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Comparing Woodstove BTU Ratings

Q: I am looking at two woodstoves, both about the same size, both recommended to heat up to 2000 sq. ft. of 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, a measurement of heat. A BTU/hr rating tells how much heat is produced per hour. All things being equal, wood stoves with higher BTU/hr ratings will produce more heat than lower-rated appliances.

The difference in BTU ratings you note is a result of different test protocols. The EPA test laboratories are concerned only with particulate emissions, not maximum heat output. In fact, the EPA label that comes inside 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 exact same load of fuel in all tests. Same shape, same size, same weight, same moisture content. To accomplish this, they use nailed-together "charges" of kiln-dried pine in a size and shape that will fit any woodstove. These "charges" are fairly small, and contain far less wood fiber (and heat value) than a full load of conventional cordwood. Also, EPA testing is accomplished with the draft control of the stove adjusted to its lowest (smokiest) setting, which is also its lowest BTU output setting. Thus, when the EPA lab tests a woodstove for emissions, their charge produces much lower BTU output ratings than could be achieved with a firebox full of hardwood and a wide open draft control (hence the disclaimer). This is true of all woodstoves, but especially evident in woodstoves with larger fireboxes.

In addition to the EPA emissions testing, a woodstove manufacturer may elect to take the stove to an independent test laboratory for "Cord Wood" testing. These tests are performed with full loads of air-dried wood, 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 short-duration fire with a full load of wood and the draft control cranked wide open. This is exactly the opposite of how most people actually operate their woodstoves, but gives the manufacturer a BTU output rating that makes their stove look much more powerful than the stoves whose brochures only give the output produced by the pine charge at low draft setting used in EPA testing. 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 wood over an 8-hour burn. For a chart comparing our woodstoves using this real world rating, click here.

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