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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) ||
Informational Sheet (4pgs)
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Vermont Department of Environmental
Conservation
Air Pollution Control Division
©
Copyright VT DEC, All Rights Reserved
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