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A pellet stove is a stove 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. Todays central heating systems operated with wood pellets as a renewable energy source can reach an efficiency factor of more than 90%.
Basically there are two different operating systems for pellet stoves. One being single ovens with direct heat release into the living space and the other being central heating systems (pellet central heating systems) including control and feedback control systems.
Pellet single ovens are generally plants in the power range of max. 6-8 kW and less. They are usually placed directly in the living room. They usually have a small reservoir of pellet fuel which can last for one or more days. Fuel supply and the control of combustion are controlled automatically and the ash removal is done manually. The heat is usually delivered directly into the room. The range of individual pellet stoves is similarly diverse as wood burning stoves, including versions with viewings window which provide a view of the fire.
Central heating systems
Pellet central heating systems are used for heat requirements (heating load) of more than 3.9 kW (Kilowatt). But pellet central heating systems are not only used in single or two family homes (up to 30 kW), but also for larger residential or business units with heat requirements of a few hundred kW. Pellet burner systems run most efficiently at full load and can usually be regulated down to up to 30% of its full capacity. Since the warm up phase of pellet ovens usually takes longer than for oil or gas firing systems, short burning phases have negative effects on the fuel efficiency. In order to improve energy efficiency and reduce harmful emissions, pellet ovens are usually combined with buffer systems like water tanks for example.
Separate pellet burner, which can be mounted to an existing oil- or wood-fired boiler, are sometimes offered as an inexpensive alternative to a complete heating changeover. But the efficiency of these add-on furnaces is reduced. Usually add-on furnaces are not subsidied by government funds.
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 20th 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 for domestic heat. From this came the miniaturized pellet stove, which emerged from Washington State 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, steel or 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. Between 1998 and 2010, 824,410 pellet appliances (stoves and fireplace inserts) were made in the US.
While some stoves are UL listed for fuels other than pellets, such as wheat, corn, sunflower seeds, and cherry pits, many pellet stove manufacturers recommend the use of a corn and pellet mixture.
The pellet fuel is automatically delivered from the storage facility or the day tank (single stoves) into the combustion chamber.With the heat generated circuit water is heated in the pellet boiler. In central heating systems the hot water then runs through the heating circuit. in pellet central heating systems. The heat distribution is the same as in other systems of central heating. Unlike for oil or gas heating for pellet heating the inclusion of a hot water tank in the heating system is recommended in order to save the hot water until it is needed.
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 monitor various safety conditions, and can 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. However, many pellet stoves will also burn fuels such as grain, corn, seeds, or woodchips. In some pellet stoves, these fuels may need to be mixed with wood pellets. Pelletized trash (containing mostly waste paper) is also a fuel for pellet stoves.
Unlike wood stoves which operate exclusively on a principle of chimney draft, a pellet stove must use specially sealed exhaust pipe to prevent exhaust gases escaping into the living space due to the air pressure produced by a combustion blower. 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 have the advantage of not always requiring a vertical rise to vent, although a three to five foot vertical run to induce some draft 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. Modern building techniques have created tighter sealed homes, forcing many pellet stove manufacturers to recommend their stoves be installed with outside air intake. This ensures their stoves will run efficiently, and prevents potential negative pressure within the home.
Pellet stoves are approved for use in mobile homes, while standard wood burning stoves are not.
In many states pellet fuel is exempt from sales tax.
Principles of operation
A pellet stove normally consists of these components, whether basic or complex:
- A hopper
- An auger system
- Two blower fans (combustion and convection)
- A firebox (burn-pot and ash collection system), sometimes lined with ceramic fiber panels
- Various safety features (vacuum switch, heat sensors)
- A main control box/board
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 or steel 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 in the small area of the burn pot as a combustion blower introduces air into the bottom of the burn-pot, while also forcing exhaust gases up and out the chimney. While some pellet stoves will be hot to the touch (especially on the viewing window), most manufacturers utilize a series of cast-iron or steel 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, sides and back 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, and sometimes vacuum sensors, enabling the controller to shut down if an unsafe condition is detected.For maintenance (daily cleaning) it is best to use an ash vac. These are similar to shop vacs, but are designed for the removal of ash materials. These vacs are available with a pellet stove kit which enables the cleaning of the interior areas of the stove which improves efficiency.
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.
A corn stove is designed for whole kernel shelled corn combustion and is similar to a pellet stove. The chief difference between a pellet stove and a dedicated corn stove is the addition of metal stirring rod within the burnpot or an active ash removal system. These vary in design slightly, but usually consist of one long metal stalk with smaller rods welded at a perpendicular angle, in order to churn the burn-pot as it spins. An active ash removal system consists of augers at the bottom of the burn pot that evacuate the ash and clinkers. During a normal burn cycle, the sugar content within corn (and other similar bio-fuels) will cause the ashes to stick together, forming a hard mass. The metal stirring rod, which is usually connected to a motor by a simple chain system, will break apart these masses, causing a much more consistent burn. While there is a push to create stoves that are able to burn multiple fuels with minimal adjustments, some pellet stoves are not designed to stir fuel and will not burn corn.
- Corn kernels
- Hearth, Patio & Barbecue Association
- Renewable heat
- Pellet Fuels Institute
- Pellet Fuel
- Pellet Mill
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- 2009 Wood Stove Tax Credits You Should Be Taking Advantage Of. Retrieved 2009-12-22.
- The Alliance for Green Heat - Federal Tax Incentives for Wood and Pellet Stoves - Carbon Neutral, Sustainable, Local and Affordable Heating