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FIREPLACE COMPARISON PAGE
Sorted By: EPA EMISSIONS RATING
A few short years ago, typical airtight woodstoves dumped 40 - 60 grams of unburned wood particles into the atmosphere every hour. Today,
all woodstoves sold in the US must comply with EPA regulations limiting particulate emissions to 7.5 grams/hour. The woodstoves we sell are
even cleaner burning, with emissions ratings of 4.5 grams or less. To read more about EPA woodstove regulations, click here.
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Note: Woodstove EPA emissions ratings don't tell the whole story.
A difference of a couple of grams or so in the emissions rating wouldn't be a good reason to choose one woodstove over another. Great care is
taken by the EPA testing laboratories to ensure that each test is as close as possible to the next (for example, identical "charges" of pine wood are
used in all tests), but uncontrollable factors such as atmospheric conditions and variables in personnel and testing equipment from one lab to
another can affect the results of a given test burn. To compensate for these minor variances, the final EPA grams/hr rating is derived by averaging
several monitored burns. As a result, differences of even a gram or two in the final emissions rating between one model and another are not
considered significant (except, of course, by the manufacturer of the model that lucked out and got the lower rating).
Here's an exerpt from an article written by Dr. James Houck, President of OMNI Testing Labs, where EPA wood stove emissions testing is
"... the EPA certification testing protocol has an uncertainty of at least ±20 % at low emissions rates; models with emission values of less than 2.5
grams/hour could easily be equivalent to [models] with emission values of less than 2 grams/hr."
So, let's look at three EPA approved low-emission wood stoves, with just that minimum ±20 % margin of uncertainty in mind:
Stove A: 7.5 grams/hr
Stove B: 6.0 grams/hr
Stove C: 4.8 grams/hr
As you can see, Stove A scored 20% higher than Stove B, and Stove C scored 20% lower. According to Omni Test Labs, all three could have
produced equivalent emissions during testing. Yet the stove shopper who is hung up on emissions ratings perceives Stove C as having a whopping
2.7 gram/hr advantage over Stove A!
I'm just beginning to educate myself on modern-day wood stoves (been using a Fisher for decades) and I've seen stoves online for example, where
one has an efficiency rating of, say, 75% with emissions at 3.5 gm/hr., and another stove will have efficiency of 72% w/ emissions at 2.3. I can't
make sense of that. Can you shed some light on how these calculations are derived?
Thanks for the inquiry! A woodstove's heating efficiency rating doesn't correlate directly to its emissions rating. In fact, those numbers are arrived
at in two different tests:
Emissions testing is performed in EPA-approved test labs using the EPA's prescribed protocol. When testing for emissions, a standardized
"charge" of kiln-dried Pine is burned, and the particulate matter in the exhaust is measured throughout the duration of several fires at various
draft control settings. In this way, an average grams/hour particulate emissions rating is derived. Heating efficiency is not measured during EPA
Heating Efficiency testing is performed using full loads of seasoned cordwood, and is designed to measure how much of the heat value contained in
the wood is extracted and delivered into the living space. When testing for heating efficiency, the following criteria are examined:
Extraction Efficiency: the load is weighed going in, and the particulate emissions and ashes are weighed after the fire to determine how effectively
a given firebox design breaks down the fuel to extract the available heat.
Heat Transfer Efficiency: this testing is performed in calorimeter rooms equipped with temperature sensors. Similar temperature sensors are
installed in the exhaust flue. The degree changes in the room and flue are monitored during the test fires to determine how much of the heat
extracted by the fire is delivered into the room, as compared to the heat lost up the flue.
Although a low emissions weight can improve a given stove's Extraction Efficiency score, the total amount of particulate emissions produced by
today's EPA approved woodstoves is so small that the affect of particulate emissions on the overall heating efficiency score is negligible. Thus,
even a model with an unusually low Emissions rating doesn't necessarily score a high Heating Efficiency rating.
Consider the two models you mention: in Extraction Efficiency testing, the 1.2 gram/hr difference would lighten the total weight of emissions from
the cleaner-burning stove by about 10 grams at the end of an 8-hour test burn. Most likely, those 10 grams would be found in the ash remaining in
the firebox, eliminating any advantage in Extraction Efficiency. Even if the 10 grams weren't found in the ashes, a difference of 10 grams of
particulates from a 40 lb. load of wood would only amount to a .06% advantage in Extraction Efficiency, which would have virtually no affect on the
overall Heating Efficiency score.
Q: I'm shopping for a new EPA approved wood stove, and am intrigued by my neighbor's catalytic model that scored 0.8 grams/hr in the EPA
emissions test. This seems far lower than the non-catalytic models you show on our emissions comparison chart. My neighbor admits he has to
"blow out" the converter every once in awhile and soak it in vinegar water once a year to keep it working, but this doesn't seem like much of a
price to pay for much cleaner burning. Am I missing something, or are catalytic woodstoves just that much better than non-catalytics?
Actually, the EPA requires new woodstoves with catalytic converters to test with lower emissions levels than non-catalytic models. This is because
the catalyst degrades over time, becoming progressively less and less effective at reburning the wood gases until the reburn finally stops
altogether, at which point the converter must be replaced. The predicted lifespan of the catalyst varies from manufacturer to manufacturer, but
seems to average about 3-5 burning seasons or so. Periodic cleaning with blasts of air or vinegar baths might help improve the performance of an
aging catalytic converter by removing flyash deposits which impede the catalytic reaction, but won't restore the converter to its original state: the
degradation process continues with every fire. In contrast, non-catalytic burn chambers don't become less effective over time, and retain the same
emissions level throughout the lifetime of the stove. Any "catalytic stove emissions vs non-catalytic stove emissions" debate should take this into
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