For a list of stove types see Stove (disambiguation).
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 gases once the fuel is ignited. It is critical that the chimney or flue gases be hotter than the outside temperature as this will result in combustion gases being drawn out of the fire chamber and up the chimney.
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Both hardwood and softwood have the same energy content (by mass) and will provide similar energy outputs. However, the essential difference will be 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, which are fast growing. They burn at a far greater rate. Another disadvantage of soft wood is it creates more soot and other deposits on the inside of the wood stove, chimney, and flue therefore causing more frequent cleanings.
One of the most critical factors in wood burning is the moisture content of the wood, as any water in firewood has to be boiled off during the burning process. Freshly cut wood (known as green lumber) has a high moisture content. Green wood of ash trees contains 35% moisture and green poplar contains 65% moisture. Apart from producing very low heat outputs, the flame temperature is also reduced. This causes unburned creosote. Creosote leaves the wood burner as a gas and cools to a liquid in the chimney. This oily liquid coats the inside of the chimney and collects soot and other combustion particulates. It can build up and reduce the draft in the chimney. It can also be ignited by a subsequent Chimney fire. This can damage the chimney and spread fire to the surrounding structure.
For best results firewood should have a moisture content of less than 20%. The process of removing the excess moisture is called seasoning. Seasoning by air-drying the wood can take up to two years. Wood should be stored in an outdoor well-ventilated, but covered, structure. A recent innovation is kiln-dried wood. With interest and usage of wood burners at an all-time high,[1] some companies are now using large kilns to quickly dry their wood.
Hardwood must be well-seasoned, with some species taking up to four years to dry out. Hardwood is seasoned by being left out to the elements for three to four winters, then for a subsequent summer under cover with ventilation to dry out. Softwood is left out to the elements for two winters, then for one summer to dry out with ventilation.[2]
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, except when helping the chimney/flue heat up initially. A fully open air control will lead to more heat being sent straight up the chimney rather than into the room (which reduces 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, which will lead to warping, buckling and general damage to the stove and its internal components. Individual stoves will have their own quirks, so take a little time to get used to a new stove's settings.
Modern building techniques have created tighter sealed homes, forcing many stove manufacturers to recommend their stoves be installed with outside air intake. Outside air ensures that stoves will run more efficiently, also removes the need for cold air for the combustion process to pass through the living space reducing "draughts" and improving comfort of the occupants. This design is called "room sealed stoves" or "external air supply stoves."
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 1 "Air Supply to solid fuel appliances" [3]
Under the United Kingdom's Clean Air Act, local authorities may declare the whole or part of the district of the authority to be a smoke control area. It is an offence to emit smoke from a chimney of a building, from a furnace or from any fixed boiler if located in a designated smoke control area. It is also an offence to acquire an “unauthorized fuel” for use within a smoke control area unless it is used in an “exempt” appliance (“exempted” from the controls which generally apply in the smoke control area). The current maximum level of fine is £1,000 for each offence.
In order to comply with the Clean Air Act in "smoke control areas," an exempt appliance or fuel must be used.[4]
The United States Clean Air Act requires that wood stoves and wood heating appliances be certified by the Environmental Protection Agency (EPA). These devices meet a particulate emissions standard of no more than 7.5 grams per hour for noncatalytic wood stoves and 4.1 grams per hour for catalytic wood stoves.[5] Washington State has stricter requirements of a maximum of 4.5 grams per hour.
Italy is one of the biggest markets for wood-burning stoves in Europe, having around 30% of all homes using wood for some heat. This means about 5 million homes have a wood fuelled stove or cooker.