The following article is excerpted from Natural Home Heating: The Complete Guide to Renewable Energy Options by Greg Pahl.
During the energy crisis of 1973, the Boeing Company asked Dr. Jerry Whitfield, a fuel efficiency engineer, to develop alternative fuels for industry. The idea was that if industry could switch to other fuels, then more of the scarce petroleum-based fuel could be reserved for aircraft. As Whitfield embarked on his research, he could not have foreseen that he was about to become a pioneer in the pellet stove industry.
Whitfield, who had a background in fluid-bed (forced-air) furnace technology, eventually met Ken Tucker in Idaho. (Tucker is now the president and CEO of Lignetics, Inc., a leader in the wood pellet fuel industry.) Tucker was intrigued by the way alfalfa could be turned into pelletized animal feed. He hypothesized that the same general process could be used to manufacture pellet fuel from sawdust and that the pellets could be burned in industrial furnaces. But Tucker’s experiments had not been successful until Whitfield arrived on the scene.
Whitfield was impressed with Tucker’s work and quickly realized that the heating potential of pellet fuel extended well beyond industrial applications. Whitfield applied his knowledge of forced-air furnace technology to smaller-scale residential heating systems, and in 1983, he introduced the first forced-air pellet stoves. Slow to catch on at first, pellet stoves were selling at about seventy-two thousand units annually by 1994. Despite some variability in the market since then, pellet stove sales remain strong, and the number sold in 2006 has increased to 133,000. More and more people are discovering the many benefits of these remarkable heating appliances.
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How They Work
Unlike its cordwood relatives, a pellet stove limits much of the physical labor of operation through automation. The best part of this automated design is in the fueling process. An auger feeds pellets to the fire in carefully controlled amounts; it draws the pellets from a storage bin or hopper (which is usually built into the main part of the stove).The speed at which the auger turns controls the amount of fuel added and the amount of heat produced. Not only does this technology eliminate the chore of hand-fueling, but also it allows pellet stoves to generate just the right amount of heat to maintain a constant temperature, which can be regulated by a thermostat. This ability to produce steady heat is generally not possible with most other heating appliances. Even most oil- and gas-fired central furnaces cycle on and off, allowing for considerable fluctuations in temperature between firings. Most pellet stoves eliminate these annoying fluctuations.
Better yet, most pellet stoves only need to have their storage hoppers filled once a day, making these heaters an excellent choice for people who want to burn wood but who are away from home most of the day. The frequency of refueling does depend on the size of the storage hopper and on the weather. The pellets are combusted in a burn pot (a small combustion chamber) with the aid of a blower, creating a mini-furnace, which generates an extremely hot and efficient fire with very low emissions. The combustion efficiency of many pellet stoves is around 95 percent. They produce lower emissions than any other type of solid-fuel-burning hearth appliance and are an obvious choice in locations where air quality in the winter is an issue.
The blower, which can have more than one speed setting, also directs the combustion gases out of the stove and into a small vent pipe instead of a normal chimney. Before exiting the stove, the heat from the fire normally passes through a high-efficiency heat exchanger, which warms the air in your living space, either by convection or with the assistance of a circulating blower. The heat exchanger and forced combustion-air design allows pellet stoves to achieve overall heating efficiencies between 60 and 80 percent.
Pellet stoves are available in a wide variety of styles, sizes, and finishes. The three main categories of these stoves are freestanding stoves, fireplace inserts, and built-in appliances.
The most popular pellet stove is the freestanding design. Legs or a pedestal supports this type of stove, which offers the most flexibility in terms of installation; it can be located almost anywhere in the living space of your home. Although it must be placed on a noncombustible floor protector, a freestanding pellet stove can usually be installed much closer to walls and other combustible surfaces than a typical cordwood stove.
Like their cordwood-burning cousins, pellet-burning fireplace inserts can be installed in most functional masonry fireplaces. A metal shield (or shroud) seals off the rest of the opening between the insert and the fireplace mouth. Some pellet inserts are approved only for use in masonry fireplaces, while others can also be installed in approved factory-built metal fireplaces.
A pellet-fired built-in (or zero-clearance) appliance offers the warm ambiance of a fireplace without much of the expense of a traditional masonry fireplace. Although a noncombustible floor protector is required, these units can be framed in with fairly tight clearances to combustible materials. When faced with brick, tile, or stone, a built-in appliance looks similar to a traditional fireplace. Many pellet fireplace inserts have zero-clearance kit options.
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Pellet appliances are further subdivided into two additional categories based on their burner design: top feed and bottom feed. Top-feed stoves have an auger that delivers the pellets to a tube or chute above the burn pot. In this design, the pellets simply fall into the burn pot from above. In a bottomfeed stove, the pellets are augered to the fire from below or behind the burn pot. Although there are variations, the bottom-feed design generally allows for more flexibility in the grade of pellet that can be burned, because the feeding action of the pellets pushes the ash and clinkers (fused minerals) away from the burn area. This self-cleaning action of the bottom-feed design reduces the amount of manual cleaning required. Some top-feed stoves have specially designed grates that allow heavier ash and clinkers to fall through to the ash pan; other are designed with a burn pot that rotates to help keep the air inlets open. Grates that can be cleaned without interrupting the operation of the stove are especially convenient. Although there are some exceptions, top-feed designs are common in pellet stoves, while bottom-feed designs are more common in pellet furnaces and boilers.
One important feature to consider when comparing pellet stoves and pellet fireplace inserts is the pellet capacity of the storage hopper. Capacity varies from 40 to over 100 pounds in some models. The quantity of pellets you can load into the hopper at one time is a key factor in determining the frequency of refueling. Ideally, you shouldn’t have to refuel more than once a day. Another important feature is whether your pellet stove has manual or automatic controls. Some people prefer to just “set it and forget it,” while others enjoy a more hands-on relationship with their heating appliances. If cost is a major issue, note that manually controlled pellet stoves are less expensive than their automatic counterparts.
Pellet stoves use three types of ignition systems: standard, self-starting, and fully automatic. Standard ignition requires the use of a starter gel and a match. Self-starting ignitions employ a button or remote control to light the fire. Fully automatic ignition is controlled by a thermostat and allows the stove to cycle on and off, depending on the heat level selected. Automatic ignition, available on many newer-model pellet stoves, may be more useful on stoves that are used intermittently rather than continuously. Automatic ignition pellet stoves can switch between high and low settings and turn themselves off when heat is not required; standard and self-starting stoves can switch between settings but cannot shut themselves off. Remote thermostats are also available for many pellet stoves.
One of the nicest features offered on both freestanding stoves and fireplace inserts is your ability to watch the fire through an airtight glass door (the firebox door should be kept closed while the stove is operating). The glass is kept clean by the same type of air-wash design that is used on many woodstoves. Watching the fire in a pellet stove or insert offers an ambiance similar to that of sitting in front of a woodstove or fireplace. However, watching the flames from a superheated pellet fire in an otherwise blank scene is underwhelming for those who are used to watching a real cordwood fire.
Most pellet-stove manufacturers realize this and offer a variety of imitation log sets that can be placed in the firebox area. Some of these log sets can be reasonably convincing with flames dancing up through them. Others look obviously artificial. Regardless of their appearance, the best feature of these logs is that they don’t ever have to be replenished. That’s something that a cordwood stove can’t duplicate. Find out if the log set for your stove is easily removable during cleaning procedures; if possible, avoid those that aren’t. Other optional features include self-cleaning burn pots, oversize pedestal ash pans, gold or nickel door trim, and special porcelain or cast-iron finishes.
Although pellet stoves produce some direct radiational heating through the glass window in the door, the majority of the heat is generated by convection, usually with the aid of an internal fan or blower. This means that the exterior surfaces of most pellet-stove cabinets tend to stay relatively cool, reducing the chance of accidental burns from touching them. But the fans can be noisy. While you are at a stove specialty shop, listen carefully to the blower on the model you are interested in while it is operating. Some models are quieter than others. Blower noise can be especially important to consider if the stove or insert is installed in a prominent location in your living area.
Sizing a Stove
A pellet stove always burns at high efficiency regardless of the thermostat setting. Because the heat output of pellet stoves is easily controllable, sizing is not quite as crucial as it is with a cordwood stove. Nevertheless, most pellet-stove manufacturers offer several sizes, and it is important to pick the heat output that best matches your home’s heating requirements and your weather zone, as well as your expectations and needs. Remember that a stove, regardless of its fuel, is primarily a heating appliance for a single room or zone. Because of their hot-air circulating fans, however, most pellet stoves do a better job of moving warm air through a home than a traditional woodstove.
Pellet Stove Pros and Cons
Pellet stoves and inserts can be used in new construction or renovations.
They provide steady, easily controlled heat.
They only need to be refueled about once a day.
They do not require a masonry chimney.
The stove exterior (except the glass door) remains relatively cool, reducing the risk of accidental burns.
Sitting in front of a pellet stove or insert is relatively romantic.
You need a place to store the pellets.
Most pellet stoves and inserts require electricity to operate.
The sound of the blower(s) may bother some people.
Some light physical labor is required.
You will need some form of backup if you are going to be away from home for more than a day.
Combustion gases may spill into your living space during a power outage.