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 are capable of producing 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 two very different test protocols: the EPA emissions test protocol, and maximum heat output testing.

The EPA test protocol is primarily designed to determine particulate emissions, not maximum heat output.  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. 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. 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. Click to view a chart comparing our woodstoves using this real world rating.

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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 list, and it is listed at just 63%!

The short answer: Until recently, heating efficiency wasn't even included in the EPA's emissions testing protocol. For that reason, the following disclaimer appeared 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 appeared on the EPA's list for every non-catalytic woodstove.

The answer gets longer yet: Because catalytic converters become less effective with age, the EPA required catalytic woodstoves to score higher efficiency numbers than non-cats when new: the number the EPA came up with waybackwhen was 72%, and that's the number that appeared on the EPA listing for every catalytic woodstove.

Hang in there, patient reader, we're almost done: In the ensuing 30+ years, thermal design engineers continued to tweak their designs, until most woodstoves subjected to efficiency testing achieved scores between 70% and 80%. As a result, most manufacturers ignored the EPA's arbitrary, outdated averages and published the "real world" number derived by independent efficiency testing.  This resulted in the situation you describe, where nearly every wood stove model had two very different efficiency ratings.

Update January, 2015: The EPA has apparently seen the light, and dropped the default efficiency numbers from their list of certified stoves.  Instead, they are requiring wood and pellet stove manufacturers to re-test their models at an EPA accredited test lab to the same protocol, calculating the results using the High Heat Value of the fuel wood.  All manufacturers are required to submit their ratings to the EPA by 2020.  In the meantime, the new "real world" efficiency numbers will begin to appear on the EPA's emissions list and voluntary hang tags as each model completes testing.  Eventually, this number will be published on all marketing brochures as well.