A good, well-seasoned hardwood will provide approximately half as much heat value per pound as does good coal. Wood is easy to ignite, burns with little smoke, and leaves comparatively little ash. On the other hand, it requires a larger storage space (commonly outdoors) and more labor is involved in its preparation.
Wood is commonly sold by the cord for heating purposes. A standard cord measures 4 feet high by 8 feet wide and contains wood cut to 4 foot lengths for a total of 128 cubic feet. Only about 70% of a standard cord actually represents the wood contents, however, because of the existence of air spaces between the wood.. The heat value of wood depends upon the type of wood being burned. The wood heat values are determined for cords containing approximately 90 cubic feet of solid wood (i.e. about 70% of a standard cord). It is important to remember that the amount of moisture in a wood will effect its burning characteristics. A green wood will burn much more slowly than a drier one, and its fire will be more difficult to start.
All woods can be divided into either hardwood or softwood types. Contrary to popular belief, the basic difference between the two groups is not the hardness of the wood. In other words, a wood that can be classified as a softwood is not necessarily softer than a hardwood. These terms do not refer to the physical properties of the wood but rather to its classification as a coniferous (needle or cone bearing) or deciduous (broad-leafed) tree. A softwood, for example, comes from a coniferous tree, whereas a hardwood is obtained from a deciduous tree. Some soft wood trees produce wood that is harder than the wood obtained from some hardwood trees.
Chimney or flue fires are always a hazard when using wood as a heating fuel. Resins and soot collect in these areas over a period of times, and can be ignited if there is a flare-up of the fire. The possibility of this happening can be eliminated or greatly reduced by cleaning the chimney or flue time to time. Softwoods (e.g. white pine, yellow pine, etc.) contain greater amounts of resin than do the hardwoods. This tends to give them a higher calorific value than the hardwoods and increased inflammability, but it also results in more smoke.