* The Empyre Pro 200 is identical to the Greenwood Aspen
** Average output when burning a load of wood over 8 hrs. Maximum outputs
may be considerably higher.
*** Must be installed with a properly sized buffer tank as described
in the owner's manual.
NOTE: All of the OWBs listed above meet the definition of "Outdoor
Wood-Fired Boiler" in Vermont's regulations because the manufacturer has
indicated they should or may be installed outdoors or in uninhabited structures.
However, some of the listed boilers (the Flex-Fuel, Pellematic and Garn models)
are primarily sold for installation inside residences or other climate controlled
noted on US EPA's Hydronic Heater website
(http://www.epa.gov/burnwise/owhhlist.html) the "energy efficiency numbers
that have been calculated using the current test procedure are generating
numbers that do not represent actual efficiencies." For this reason and to
be consistent with US EPA, the efficiency column has been removed from the
above table. Please bear with us while US EPA reviews this issue.
Choosing an OWB: All the Phase II OWBs are far more efficient and
less polluting than the uncertified OWB models. Although some of the efficiency
based test results are questionable, the grams per hour emission rates listed
above should be correct. For comparison, the US EPA standards for indoor
wood stoves are 7.5 grams/hr for non-catalytic and 4.1 for catalytic stoves.
The emissions from most recently designed indoor woodstoves are approximately
half the level of the wood stove standards during tests. The grams/hr emissions
from most of the Phase II OWBs approach or exceed the woodstove standards
during laboratory testing, even though they produce at least twice to several
times the heat output (in BTUs).
THE NUMBERS GAME AND CHOOSING AN OWB
What do the numbers mean?
8-hr. heat output rating: If an OWB is loaded with wood and
burned at a rate such that the whole load took 8 hours to burn, it will have
produced heat at this hourly rate. Note that this rating may not be very
useful and it does not reflect the maximum heat output which may be very
much higher than the 8 hour average output rating. In most cases, burning
continuously at the maximum rated output would burn the load of wood in less
than 8 hours. Important Note: Purchasers of OWBs should always check with
the manufacturers, a knowledgeable boiler dealer or a heating specialist
to determine proper sizing for their heating needs.
Emission Rates: The following three paragraphs describe the
tested particulate (i.e. smoke) emission rates for the boilers listed in
the table. In all cases, the lower the number, the "cleaner" the boiler.
Average Emission Rate (grams/hr): A gram is a measure of weight
and this number describes the weight of solid particles (particulate matter)
emitted per hour, as determined during a specific laboratory test. The greater
the particulate emissions, the denser the visible smoke emitted from the
boiler. For comparison, the EPA standard for indoor woodstoves is 7.5 g/hr
for noncatalytic woodstoves and 4.1 g/hr for stoves with catalysts. Most
new woodstoves on the market do much better than this with some emitting
less than 1 g/hr of particulate.
Average Emission Rate (lb/mmBTU heat output): A BTU
(British Thermal Unit) is a unit of heat. Each cord of good dry hardwood
fuel contains about 30 million BTUs of heat but some heat is lost to gases
going out the stack. The "lb/mmBTU heat output" is a measure of the pounds
of particulate (smoke) emitted for each million BTU of heat output (the heat
that can be used to heat your home and water). The cleaner the OWB, the less
lbs of particulate (smoke) is emitted per million BTUs (mmBTU) of usable
heat produced. This is the critical number to compare to Vermont's Phase
II standard of 0.32 lb/mmBTU of heat output (see column 7 in the table).
All certified OWBs must emit less than this standard during laboratory testing.
Furnaces with the lowest number in this column not only have to be clean
burning, but are also good at transferring the heat into the water that gets
pumped into your house (i.e. they are most efficient).
Important Notice: Vermont has adopted a "Phase II"
particulate emission standard for OWBs. Vermont's "Phase II" emission standard
is 0.32 lb/mmBTU of heat output. This is the same as other
northeastern states that have already adopted this stricter standard as well
as the US EPA voluntary program. OWBs meeting this standard are cleaner and
more efficient, requiring less wood to heat your home.
Choosing an OWB
Choosing a wood heating source is an important decision. Consider having
an energy audit and increasing the efficiency of your home no matter what
device you choose. Burning wood is labor intensive and you want the most
benefit from your efforts.
There are many wood heat options including indoor woodstoves, indoor
wood boilers, masonry heaters, pellet stoves or boilers and outdoor wood
and pellet boilers.
Burning regular firewood is not only labor intensive but requires a
great deal of good storage space to dry the wood properly. Wood that is not
dried properly will not give the maximum heat and will produce more particulate
Pellets can be purchased in smaller or larger quantities and may be
easier to handle, though supplies may be uncertain. Woodstoves can heat without
power but pellet stoves and outdoor units require electric power to operate.
Also consider that some energy is lost in the underground piping for outdoor
installations and some energy is required to run the pumps and other electrical
equipment on OWBs.
Sizing a wood burning device:
In general, for any wood burning device the smaller the better. Oversized
units tend to burn at lower temperatures (less efficiently) much of the time.
One exception is the use of very large water tanks to store the energy from
your boiler. In this case, the heating device can burn at a maximum rate
(usually the most efficient rate) and the excess energy is stored in the
water tank for use over time. Your heating requirements are unique to your
house and your family, especially if you're heating domestic hot water. It
is best to discuss your heating needs with a professional.
If you have been burning oil or gas, you can calculate the total yearly
energy use from those fuels as an estimate of your heating demand. It is
important to include the overall efficiency of the heating devices when
performing these calculations.
Exhaust from any combustion device should be considered a potential
problem. Locate your OWB so there is little chance that the exhaust will
impact on your house, your neighbors' homes or where children are likely
to play. Even the cleanest wood burning device will emit toxins, including
potentially high levels of carbon monoxide.
Locate the OWB reasonably close to your house to minimize the loss
of heat from the underground piping. The location should also be readily
accessible even when the snow gets deep. Good covered wood storage near the
boiler is also essential.
By Vermont regulation, any uncertified or Phase I OWB installed after
October 1, 1997 must be at least 200 from any neighbor's residence. It may
not be legal to install an OWB in a tight residential neighborhood. The setback
requirement for installation of Phase II OWBs is 100 feet from another person's
residence, school or healthcare facility.
Questions to ask your dealer:
Is the OWB model certified by the State of Vermont?
How long is the warranty and what is covered?
How long have the units been on the market?
What is the thermal efficiency of the unit?
What kind of maintenance is required?
What are the installation requirements?
How large a space will the unit heat?