What it is:
How to build one:
Type of wood - whether it is hardwood or softwood - burned in the combustion process is important for the heat value and the energy efficiency.
Hardwoods have less resin and burn slower and longer. Softwoods burn quickly. In addition the seasoned length influences on the fuel efficiency. Seasoning the wood refers to the allowed drying time before combustion.
Wood need to be dried at least 4 to 6 months before use.
Densities and heat values of some common wood species are indicated in the table below. Note that the volume of a stack of firewood varies considerably on whether or not it is split and how it is stacked. The moisture content also play a role - the values below are based on a average moisture content of 20%.
Note that in the table above 1 cord = 85 ft3 is used to convert between the "Density" and "Weight of Cord" column. Be aware that the densities used for the wood species varies significantly. The densities used above is for natural dried wood where the average moisture content is approximately 20%.
Heat values of Cords with dry wood can be estimated by adding the green wood cords values with approximately 10%.
Recoverable heat values are calculated with a stove efficiency of approximately 65%.
good stuff, but remember in a SHTF scenario you want hard copies of your DIY docs in case you don't have access to interweb, or even electricity.