Wood-burning stove
A wood-burning stove is a heating or cooking appliance capable of burning wood fuel, often called solid fuel, and wood-derived biomass fuel, such as sawdust bricks. Generally the appliance consists of a solid metal closed firebox, often lined by fire brick, and one or more air controls. The first wood-burning stove was patented in Strasbourg in 1557. This was two centuries before the Industrial Revolution, so iron was still prohibitively expensive. The first wood-burning stoves were high-end consumer items and only gradually became used widely.
The stove is connected by ventilating stove pipe to a suitable flue, which will fill with hot combustion gases once the fuel is ignited. The chimney or flue gases must be hotter than the outside temperature to ensure combustion gases are drawn out of the fire chamber and up the chimney.
Operation
Air supply
Keeping the air flowing correctly through a wood-burning stove is essential for safe and efficient operation of the stove. Fresh air needs to enter the firebox to provide oxygen for the fire; as the fire burns, the smoke must be allowed to rise through the stove pipe, creating negative pressure in the firebox, and exit through the chimney. To regulate air flow, there may be damper devices built into the stove, flue, and stove pipes, and there is usually an air intake adjustment to regulate combustion.By opening or closing the dampers, air flow can be increased or decreased, which can fan the fire in the firebox, or "dampen" it by restricting airflow and reducing the flames. The dampers can usually be accessed by turning knobs or handles attached to the damper. Some stoves adjust their own airflow using mechanical or electronic thermostatic devices.
The highest heating efficiencies on closed appliances can be attained by controlling the various supplies of air to the stove. On modern stoves, owner's manuals provide documented procedures. Fully open air controls may lead to more heat being sent straight up the chimney rather than into the room. The biggest problem with leaving the air controls fully open on many stoves 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. Different stoves have different numbers and types of air controls. More efficient models have been developed in recent years, employing catalytic combustors to burn remaining particles and gasses within the smoke before it exits the flue, resulting in more heat output, longer burn times and less harmful effect on the environment.
Modern building techniques have created more airtight homes, forcing many stove manufacturers to design their stoves to permit outside air intakes. Outside air can improve the overall efficiency of the stove as a heater by drawing cold combustion air directly from the outside instead of drawing preheated air from the room that the stove is in. Many modern stoves can optionally use an outside air intake. Many manufacturers supply the necessary parts in kit form. When considering an outside air kit, it is important to know that the air must come in from below the level of the stove. For example, a basement stove may not safely use an outside air kit. This is to prevent a reversal of venting in which very hot flue gasses are exhausted through a air intake pipe, which could lead to a structure fire and/or hot flue gasses being released into the structure.
Fuel
Hardwood or softwood
is usually measured and traded by volume or mass. In English-speaking countries not fully metric it is often measured in cords. A face cord is 4' x 8' x the length of the log. A full cord is 4'x4'x8'. When purchasing, cutting, or collecting firewood, it is good to be aware of the difference between hardwood and softwood. Both hardwood and softwood have similar energy contents by mass, but not by volume. In other words, a piece of hardwood would usually be heavier and have more available energy than the same sized piece of softwood. Hardwoods, derived from trees such as oak and ash, may burn at a slower rate, resulting in sustained output. Many softwoods are derived from conifers, which are fast growing and may burn at a faster rate. This is one reason why softwood pellets are popular.The primary advantage of hardwoods is that they tend to contain more potential energy than the same volume of a softwood, thus increasing the amount of potential heat that can be stacked into one stoveload. Hardwood tends to form and maintain a bed of hot coals, which release lower amounts of heat for a long time. Hardwoods are ideal for long, low burns, especially in stoves with a poor ability to sustain a low burn, or in mild weather when high heat output is not required.
Softwoods, in contrast, tend to burn hot and fast with little coaling. They may leave less ash than hardwoods. Softwoods are ideal for fast, hot burns. They produce excellent heat and do not fill the stove with coals, a frequent problem for those pushing their hardwood-fired stoves hard to get the maximum possible heat out of them.
Not all hardwoods have a higher potential energy content than all softwoods. Wood varies by species and even individual trees. Osage orange, also known as hedge, is perhaps the highest-BTU wood that is common in North America.
Many softwoods will season much more quickly than many hardwoods. For example, pine that has been cut, split, stacked and topcovered will usually be ready to burn in one year; oak may be expected to take three years under the same conditions.
Softwood mythology
Softwood is often said to be dangerous to burn because it generates more dangerous creosote than hardwood. This myth is pervasive in the North American northeast, where both types of wood are commonly available. It is not common in the northwest, where most full-time wood burners burn pine and fir exclusively.It is possible that this myth originated with old-fashioned stoves and fireplaces. These appliances did not require seasoned wood, and frequently did not receive it. As a result, they often experienced very low flue temperatures — usually in flues that were not insulated as modern flues are. The combination of low firebox temperatures due to high moisture content in the wood and low flue temperatures due to lack of insulation led to high levels of creosote accumulation. Burning a wood that emits a lot of sparks in an old-fashioned fireplace or stove will lead to sparks going directly into the flue, which can lead to a dangerous chimney fire if the flue is coated in creosote.
Modern stoves which are operated properly do not cause this high level of creosote accumulation. While different wood species do contain varying levels of volatile organic compounds, the difference is inconsequential since all woods produce creosote which accumulates if burned improperly. So-called dangerous woods such as pine are in fact safer than woods such as oak, as they will burn hotter and thus help keep flue temperatures up, and their fast seasoning will help ensure that novice wood burners are burning reasonably dry wood.
Moisture content and creosote
Dry wood produces more usable heat than wet wood, since the energy isn’t being used to evaporate the water from the wood before it can burn. Freshly cut wood has a high moisture content. Different wood species have different moisture contents, which also vary tree to tree. Burning fuel that is mostly water uses much of the combustion energy to evaporate the water. This results in low firebox temperatures and low flue temperatures.Firewood with a moisture content below 20 per cent by weight can burn efficiently. This is the "free" moisture content absorbed in the wood fibers, and does not include the chemically-bound hydrogen and oxygen content. Moisture content can be reduced by outdoor air-drying, for a period of several months in summer weather. Solar-powered or fuel-fired kilns can accelerate the drying process.
The most common process of removing the excess moisture is called seasoning. Seasoning by air-drying the wood can take three years or more. Wood is dried in outdoor well-ventilated covered structures, or in a kiln.
All wood will release creosote vapors when burned. Modern stoves will burn the vapors, either via direct secondary combustion or via a catalyst. Very little, if any, creosote will escape a properly operating modern stove's secondary combustion.
Creosote that does escape may still not be harmful if it leaves the wood in gaseous form. It will not condense on surfaces above 250 degrees Fahrenheit. Modern flues are insulated to help ensure that they do not fall below this temperature during normal stove operation. Creosote accumulation can be dangerous, as it is flammable and burns hot. If a flue is coated with creosote and ignited, perhaps by a spark going up the flue, it can cause a serious chimney fire that can lead to a structure fire. This can be avoided by using modern stoves and flue standards, burning dry wood, keeping fires hot enough to maintain flue temperatures of at least 250 degrees F at the top of the flue, and proper chimney cleaning as needed.
Multi-fuel models
designs are common in Europe. They burn solid fuels only, including wood, wood pellets, coal and peat. They are typically made of steel or cast iron. Some models are also boiler stoves, with an attached water tank to provide hot water, and they can also be connected to radiators to add heat to the house, though they are usually not as efficient as a dedicated wood boiler.There are also stove models that can switch from wood fuel to oil or gas sources that are installed in the house to supply heat to a separate water boiler. Stoves that readily convert to either oil or gas in addition to wood fuel have been manufactured in North America and Europe since the early 20th century, and are still manufactured. In some models, the oil or gas may fuel the stove through a pipe connection leading to a "pot burner" in the rear of the firewood compartment in the stove.
Multi-fuel stoves are versatile, but usually perform poorly compared to a stove that is designed to burn one specific fuel as well as possible.