

Since about 1050 Btu are necessary to boil or evaporate a pound of water, and 1 Btu additional is necessary to raise the pound's temperature 1°F, it is possible to determine the latent heat fairly easily by knowing the total weight of water vapor given off by the fire. We had the 0.25 pound of moisture content. Add about 0.54 pound of water vapor as products of combustion. If we assume low humidity conditions that contribution is small. We now have 0.79 pounds of water vapor that started at say 60°F average temperature and was heated to say 400°F. The latent heat is then 0.79 times (1050 plus 340 temp rise) or 1098 Btu per 1.25 pound piece, or 880 Btu/pound. Therefore, the "low heat value (LHV)" of wood fuel is less than the high heat value (HHV) by this amount. The result is that the available energy in seasoned (20% moisture content) wood used in an actual usage environment (400°F flue gases) is about 6050 Btu/pound. We feel that this is the most realistic number to use for domestic wood burning as it is the number that would apply if the user weighed his wood as part of determining efficiency of his appliance.
Some charts you may run across use a figure described as an output per cord or pound of wood. This is always based on some assumption about the efficiency of the device being used. Often 50% or 40% is assumed, so that if the actual device had substantially different efficiency the figures would be wrong. Even the LHV must be slightly compensated for if the flue gas temperature is not as assumed, but these changes are relatively small and generally will not materially affect comparison results.
There are also differences in types of wood. Softwoods usually have a lot of resin content that has high energy content so their total energy content is usually higher than for hardwoods (often by about 5%). The softwoods tend to burn up faster than hardwoods and have other characteristics that reduce their attractiveness as fuel. The fact that their average density is usually lower than hardwoods means that you get less weight of wood in a cord and the extra 5% of volatile fuel will not make up for this.
For comparison sake, using LHV gives results about 8% higher that the same results using HHV. Thus, an 80% device (LHV) is 74% efficient using HHV. A 45% HHV reading is equivalent to about 49% LHV.
.It is also useful to note how these concepts apply to unseasoned (green) wood fuel. If only seasoned a short time, 50% moisture is a realistic figure. Then a twopound piece has one pound of wood fibers (worth 8660 Btu). There will be 1.54 pounds of water to vaporize and heat up (taking away 2200 Btu). The twopound piece has a net available energy content of 6460 Btu or 3230 Btu/pound. This is only HALF of the available energy present when burning seasoned wood. Green wood consumes the bulk of its energy just to keep itself going, and is obviously subject to easily going out.
A freshly cut tree has even higher moisture content, often above 60%. Similar calculations show that this fresh wood has only 2000 Btu/pound of energy available. This explains why it is so difficult to burn freshly cut trees.