating OWBs. According to Tom Todd of the Air Quality Program at the Washington Department of Ecology, that state has banned the sale of all solid-fuel-burning devices that do not meet its particle emissions limits; this includes OWBs, which could not be effectively tested when the law was written.

The new voluntary program announced by EPA would allow manufacturers whose OWBs meet emissions requirements to mark their products with a tag identifying them as EPA-certified. To qualify, manufacturers must subject their products to a third-party test adapted from EPA’s method for testing woodstoves. Qualifying OWBs must meet a particulate emissions limit of 0.6 lbs/MMBtu (10.25 g/MJ)—higher than NESCAUM’s model rule—making them up to 70% cleaner than many boilers currently on the market, according to EPA. Ten manufacturers have signed agreements, and EPA says the first certified products should appear on the market in April 2007. “Manufacturers are aware that states want to regulate these units,” Etter said, so they’re willing to work with EPA on the standards.

The EPA standard “is a notch less stringent than the NESCAUM model rule,” Etter told EBN, “but there will be cleaner, more efficient units available to consumers.” Although the standard does not explicitly require increased efficiency, Etter said that decreasing emissions requires more efficient burning. Todd was more skeptical about the effectiveness of the EPA program, arguing that NESCAUM’s regulatory approach stood a better chance. He said, “If the customer has the choice to buy a cheaper and dirtier OWB, versus a more expensive but cleaner one, I fear that not enough of the cleaner ones will be purchased.”

For more information:

Environmental Protection Agency Wood-Fired Hydronic Heaters Program

Northeast States for Coordinated Air Use Management

Air Pollution Control Division
Vermont Department of
Environmental Conservation

March 1, 2007


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