Some confusion can arise when comparing lists of energies per cord of various woods because the actual amount of solid wood in a cord depends on the straightness and length of the pieces, and how it is piled. Some authors assume 80, others 90, cubic feet of solid wood in a cord (a cord has an overall volume of 128 cubic feet including the spaces between the pieces of wood). The difference between the assumptions of 80 and 90 cubic feet per cord result in a difference of about 12 percent between reported energies per cord. Actual cords may actually contain from 60 to 100 cubic feet of solid wood. Thus, in practice, there is a very large variability in the amount of energy per cord even for a given kind of wood.
The larger (20-40 percent) discrepancies between some lists of woods are due to the different ways of reporting energy contents. All the wood energies given up to this point have been the total chemical energy in wood as measured in a bomb calorimeter, where the final temperature of the combustion products is essentially room temperature, and where virtually all water vapor generated condenses into liquid water. The heat value measured this way is the high (or gross) heat value. It represents the most heat that could possibly be derived from the burning of wood.
But when wood is burned in a stove or fireplace, the water vapor in the flue gases rarely condenses, especially where the released heat can be used. In fact, such condensation anyplace in the heating system is to be avoided for reasons of safety (related to creosote - see Chapter 12), and chimney life expectancy (due to corrosion). Since the latent heat part of the energy is essentially unavailable for heating, its contribution is frequently subtracted out from the high heat value, yielding the low heat value.
The difference is significant. As mentioned previously, there are two sources of water vapor in the exhaust of a fire, and low heat values take both into account. Wood contains water (its moisture content), and water vapor is also manufactured in the combustion process. When burned completely, each pound of ovendry wood produces about 0.54 pound of water vapor. For wood with a moisture content of 25 percent there is another quarter pound of water vapor going up the chimney for each piece burned whose ovendry weight would be 1 pound. The total amount of water vapor is 0.79 pound, which represents about 830 Btu of potential energy. The assumption behind the concept of low heat value is that this energy is not usable. The low heat value of wood with a 25 percent moisture content is thus about 830 Btu less than its high heat value, or about 7770 Btu per piece whose ovendry weight would be 1 pound, a decrease of about 9 percent.
The water vapor in the flue gas, along with everything else, also carries away sensible heat. As long as the gases leave the house at any temperature above room temperature, some of the heat generated in the fire was not recovered as useful heat in the house. Some authors have incorporated an estimate of this loss in their lists of available energy of different woods by making quite arbitrary assumptions about flue-gas temperatures and the amount of combustion air. This is not appropriate. The amount of heat going up a chimney is not a property of the wood burned, but of the heat-transfer properties of the stove and chimney, and thus belongs rather in a discussion of the energy efficiencies of stoves (Chapter 6), not in a list of wood types.
When assessing the energy content of a cord of firewood, the most important parameter is the ovendry density of that kind of wood, since a pound of dry wood of any kind has nearly the same energy. The densest woods have the most energy per cord (at equal moisture content). Moisture in wood decreases its useful energy. If all types of wood had the same cost per cord, the better buy would be the denser woods. If wood were sold by the ton, as is sometimes the case, the best buy in terms of energy would be the driest wood. No fuelwood dealer I am aware of sells wood by its energy content. Reasonably accurate BTU assessments would require both weighing and a determination of moisture content.