By definition, a British Thermal Unit (BTU) is the amount of energy needed to heat one pound of water by one degree Fahrenheit. BTU’s are commonly used to describe the energy content of fuels and are used as a loose standard of comparison for the heat output from heating appliances such as propane furnaces and wood stoves.
While BTU/hour ratings may be fairly consistent when shopping for propane furnaces, BTU/hour ratings mean little when shopping for a wood stove.
In theory, one would think that wood stoves with a higher BTU/hour rating would produce more heat than lower rated units. However, BTU/hour test protocols do vary, causing a great deal of confusion for the general public. Therefore, to fully understand a particular wood stove’s BTU/hour rating, one must first understand the test protocol which produced it.
First of all, it is critical to realize that the EPA test protocol for wood stoves was established to measure emissions, not a BTU/hour heat output. The EPA emissions test protocol is standardized to burn milled pine lumber of the same size, shape, and moisture content, relative to the size of the firebox for each stove. The EPA emissions test protocol requires that each stove’s draft control be set to its lowest and smokiest setting.
Therefore, the BTU/hour measured during an EPA emissions test will always be lower than what would be produced if burning a firebox full of seasoned hardwood fueled by oxygen from a more open draft. It is for this reason that the following disclaimer appears on every EPA label in upper case letters:
EPA Disclaimer: *NOT TESTED FOR EFFICIENCY. THE VALUE INDICATED IS FOR SIMILAR WOOD HEATERS.
It is therefore misleading for one to compare the BTU/hour heat output of one wood stove against another wood stove, because the BTU/hour ratings are not the subject of the EPA emissions test, and are therefore not trustworthy. Furthermore, other test protocols are not standardized.
Comparing BTU/hour ratings on wood stoves would be like comparing apples to oranges and oranges to bananas. While they are all fruits, they are all very different from one another.