A wood-burning stove is a heating appliance capable of burning wood fuel and wood-derived biomass fuel. Generally the appliance consists of a solid metal (usually cast iron or steel) closed fire chamber, a grate and an adjustable air control. The appliance will be connected to a suitable chimney or flue which will fill with hot combustion gasses once the fuel is ignited. It is critical that the chimney or flue gasses are hotter then the outside temperature as this will result in combustion gasses being drawn out of the fire chamber and up the chimney.
Clearly there are different types of wood, but they will usually fall into either the hardwood or softwood types. Both types of wood have the same energy content (by mass) and will provide similar energy outputs, however the essential difference is in the rate at which the fuel burns. Hardwoods derived from slow growing broadleaf trees will burn at a slower rate for sustained output. Softwoods are derived from evergreen trees such as conifers, they are fast growing but burn at a far greater rate. Softwood burns at nearly twice the rate of hardwood meaning that in the course of an hour you would use roughly twice the volume of softwood (as opposed to hardwood) to maintain the heat in the room.
One of the most critical factors in wood burning is the moisture content of the wood. This is where wood seasoning comes into play. Freshly cut wood will contain a moisture content of around 65-90%. This wood should never be used. Apart from producing very low outputs this wet wood will also generate large amounts of soot and tar, which can potentially lead to chimney fires (as these particles will coat your chimney and will fuel a chimney fire). For best results wood should have a moisture content of less than 20%. The process of removing the excess moisture is called seasoning. Seasoning is air drying the wood and can take up to two years. Wood should be stored in a well ventilated (but covered) structure, outdoors.
High heating efficiencies on closed appliances can only be attained by controlling the supply of air to the fire chamber (operating the air control correctly). It is not recommended to leave the air control fully open, beyond the point of getting the chimney/flue hot initially. A fully open air control will lead to more heat being sent straight up the chimney rather than into the room (reduced efficiency). The biggest problem with leaving the air control fully open is “overfiring”. Overfiring is caused when too much heat is generated within the fire chamber, this will lead to warping, buckling and general damage to the stove and its internal components.
Correct air flow and ventilation is also critical to efficient and safe wood burning. Specific requirements will be laid down by the stove manufacturer. Legal requirements for new installations in the UK can be found in Building Regulations Approved Document J, Section 2, Table 2.1 "Air Supply to a solid fuel appliance" .
