Pellet stove
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A pellet stove is an appliance that burns compressed wood or biomass pellets to create a source of heat for residential and sometimes industrial spaces. By slowly feeding fuel from a storage container (hopper) into a burn-pot area, they create a constant flame that requires little to no physical adjustments.
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[edit] Background and History
Scrap wood and ship-lap burners have been around for decades. Barrel stoves, braziers, and oil drum fires in Depression-era Hoovervilles support this. Professionally built wood ovens with sawdust hoppers were used in the early part of the century. All of these units used scrap and loose wood, or sawdust. In 1930, the Presto-Log was invented reusing scrap sawdust from the Potlatch pine mill in Lewiston, Idaho as domestic heat. From this came the miniturized pellet stove, which emerged from Scandinavia in the 1980s.
The pellet stove has changed in appearance over the years from a simple, boxy workhorse design, to a decorative heating appliance. Pellet stoves can be either free-standing units or fireplace inserts vented into an existing chimney. Most pellet stoves are constructed using large, conductive, cast-iron pieces, with stainless steel to encase circuitry and exhaust areas.
Pellet furnaces and pellet boilers are also available in addition to the decorative stove. These units can be retrofitted into existing home heating systems with only minor changes to existing ductwork and or plumbing.
The heating industry has considerably shifted toward biomass stoves and heating devices based on efficient combustible and renewable resources. This was a trend that began during the 1973 oil crisis causing the creation of the first pellet stoves. Even so, pellet stoves have become a viable, economical, and popular option for home heating systems only in the last ten years. Many pellet stove manufacturers recommend the use of a corn and pellet mixture, though some are UL listed for fuels other than pellets, such as wheat, corn, sunflower seeds, and cherry pits.
[edit] Benefits
Pellet stoves are relatively versatile appliances. Most pellet stoves are self-igniting and cycle themselves on and off controlled by a thermostat. Stoves with automatic ignition can be equipped with remote controls. Recent innovations have created computer systems within pellet stoves which run diagnostic tests if an imminent problem arises.
A properly cleaned and maintained pellet stove should not create creosote, the sticky, flammable substance that causes chimney fires. Pellets burn very cleanly and create only a layer of fine fly-ash as a byproduct of combustion. The grade of pellet fuel affects the performance and ash output. Premium grade pellets produce less than one percent ash content, while standard or low grade pellets produce a range from two to four percent ash. Pellet stove users should be aware of the extra maintenance required with a lower grade pellet, and that inconsistent wood quality can cause serious effects to the electronic machinery over a short period of time.
A pellet stove is normally associated with pelletized wood. Up to 50% grain, seeds, or corn may be mixed with wood pellets in some pellet stoves. But today many will also burn straight corn, wheat or other grain or seeds. Pelletized trash is also a fuel for pellet stoves.
Pellet stoves require certified double walled venting, normally three or four inches in diameter with a stainless steel interior and galvanized exterior. Because pellet stoves have a forced exhaust system, they do not usually require a vertical rise to vent, although a three to five foot vertical run is recommended to prevent leakage in the case of a power outage. Like a modern gas appliance, pellet stoves can be vented horizontally through an outside wall and terminated below the roof line, making it an excellent choice for structures without an existing chimney. If an existing chimney is available, manufacturers urge use of a correctly sized stainless steel liner the length of the chimney for proper drafting.
In many states in the U.S., pellet stoves and fuel are exempt from sales tax.
[edit] Principles of operation
A pellet stove normally consists of these components, whether basic or complex.
- A hopper
- An auger system
- Three blower fans (combustion, convection, and exhaust)
- A firebox (with refractory panels, burn-pot, and ash collection system)
- A vacuum pump
- A main control box/board (or "brain")
To properly function, a pellet stove uses electricity and can be plugged into a normal wall outlet. A pellet stove, like an automatic coal stoker, is a consistent heater consuming fuel that is fed evenly from a refillable hopper into the burn-pot (a perforated cast-iron basin), through a motorized system. The most commonly used distributor is an auger system that consists of a spiral length of metal encased in a tube. This mechanism is either located above the burn-pot or slightly beneath and guides a portion of pellet fuel from the hopper upwards until it falls into the burn-pot and begins to combust.
Fan systems are necessary for clean, economical performance. The flame produced is concentrated and intense as a combustion blower introduces air into the bottom of the burn-pot. While some pellet stoves will be hot to the touch (especially on the viewing window), most manufacturers utilize a series of cast-iron heat exchangers that run along the back and top areas of the visible firebox. With a convection blower, room air is circulated through the heat exchangers and directed into the living space. This method allows for a much higher efficiency than the radiant heat of a hand-fed wood or coal stove, and will in most cases cause the top of the stove to be not more than warm to the touch. Along with convection air, an exhaust fan forces air from the firebox through special venting specifically made for pellet fuel. This cycle of circulation is an integral part of the combustion system as well, for the concentrated high-temperature flame will quickly overheat the firebox. The possible problems associated with overheating are electrical component failure and flames traveling into the auger tube causing a hopper fire. As safeguards, all pellet stoves are equipped with heat sensors enabling the controller to shutdown if any safe conditions are exceeded.
Pellet stoves can either be lit manually or through an automatic igniter. The igniter piece resembles a car's electric cigarette lighter heating coil. Most models have automatic ignition and can be readily equipped with thermostats or remote controls.
[edit] See also
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