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EPA efficiency & heat output ratings:
Why do they differ from what I see on manufacturers' brochures?
Q: I am looking at two woodstoves, both about the same size, both recommended to heat up to a 2000 sq. ft. house. One stove's brochure lists a
"Cord Wood Maximum Heat Output" rating of 79,000 BTU/hr., while the other stove's brochure lists an "EPA" rating at a max 42,000 BTU/hr.
Exactly what is a BTU/hr rating, what's the diff between "EPA" and "Cord Wood" ratings, and how can these two stoves heat the same area
when their maximum output ratings are so different?
A BTU is a British Thermal Unit, used to measure heat: a stove's BTU/hr rating tells how much heat it produces per hour. All things being
equal, wood stoves with higher BTU/hr ratings will produce more heat than lower-rated models.
But what if the same stove has two different ratings? The difference in BTU ratings you note is a result of different test protocols. The EPA test
protocol is concerned only with particulate emissions, not maximum heat output. In fact, the EPA label that comes with every woodstove has a
disclaimer on it, stating the EPA lab didn't test the stove for heating efficiency. Here's why:
The only way the EPA can test emissions fairly is to burn the same load of fuel in all tests, relative to the size of the firebox being tested (the test
loads must be the same size, same weight, same moisture content). To accomplish this, they use milled 2x4's and 4x4's of C-grade Pine, air-dried to
19% - 25% moisture content and nailed together with 3/4" x 1.5" spacers in exactly the same size and shape. These softwood "charges" contain
far more airspace, far less wood fiber and far less heat value than a full load of conventional hardwood. Further, the EPA protocol requires testing
with the draft control of the stove adjusted to its lowest (smokiest) setting, which is also its lowest BTU output setting. Thus, the heat output
recorded by the EPA lab during emissions testing is far lower than could be achieved with a full load of hardwood and a more open draft control
(hence the disclaimer). This is true of all woodstoves, but especially evident in woodstoves with larger fireboxes, where the EPA "charge" can
leave LOTS of space that could be packed with wood.
So, where do the higher advertised btu output and efficiency numbers come from? In addition to the EPA emissions testing, a woodstove
manufacturer may elect to subject a stove to "Cord Wood" testing. These tests may be performed at an independent testing laboratory, or at the
manufacturer's own accredited facility. Instead of nailed-together softwood "charges", these tests are performed with full loads of hardwood, and
produce ratings that are quite different from the EPA numbers. One of the results produced during cord wood testing is Maximum Output, which is
the heat output of a full load of wood with the draft control adjusted to provide optimum air to the fire. This isn't exactly how most people actually
operate their woodstoves, but gives the manufacturer a BTU output rating that makes the stove look much more powerful than the stoves whose
brochures only give the output produced by the EPA's pine charge at low draft setting. For example, the stove you mention with a 79,000 BTU/hr
"Cord Wood Maximum Output" rating scored just 40,000 BTU/hr in EPA testing.
So, which is the best number to use when comparing woodstoves, the EPA Maximum Output rating or the Cord Wood Maximum Output rating?
The answer is, neither. Since most of us operate our woodstoves with the draft control set for an all-night burn (neither all the way open nor all the
way closed), what we really need to know is the average BTU ouput of one full load of cord wood over an 8-hour burn. For a chart comparing our
woodstoves using this real world rating, click here.
Q: I read your Q&A about the difference between EPA heat output ratings and Cord Wood heat output ratings, and it explained a lot; thank you.
But I still have a question: what about heating efficiency? When I bought my new woodstove, the brochure (and owner's manual) claimed an
efficiency rating of 79%. I just looked up the same model on the EPA's emissions chart, and it is listed at just 63%!
The short answer: Heating efficiency isn't even included in the EPA's emissions testing protocol. For that reason, the following disclaimer
appears on every EPA label, in all caps:
*NOT TESTED FOR EFFICIENCY. THE VALUE INDICATED IS FOR SIMILAR WOOD HEATERS.
The long answer: Back in the 1980's when emissions testing was in its infancy, the EPA decided (for some reason we can't even fathom) to put an
average heating efficiency rating on each EPA label. The average efficiency of the non-catalytic stoves of the day was presumed to be about 63%,
and that's the number that appears on the EPA's approved stove list for every non-catalytic woodstove to this day.
The answer gets longer yet: Because catalytic converters become less effective with age, catalytic woodstoves are required to be more efficient
than non-cats when new: the number the EPA came up with waybackwhen was 72%, and that's the number that appears on the EPA listing for
every catalytic woodstove today.
Hang in there, patient reader, we're almost done: In the ensuing 30+ years, thermal design engineers have continued to tweak their designs.
Today, most woodstoves that are subjected to efficiency testing achieve scores between 75% and 85%. As a result, most manufacturers ignore the
EPA's arbitrary, outdated averages and publish the "real world" number derived by efficiency testing for each model.
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