HOW IS WOOD STOVE BTU OUTPUT DETERMINED?

Q: I have been trying to find a description of exactly how the heat output of wood burning stoves is measured.

My friend and I are trying to decide between a number of wood burning stoves. Each brochure lists the output in BTUs, but we have been unable to find any information about how these figures are arrived at. I recognize that these measurements must be difficult to carry out and I suspect that only a few laboratories do this testing for all manufacturers of wood stoves.

What I want is a detailed protocol of the measuring methods so I'll know that I can make an "apples to apples" comparison between one manufacturer and another based upon BTU ratings.

Any help you can provide would be much appreciated.

Best Regards,
Jim Stopps

A: We don't put much stock in manufacturers' heat output claims, for two reasons: first, heat output testing is not regulated by any government agency. Second, the hearth product industry has never established a uniform testing procedure. There are actually many test protocols available, the two most prevalent being “calorimeter room” and “stack loss” testing, but a manufacturer may choose to test to either of these protocols, a combination of both, or another protocol altogether. This testing may be performed in an independent lab, or in the manufacturer’s own lab. The only assurance that actual test results of any kind exist to back up a manufacturer’s published numbers is provided by the truth in advertising laws.

The lack of a uniform testing procedure can result in some seemingly wacky manufacturer's claims. For example, Travis Industries lists the heat output for their smallest model, the Avalon Pendleton, which has a shoebox-size (1.3 cu.ft.) firebox, at 64,200 btu/hr. How Travis managed to squeeze 64,200 btu/hr out of one cubic foor of wood is beyond us, but nonetheless, we're betting they can produce verifying test results if prodded.

Travis Industries is not alone. Whatever test protocol is used, woodstove manufacturers tend to publish the highest output achieved in their best test burn, usually qualified with the phrase “maximum output” or “up to.” While not reflective of real-world performance, this number can provide a simple basis of comparison from one model to another, particularly within a given manufacturer’s product line, where all other factors are often equal (same basic design and materials, different size firebox).

The problem is, all other factors are often not equal. Soapstone stoves, for example, don’t get nearly as hot as plate steel or cast iron stoves, so they don’t tend to score as high in maximum output testing. However, a soapstone stove will continue to radiate meaningful heat hours after a plate steel or cast iron stove has gone cold, so the net heat output to the room over time might be equal or even greater.

When shopping for a wood stove, it is important to keep these facts in mind, as well as mitigating factors such as size of house, number of rooms, quality of insulation, species of wood to be burned, etc. To read our take on this subject elsewhere in our Sweep's Library, check out our Wood Stove Heat Output Comparison Chart.

Perhaps the most accurate way to compare one woodstove to another doesn’t consider maximum output at all. You’ll find a formula to determine the heating ability of a given stove for the duration of an all-night burn based upon its firebox size and efficiency rating in our Wood Stove Sustained Burn Comparison Chart. Note that the same mitigating factors outlined above should be considered when using this technique.

Here's a link to an article about wood stove heat output testing from a UL laboratory's point of view.