Frequently Asked Questions
        I      EPA's Phase 2 Voluntary Partnership
                       Program: Hydronic Heaters
In January 2007, EPA launched a voluntary program to make cleaner hydronic heaters (also
called outdoor wood heaters or outdoor wood boilers) available for consumers. In the first phase
of the program, manufacturers produced 15 units that are 70 percent cleaner than unqualified

Less than two years later, EPA moved to Phase 2 of the program, aimed at making models
available that are 90 percent cleaner than unqualified units. Six of the original 15 models met the
Phase 2 levels when EPA announced the program in October 2008. Manufacturers have pledged
to produce even more Phase 2 units in the near future.
About Cleaner Hydronic Heaters

What is a Hydronic Heater?
     Hydronic heaters heat water that is piped to a nearby building (usually a home),
      providing both heat and hot water to the structure. An outdoor wood-fired boiler, which
      is sometimes called an outdoor wood heater, is an example of a hydronic heater.

     These heaters can be located inside or outside of the building to be heated. Hydronic
      heaters may use other biomass as fuel, such as corn or wood pellets. The EPA voluntary
      program does not include gas, oil, or coal at this time.  An outdoor hydronic heater
      resembles a small shed with a short smokestack. An indoor hydronic heater typically is
      located in the basement, but some are located in the living area.

     Most hydronic heaters are sold for use in rural, cold climate areas where wood is readily
      available; however, they can be found throughout the United States.

Are outdoor wood-fired heaters a problem?
     They can be. Many outdoor wood heaters are significantly more polluting than other
      home-heating devices. They can create heavy smoke, which can be a nuisance, in
      addition to posing risks to public health in populated areas. This is most likely when
      heaters are used improperly or located too close to homes.  Heaters that are qualified
      under EPA's voluntary hydronic heaters program are significantly cleaner.

How much cleaner are the new heaters than current outdoor wood heaters?
     Units that have a white hangtag meet EPA's Phase 2 emission levels and are 90 percent
      cleaner than unqualified units.
     Units that have an orange hangtag meet EPA's Phase 1 emission levels and are 70

       percent cleaner than unqualified units.

What kind of pollution comes from outdoor wood heaters? How much?
      Wood smoke contains a number of pollutants, the more prevalent of which is fine
       particles (PM 2.5).
      Most unqualified outdoor wood-fired heaters emit about 2 pounds of fine particle
       pollution per million BTUs of heat input (i.e., wood burned)  or about 0.83 tons per
      White hangtag units (those qualified under Phase 2 of EPA's voluntary program), must
       emit no more than 0.32 pounds of fine particles per million BTUs of heat output.

What are the health effects of smoke from hydronic heaters?
      Wood smoke contains both fine particle pollution (PM 2.5) and a number of air toxics.
       Fine particle pollution is linked to a variety of health problems, including aggravated
       asthma, reduced lung function, development of chronic bronchitis, irregular heartbeat,
       non-fatal heart attacks and premature death in people with heart and lung disease.
       Children, people with heart and lung disease, and older adults are the most vulnerable to
       the effects of fine particle pollution.

      Residential wood combustion emissions also contain the harmful air pollutants sulfur
       oxides, nitrogen oxides, carbon monoxide, and air toxics such as potentially  cancer-
       causing compounds including polycyclic aromatic hydrocarbons, benzene, formaldehyde
       and dioxins. Some of these pollutants are known to cause cancer but their effects on
       human health via exposure to wood smoke have not been extensively studied.

Several manufacturers of outdoor wood heaters have said the existing heaters are as clean
as indoor wood stoves. Is  this true?
      No. When we determine how clean or polluting a unit is, we look at total emissions per
       hour, along with stack heights and proximity to other structures to estimate emissions and
       their impact on air quality and health. Current outdoor wood heaters burn significantly
       more wood than wood stoves  often 10 times as much or even more. As a result, they
       emit much more smoke - and that means more particle pollution. Outdoor wood heaters
       also typically have short stacks that are close to nearby structures, meaning that the
       stacks are not usually above the roofline. As  a result, wood heater emissions do not
       disperse as well as the emissions from typical wood stoves.

Is it true that to qualify under the  EPA voluntary program, a hydronic heater must emit
less than a typical EPA-certified non-catalytic indoor wood stove?
      No. The test methods used for hydronic heaters in EPA's voluntary program and for
       indoor wood stoves  in EPA's New Source Performance Standard (NSPS) are closely
       related, and both originated from the same method but there are differences.  The wood
       stove NSPS emission standard cannot be directly compared with the hydronic heater
       qualifying emission level, however, because  the measurement units are different. Over
       85% of non-catalytic woodstoves sold in the US today  not only meet the NSPS but emit

       less than 4.5 grams per hour and some models emit only 1 or 2 grams per hour. A typical
       Phase 1 qualifying heater emits 4 to 10 times more particle pollution than a typical new
       non-catalytic wood stove and a typical Phase 2 qualifying heater emits 1.5 to 3 times
       more than a typical non-catalytic wood stove.

Do all areas allow hydronic heaters? How do I find out?
      Not all areas allow the use of hydronic heaters. And some areas allow their use but
       regulate emissions, opacity, location, and stack height. Your state or local air agency can
       tell you what your area allows. For information on how to reach your air agency, visit
       EPA's website at and click on "Where You Live."

What states and local governments  regulate or ban hydronic  heaters now?
      Many states and local governments use nuisance or opacity regulations to regulate
       hydronic heaters. A number of local governments ban new hydronic heaters and/or
       regulate the minimum distance they can be sited from neighbors or public areas. Five
       states - Connecticut, Maine, New Hampshire, Washington  and Vermont - have new
       regulations specific to heater use, and numerous states are considering regulations. Visit, and click on "Where  You Live," to learn more about state air
       programs and regulations for hydronic heaters.
About the Voluntary Program

Why has EPA created a voluntary program instead of regulating these heaters?
     EPA wants to reduce emissions from these heaters as quickly as possible. EPA's
      voluntary program has helped bring cleaner heaters to market several years faster than a
      traditional federal regulation process.

     EPA also provided technical and financial support for NESCAUM to develop a model
      rule that state, local and tribal governments can tailor to address their specific needs and

Is EPA encouraging the use of hydronic heaters?
     EPA does not encourage the sale of any particular heating device. However, we urge
      consumers who choose to purchase a hydronic heater that uses wood or other biomass as
      fuel to buy one of the cleaner units that qualify for EPA's voluntary program.

How many manufacturers have agreed to make cleaner units? How many models have
orange hangtags? How many models have white hangtags?
     Twenty manufacturers have agreed to use their best efforts to make cleaner models.
      These manufacturers represent more than 80 percent of the current sales in the United
      States.  To date, 15 models have qualified at the Phase 1 orange hangtag level; six of
      these already meet emission levels at the Phase 2  white hangtag level, meaning they are
      90 percent cleaner than unqualified units.

      Currently qualified models and their emission levels are listed on the EPA web site.
       Check the list frequently for updates.

Who verifies the manufacturers' emission performance results?
      Participating manufacturers have their heaters tested by independent EPA-accredited
       laboratories and the test reports are reviewed by EPA staff and/or EPA's Environmental
       Technology Verification Center.

How can a manufacturer join the partnership?
      Review the Partnership Agreement on the website and then contact EPA at to apply as a new partner.
Information for Consumers

Should I replace my old outdoor wood boiler or hydronic heater?
     Yes. If you have an old model, consider replacing it with a cleaner heating device that
      qualifies for the EPA white hangtag.

How do I know which units are the cleanest?
     Look for a white hang tag that indicates that a unit meets the Phase 2 performance level
      of EPA's voluntary program. These heaters are 90 percent cleaner than unqualified
      models. These heaters are also marked with a permanent label. Pictures of the labels and
      tags are on EPA's website.

     Units that have an orange hangtag are Phase 1 models and are 70 percent cleaner than
      unqualified models.

Where can I purchase a cleaner hydronic heater?
     Manufacturers sell directly to the public and also through dealers. A list of qualified
      models and manufacturers is available on the EPA web site.

How much does a hydronic heater cost? Will a cleaner heater cost more?
     Hydronic heaters fueled by wood, pellets and other biomass cost between $8,000 and
      $18, 000, depending on the size of the unit. The cleaner units may be more expensive -
      about 15 percent more - because of the changes made to improve  the efficiency of these
      units and reduce their emissions. However, most of these new models are significantly
      more efficient - meaning they will use less wood to produce the same amount of heat,
      reducing the cost of wood purchases.

Can I retrofit my existing wood-fired heater to make it cleaner?
     Some heater manufacturers and manufacturers of emission control devices are working
      on developing retrofits for wood heaters. One model is currently available; however, it

       has not yet been tested by an accredited, independent, third-party lab. EPA is
       encouraging the manufacturers to continue retrofit development and testing efforts.

Does it matter what people burn in hydronic heaters?
      Yes! Use of the wrong fuels in a hydronic heater can decrease the efficiency of your
       heater, increase pollution, and sometimes can be dangerous. If you have a hydronic
       heater, only use fuels recommended by the manufacturer - such as seasoned, untreated

      In addition, EPA urges owners of these devices to ensure that they are properly located,
       operated, and maintained. Short stacks and improper location too close to buildings and
       too close to neighbors can increase the amount of pollution to which people are exposed.

What about driftwood?
      Never burn driftwood - in a hydronic heater or any other kind of fire. Driftwood contains
       salt (sodium chloride), which contains chlorine. When improperly burned, salt can form
       dioxin, which is toxic even at very low levels.

What can I do to burn cleaner  and more efficiently?
      Follow the manufacturer's recommendations, especially fuel quality, loading times, and
       amounts. Visit EPA's website for more information and recommendations for safe wood
       burning practices at