Checking It Out – Understanding Pellet Fuel and what to look for in appliances.
Note: The following information was prepared by HEARTH Education Foundation, in cooperation with the Hearth Products Association and the Pellet Fuel Institute.
What are pellets made of ?
All pellets are biomass materials, that is, products of commonly grown plants and trees. The most common residential pellets are made from sawdust and ground wood chips, which are waste materials from trees used to make furniture, lumber, and other products. Resins and binders (lignin) occurring naturally in the sawdust hold wood pellets together, so they usually contain no additives. Nut hulls and other materials are pelletized in some areas, and unprocessed shelled corn and fruit pits can be burned in a few pellet stove designs. Your fuel of choice and its price may depend on the waste biomass most avail- able to pellet mills in your region. In turn, your choice of appliance design depends on the fuel available.
Where do pellets come from?
Pellet mills across the country receive, sort, grind, dry, compress, and bag wood and other biomass waste products into a conveniently handled fuel (Figure 1). Today, over 100 pellet mills across North America produce in excess of millions of tons of fuel per year, a figure that has more than doubled in the last five years. Pellets are available for purchase at stove dealers, nurseries, building supply stores, feed and garden supply stores, and some discount merchandisers. Pellets are usually packaged in forty pound bags and sold by the bag or by the ton (fifty bags on a shipping pallet). Some mills offer twenty pound bags for easier handling.
What are common characteristics of all pellet fuels?
Although the chemical constituents and moisture content of different biomass materials vary, the Pellet Fuel Institute has identified common characteristics and developed fuel standards. These voluntary industry standards assure as much uniformity in the final product as is possible for naturally grown materials that become processed, but not refined fuel. PFI graded fuel must meet tests for:
Density-consistent hardness and energy content (minimum 40 pounds/ cubic foot)
Dimensions-length (1 1/2” maximum) and diameter (1/4"or 5/16”) to assure” predictable fuel amounts and to prevent fuel jamming
Fines-limited amount of sawdust from pellet breakdown to avoid dust while loading and problems with pellet flow during operation (amount of fines passing through 1/8” screen no more than .5% by weight)”
Chlorides-limited salt content (no more than 300 parts per million) to avoid stove or vent rusting
Ash-important factor in maintenance frequency.
What is the difference between standard and premium grade fuel?
All of the measurable characteristics defined by PFI standards are the same for both fuel grades except ash content. Standard grade pellet fuel (up to 3% ash content) is usually derived from materials which result in more residual ash, such as sawdust containing tree bark (which contains more impurities) or agricultural residues like nut hulls. Standard pellets should only be used in stoves designated for their use. Premium grade pellet fuel (less than 1% ash content) is usually produced from hardwood or softwood sawdust containing no tree bark. Ash content varies in premium fuels from about .3% in some western softwoods to about .7% in eastern hardwoods. Premium pellets, which make up over 95% of current pellet production, can generally be burned in stoves calling for either standard or premium fuel. Increased availability of standard fuel is anticipated as stove designs continue to improve ash tolerance. Ash content determines fuel grade because of its role in maintenance frequency. It is the prime factor that determines maintenance frequency of ash removal from the appliance and venting system. In early pellet stove designs, fuel compatibility was the critical factor that determined whether a stove worked well or not. Fuel grade and specific ash content within a fuel grade are still to be considered, but advances in pellet stove technology are making fuel choice wider and easier. The size of the ash drawer, fuel feed and grate design, proper venting, correct operation and maintenance all play a part in maintenance frequency. The experienced pellet stove professional is the best source of information about stove and fuel compatibility.
What other differences are there between fuel types and brands?
There are a number of variations in pellet fuels that are not included in PFI standards. For example, Btu (heat) content may range from just under 8,000 to almost 9,000 Btu, depending upon species and region of the country. Other characteristics like trace minerals in pellet raw materials vary not only from region to region, but even in close by growing areas. Some trace minerals promote clinkering, the formation of clumps of fused ash that can block air inlets in the burn pot. A fuel’s tendency to form clinkers in a stove cannot be predicted by laboratory analysis both because of variations in the raw materials and the different burning conditions that affect the process. Clinkering can increase routine maintenance, but professional recommendations for matching available fuels to stove design can minimize the problem. Pellet mills strive for consistency despite the nature of the raw material. Slight variations in fuel even from bag to bag are inevitable, but the differences are usually insignificant and much smaller than found in the original raw material before processing. Pellets consistently deliver enjoyable, predictable comfort when burned in well designed, operated, and maintained stoves.
What do pellets cost?
The selling price of pellets currently ranges anywhere from $120-$200 per ton and averages $150. Price varies by region, availability, and season, just like other heating fuels. Because bags of pellets stack and store easily, many prudent customers take advantage of lower off season prices and ensure their winter fuel supply by buying early. Selling price, of course, is only a part of the cost picture. The primary issue is the cost of energy, which is measured in dollars per million British thermal units ($/MM Btu). Pellets purchased at the average $150 per ton and burned in a typical pellet stove cost about $11.50 per million Btu, a figure that is less than the cost of electric heat and competitive with average energy costs of some other fuels (see fuel cost chart, Appendix B, for comparisons at other prices). While tables and charts assume average appliance efficiencies and fuel costs, real world experiences vary widely. The actual cost of heating a home must take into account the insulation and tightness of the home, its size and layout, the level of comfort desired, and local climate. Other economic factors impacting energy costs, though hard to quantify, are also worth consideration. Biomass pellets reduce the use of dwindling fossil fuels, often imported from foreign countries. Every ton of waste material used in pellets reduces the rising costs associated with waste disposal.
What fuel advantages do pellets offer?
The first appeal of pellets is their convenience. Bags of pellets stack compactly and store easily. A ton of pellets can be stacked in an area as small as four feet wide, long, and high, an area about half the space needed for a cord of wood. Bags of pellets can be stored in a small area of a dry garage, basement, or utility room or shed. Pellets are also convenient because they load easily and cleanly into the stove hopper. Loading the hopper is normally required only once a day and may be even less frequent when the stove is used on low settings. The small size of pellets allows for precisely regulated fuel feed. In turn, combustion air can be regulated easily for optimum burn efficiency since the amount of fuel in the burn pot is predictable and consistent. High combustion efficiency is also due to the uniformly low moisture content of pellets (consistently below 10% compared to 20 to 60% moisture content in cordwood). Uniformly low moisture, controlled fuel batches, and precisely regulated combustion air means high heat output and a very low level of unwanted emissions . Other environmental benefits besides clean burns result from the use of pellet fuels. As a biomass fuel, pellets offer the advantages of sustainable energy supplies through renewable raw materials. In addition, pellets are a by-product, not a primary user, of these renewable materials. Using pellets also helps reduce the costs and problems of waste disposal. In 1993-94, more than 6.5 million cubic yards of waste were diverted from landfills and converted to home heating in the form of pellets. As part of the tradition of the hearth, pellet burning offers the enjoyment of fire viewing and active participation in providing winter comfort in the home.
How do pellet stoves work?
Pellet appliances automate as many functions as possible. The most significant is fuel delivery. The heat setting made by the operator controls an auger or similar feed device that delivers regulated amounts of fuel from the hopper to the fire (Figure 2). Automatic fuel delivery from the hopper frees the operator from frequent attention and loading, while providing clean burns and the desired comfort level. The amount of air needed for optimum combustion efficiency is delivered automatically or with minor manual adjustments. In most designs, a fan delivers air to the fire and blows exhaust by-products out of a vent pipe that is smaller and typically less expensive than a chimney (Figure 3). In most designs, a fan delivers heat to the home by blowing air through heat exchangers in the stove and out into the home. Heating efficiency is greatly enhanced by removing the heat from the appliance before it can exit the system.
What types of pellet appliances are available?
Pellet stoves come in a wide range of style, size, and finish. The first choice is the basic configuration of the appliance. Freestanding pellet stoves (Figure 4) offer great flexibility in installation choices. Supported by a pedestal or legs, they are designed to be installed in almost any living area of the home (restrictions may apply to sleeping areas). Freestanding stoves are placed on a non-combustible floor protector. They are installed a specified distance from combustible surfaces that is usually smaller than that required for comparable cordwood appliances. Fireplace inserts (Figure 5) are installed in existing, working fireplaces. A decorative panel covers the space between the insert and the fireplace opening. Some pellet inserts are approved only for use in masonry fireplaces, while others can also be installed in approved factory-built metal fireplaces. Built-in appliances (Figure 6) are an economical choice that offers homes without an existing fireplace the look of an insert in a fireplace setting. A unit tested and listed as a built in can be boxed in with close clearances to combustible framing. Noncombustible materials like brick can be applied to the area around the front of the unit to give the appearance of a fireplace. A noncombustible floor protector is required, sometimes with spacers to provide an air space under the unit. Pellet furnaces are large units designed to heat an entire house through duct work. They are usually installed in a basement or other non-living area of the house. Like pellet stoves, furnaces require venting to the outside.
Pellet stoves are also categorized according to their method of delivering fuel. Top feed stoves deliver pellets from a tube or chute above the fire, and bottom feed stoves deliver pellets from behind or beside the burn pot directly to the fire (see Figure 2). There are many variations within these categories, but in general, bottom feed stoves tend to perform better with the wider range of ash content in standard grade fuel because the feeding action moves ash and clinkers away from the burn area. This action helps keep air inlets open and thereby reduces the frequency of cleaning the burn pot. Top feed stoves may have some advantage in overall heating efficiency since pellets remain in the burn pot until they are completely burned, and exhaust gases tend to move slower, allowing improved heat transfer. Special grates or rotating burn pots in some top feed designs can also move ash and clinkers from the air inlets to reduce maintenance frequency. If only standard grade pellets are available, or if there is a desire to burn a higher ash or special fuel like corn or nut hulls, special attention must be paid to the issue of fuel delivery, ash content, and stove compatibility. Otherwise, design differences are less significant considerations of relative degrees of efficiency and maintenance frequency.
How do I know what size unit to consider?
The wide range of heat output possible with pellet stoves reduces the number of different stove sizes needed for most heating situations. Within the range of pellet stove sizes, choices involve input from different sources. Information from manufacturers is of necessity general in nature given the many variables in climate, home construction, and personal comfort. Btu output and efficiency ratings must be qualified to be useful. Overall efficiency, a measurement of the percentage of the energy available in a fuel that is actually delivered as heat in the home, is more important than combustion efficiency, a measurement of the percentage of available energy that is converted to heat (some of which escapes through the vent). A knowledgeable dealer is the best source of information about sizing. You can help the dealer by providing information about factors that affect stove sizing:
Your intended purpose-primary or backup heat source for the entire house or a selected area house size and layout;
Sketch with room and house size estimate or building plans very helpful heating characteristics of your home;
Insulation and tightness of construction
Your idea of comfort;day and night.
What appliance features should I ask about?
Stove size, type, and appearance generally begin the process of appliance selection. The choice may then include considerations of performance, convenience, and cost. Operational convenience is affected by a number of design features. Hopper capacity, which ranges from under 40 to over 100 pounds, plays a role in loading frequency. Hopper size should match heating needs on a reasonable loading cycle, typically once daily. The means of adjusting controls affect convenience also. Manually controlled stoves require occasional adjustment of air inlet dampers as the fuel feed rate is changed. Stoves with more sophisticated controls are able to monitor burn conditions and make these adjustments automatically. The choice may focus on the lower cost of manual stoves versus the value of minimal interaction with stove operation. Some owners prefer the reduced attention of automatic air controls while others enjoy more active participation in tending the fire. Features which affect ash tolerance and fuel compatibility may influence both performance and convenience, particularly in regions where fuel grade choice is limited. While many newer stoves perform well with increasingly wider ranges of fuel, some designs are better at extending good performance with longer intervals between routine maintenance. Bottom fuel feeding moves ash and clinkers away from air inlets. Specially designed grates in top feed designs either allow heavier ash and clinkers to fall through to the ash drawer, or they rotate themselves to move ashes and keep air inlets open. In both stove designs, a larger ash drawer reduces ash removal frequency, and grates which can be dumped without stopping operation add convenience. As long as a compatible pellet fuel is available, the issue of ash tolerance is one of convenience, not the acceptability of stove design. Ease of maintenance is enhanced by features that make routine cleaning tasks easier. The heat exchanger can be cleaned by simply moving an external rod handle back and forth on some stoves.
Other designs demand a more involved cleaning procedure. Ask the dealer to demonstrate heat exchanger cleaning as well as access to ash traps, receptacles behind the fire chamber designed to separate fly ash from the exhaust. Although pellet stoves are safety tested as a solid fuel heater by an accredited laboratory, stoves safety tested to ASTM Standard E 1509 undergo stringent testing specifically designed for pellet appliances to assure their safety and performance reliability . Other features bring added convenience and enjoyment:
Automatic ignition. Offers simple, reliable startup. Typically more desirous on stove used intermittently than on one used continuously. Remote thermostatic control. Controls heat output based on room temperature. Ensures even temperature from unattended stove regardless of changes in the weather. Discuss with the dealer the merits of thermostats and stove startup design: automatic ignition stoves cycle on and off according to heat demand; manual start stoves keep fire at low setting and return to high. Large glass view and air wash system. Offers fire viewing, particularly enjoyable in living area installations. Air wash keeps glass clean by directing warm air over the glass during combustion. Imitation log sets (stove manufacturer approved only). Improve aesthetics of the fire. May increase frequency and difficulty of maintenance; ask if removable. Blower quietness. A concern in some areas of the house, depending on room use and personal preference. Backup power source. Consideration for primary heat stoves in areas subject to frequent power outages. Should automatically switch from utility power to battery and back again. Extended warranty for components. Low cost, high value extension of warranty on components (from normal one or two years to as many as five).
What information should I know about pellet stove dealers?
Pellet stoves are not difficult to operate or maintain with a little initial instruction. You may want to ask what owner training materials and services are available before you buy. Support after the sale adds value to the purchase price. Many dealers offer training through stove operation demonstration, “Stove School” classes, newsletters, or by telephone in addition to the instructional manuals and videos provided by manufacturers. Just as with automobiles and other mechanical products, most owners will need professional service. Dealers can provide details of warranty service, including extended warranty plans, as well as regular (usually annual) service. Dealers will discuss a fuel plan with you, detailing what locally available fuels are compatible with your stove and what sources supply them. Most dealers are glad to demonstrate their qualifications by supplying satisfied customer references. Some dealers have taken a nationally administered exam to become certified by the HEARTH Education Foundation.
Ease and expected frequency of routine owner maintenance
Availability and cost of professional service
Dealer’s owner training program and materials
Special features like self-igniter system, remote thermostatic control, glass air wash and imitation logs for fire viewing, ash drawer size
Understanding of heating system requirements and installation plan
Backup power or alternate heat source plan (if important)
Warranty detailsTotal system and installation cost