Wood Burners Unlock Energy with a Match

As breath hangs in the frosty autumn air, thoughts turn to protecting home and body from the inevitable deep freeze of the coming season. Many Alaskans choose wood heat to make the winter more bearable. Burning firewood provides warmth by releasing stored energy from the sun converted by trees to mass we can use.

The energy provided by a certain species of wood is defined by British thermal units, or Btu, according to the Solid Fuels Encyclopedia by Jay Shelton. A Btu is the amount of energy it takes to increase the temperature of one pound (one pint) of water by one degree Fahrenheit. For example, to bring a pint of water in a tea kettle from 60 degrees to a boil requires 152 Btu (212 degrees minus 60 degrees).

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 the stove, the resulting wood pile will be 32 feet long and four feet high.

New Englanders laugh at the fact that Alaskans burn birch and spruce, but hickories and oaks, generally thought of as the best firewood, aren't hardy enough to survive our winters. Hickory provides about 30 million Btu per cord. The higher the Btu, the more heat burning wood provides to the living room.

Paper birch, the first choice of Alaskans who can get it, 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, according to Shelton. Freshly cut, green wood contains 30 to 60 percent moisture.

Properly seasoned logs put off much more heat than wet wood. When a log is placed inside a stove on other burning logs, it doesn't bring instant gratification. First, the heat energy provided by the other 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 that's used to dry it out. For that reason, and because dangerous creosote deposits increase when burning wet wood, Shelton recommends drying firewood for at least six months after it's cut live and split, which brings 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.

So, which puts off more heat--one pound of oven-dry birch or one pound of oven-dry aspen? It's a trick question, because all oven-dried woods have about the same energy content, 8,600 Btu per pound. Oven-dried wood contains no moisture, an impossible feat to achieve without baking wood in an oven before burning it in a stove. Therefore, if firewood were sold by the oven-dried pound instead of the cord, 10 pounds of aspen would be as valuable as 10 pounds of birch, but the aspen would take up twice as much room in the woodshed.