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Wood burners unlock energy with a match
By Ned Rozell


September 25, 2006

As our breath hangs in the frosty autumn air, thoughts turn to protecting our fragile selves from the inevitable deep freeze. Many Alaskans choose wood heat to make the winter more bearable.

jpg burning wood

Fall Fire
Photo courtesy

Burning firewood provides warmth by releasing stored energy from the sun that trees have converted to mass we can use. British thermal units, or Btu, define the energy provided by a certain species of wood. A Btu is the amount of energy it takes to increase the temperature of one pound (one pint) of water by one degree Fahrenheit.

Firewood energy is measured in Btu per cord. A cord is 128 cubic feet, which is a four-foot by four-foot by eight-foot pile of wood. If a cord is cut in one-foot lengths to fit in the stove, the resulting woodpile will be 32 feet long and four feet high.

New Englanders might laugh at the fact that Alaskans burn birch and spruce, but hickories and oaks aren't hardy enough to survive our winters. Hickory provides about 30 million Btu per cord.

Paper birch, the first choice of Alaskans, provides 25.4 million Btu per cord, according to a table on the energy content of Interior Alaska trees prepared by George Sampson, a former Institute of Northern Forestry research forester. Tamarack, a tree often mistaken for sickly spruce because of its spindly branches, provides 24.8 million Btu per cord, followed by black and white spruce at about 20.5 million Btu, aspen at 18.8 million Btu, and balsam poplar at 17.5 million Btu. Sampson's measurements are for air-dry wood with a 20 percent moisture content. Wood is considered dry when it reaches a moisture content of 15 to 30 percent. Freshly cut, green wood contains 30 to 60 percent moisture.

Seasoned logs put off much more heat than wet wood. When a log is placed inside a stove on top of other burning logs, it doesn't bring instant gratification. First, the heat energy provided by the burning logs drives off the moisture of the unburned log, and none of the heat from the reaction warms the room. The wetter the log, the more energy required to dry it out. For that reason, and because dangerous creosote deposits increase when burning wet wood, experts recommend drying firewood for at least six months after it's cut live and split, which should bring the moisture content down to an acceptable 25 percent.

Of Alaska woods, birch has the most Btu per cord because it's dense. This means there's a lot more wood mass, and therefore energy, crammed into a birch log than the same-sized aspen log.

Given that fact, which puts off more heat-one pound of oven-dry (no moisture at all) birch or one pound of oven-dry aspen? It's a trick question. All oven-dried woods have about the same energy content, 8,600 Btu per pound. Therefore, if people sold firewood by the oven-dried pound instead of the cord, aspen would be just as valuable as birch, but it would probably take twice as many woodsheds to store the same amount of energy.


This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Ned Rozell is a science writer at the institute. This article was previously published. Select articles will be reprinted while Ned Rozell is on paternity leave.

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