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About Outdoor
Boilers
A Controversial Wood Heating Technology |
"We found that while OWBs are advertised
as a clean and economical way to heat one's house and water, OWBs may be
among the dirtiest and least economical modes of heating, especially when
improperly used."
NY Attorney General's
Report |
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What are Outdoor Wood-fired Boilers
(OWBs)?
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OWBs are residential or small commercial 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.
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Why Are the New Certified OWBs
Better?
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Old Style OWBs tend to cause dense smoke that impacts neighbors who
complain about nuisance and health problems. Most OWBs come equipped with
very short stacks. The smoke from these low stacks disperses poorly. In addition,
the owners often operate the OWBs to heat hot water or swimming pools during
the summer when neighbors have their windows open and are trying to enjoy
the outdoors.
New certified OWBs burn the wood gases more completely and extract
the heat more efficiently causing less smoke. Owners will need to burn less
wood in the new certified OWBs because they are so efficient.
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What Causes Old Style OWBs To
Smoke?
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Most old style 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.
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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.
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Are Old Style OWBs Worse Than Indoor
Woodstoves?
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Yes. Newly manufactured indoor woodstoves are required to meet
strict US EPA particulate emissions standards, 4.1 grams per hour for catalytic
stoves and 7.5 g/hr for noncatalytic stoves. Certification tests are conducted
in EPA approved laboratories. As expected, emissions during actual use are
somewhat higher.
In contrast, the New York State Attorney General's office found that
average emissions during laboratory testing of OWBs was 71.6 g/hr or roughly
ten times the particulate emission rate from indoor woodstoves. 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.
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Is Natural Wood Smoke Harmful?
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Yes, all wood smoke is harmful, but smoke from old OWBs is worse due
to the poor combustion and large amounts of smoke emitted. While smoke and
gases from burning fossil fuels, such as oil, contain air pollutants like
sulfur dioxide, the smoke from wood burning contains much higher levels of
small particles. In fact, the vast majority of particulate emitted by OWBs
is very fine (less than 2.5 microns in size) and can become trapped in the
delicate air exchange sacs deep in your lungs when inhaled. Numerous studies
have found strong relationships between high fine particulate levels and
chronic lung diseases, cardiovascular disease and premature death. According
to the American Lung Association, the fine particulate found in woodsmoke
can be linked to higher school absenteeism, emergency room visits and
hospitalizations for cardiopulmonary conditions, respiratory infections and
asthma.
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I Want To Burn Wood-What Should I
Do?
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First, consider your neighbors. Burning wood in some dense residential
neighborhoods may never be a good idea.
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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.
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If you have an older non-certified wood stove, consider purchasing a
cleaner, more efficient EPA certified woodstove or pellet stove.
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Consider an efficient indoor wood boiler that may include a large hot
water storage tank. Operated properly, these units cycle less and burn hotter
and cleaner.
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If you like the idea of an outdoor wood boiler, purchase one of the
new units certified to meet Vermont’s emissions standard. After March
31, 2008, dealers are prohibited from selling uncertified OWBs in Vermont
or for installation in Vermont. Purchasing an OWB is a big investment; make
it wisely.
For additional information about OWBs, contact
the Vermont Department of Environmental Conservation, Air Pollution Control
Division. Download Information in pdf files:
Fact Sheet (2pgs)
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Vermont Department of Environmental
Conservation
Air Pollution Control Division
©
Copyright VT DEC, All Rights Reserved
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