This article is reprinted with permission from the Energy Saver website, courtesy of the U.S. Department of Energy.
Today you can choose from a new generation of wood- and pellet-burning appliances that are cleaner burning, more efficient, and powerful enough to heat many average-sized, modern homes. Pellet fuel appliances burn small pellets that measure 3/8 to 1 inch in length.
When choosing a wood- or pellet-burning appliance, it's important to select one that's properly sized for the space to be heated. When an appliance is too big, residents tend to burn fires at a low smolder to avoid overheating, which wastes fuel and is one of the biggest causes of air pollution. An under-sized unit will not provide sufficient heat. You should discuss your heating needs with a reputable dealer. A good rule-of-thumb is that a stove rated at 60,000 British Thermal Units (Btu) can heat a 2,000-square-foot home, while a stove rated at 42,000 Btu can heat a 1,300-square-foot space.
Wood-burning appliances and fireplaces may emit large quantities of air pollutants. Wood smoke contains hundreds of chemical compounds including nitrogen oxides, carbon monoxide, organic gases, and particulate matter, many of which have adverse health effects. In many urban and rural areas, smoke from wood burning is a major contributor to air pollution. Because of this, some municipalities restrict wood heating appliance use when the local air quality reaches unacceptable levels. Others restrict or ban the installation of wood-burning appliances in new construction. Before installing a wood-burning system, you should contact your local building codes department, state energy office, or state environmental agency about wood-burning regulations that may apply in your area.
If you have an older wood-burning appliance, consider upgrading to one of the newer appliances certified by the U.S. Environmental Protection Agency (EPA). Some include a catalytic combustor that allows combustion gases to burn at lower temperatures, thereby cleaning the exhaust gas while generating more heat. All woodstoves sold today should bear an EPA certification sticker. Higher-efficiency appliances usually have lower emissions and are also often safer, because complete combustion helps to prevent a buildup of flammable chimney deposits called creosote.
The location of the appliance (and chimney) will influence how well heat is distributed and conserved in your home. Most wood- and pellet-burning appliances are essentially space heaters, and should be put in the room where you spend most of your time. Ideally, there should be a way for heat to circulate to the rest of the house, such as a fan or blower assembly.
For safety and to maximize efficiency, you should consider having a professional install your wood- or pellet-burning appliance. A professional will carefully evaluate everything from your chimney to your floor protection. A certified professional can also help you choose the best appliance to heat your home. The National Fireplace Institute maintains a list of certified industry professionals.
Types of Wood- and Pellet-Burning Appliances
The following is a brief overview of the different types of appliances available.
High-efficiency Fireplaces and Fireplace Inserts
Designed more for show, traditional open masonry fireplaces should not be considered heating devices. Traditional fireplaces draw in as much as 300 cubic feet per minute of heated room air for combustion, then send it straight up the chimney. Fireplaces also produce significant air pollution. Although some fireplace designs seek to address these issues with dedicated air supplies, glass doors, and heat recovery systems, most traditional fireplaces are still energy losers. When burning a fire, you should turn your heat down or off and open a window near the fireplace.
High-efficiency fireplace inserts have proven effective in increasing the heating efficiency of older fireplaces. Essentially, the inserts function like wood stoves, fitting into the masonry fireplace or on its hearth, and using the existing chimney. A well-fitted fireplace insert can function nearly as efficiently as a wood stove.
Studies have shown that proper installation of fireplace inserts is very important. Have a professional installer examine the fireplace and chimney to determine if they are suitable for an insert. Inserts should be as airtight as possible. The more airtight it is, the easier it is to control the fire and the heat output. The installer should use only approved fireplace insulating materials to fill any gaps between the fireplace mouth and insert shield.
Moving an insert to clean the chimney or liner can be difficult, and is a job best left to a professional chimney sweep. In some situations, a clean-out door can be installed above the insert connection so the insert does not have to be moved as often. Some models have wheels to simplify installation, cleaning, repairs, and other adjustments.
Some modern fireplaces heat at efficiencies near those of wood stoves and are certified as low emission appliances. Although designed to include the fire-viewing benefits of a traditional fireplace, this generation of fireplaces can effectively provide heat as well. Through vents under the firebox, room air is drawn in, heated through a heat exchanger, and sent back into the house either through vents at the top of the fireplace or through ducts leading to nearby rooms. Some of these fireplaces are approved to route heated air to a basement auxiliary fan. The air then travels through ducts to other rooms in the house. The fireplace should have a dedicated supply of outside air for combustion.
Flues leak heated and warm air out of your home. If you have a fireplace that you don't use, plug and seal the flue. If you use the fireplace, be sure to close the flue when the fireplace is not in use. You could also use an inflatable stopper, available commercially, to temporarily seal the chimney and avoid air leakage through the flue.
Catalytic Wood Stoves, Advanced Combustion Woodstoves, and Centralized Wood-Burning Boilers
Wood stoves are the most common appliance for burning wood. New catalytic stoves and inserts have efficiencies of up to 83% HHV (higher heating value).
Advanced combustion woodstoves provide a lot of heat but often only work efficiently when the fire burns at full throttle. Also known as secondary burn stoves, they can reach temperatures of 1,100°F—hot enough to burn combustible gases.
These stoves have several components that help them burn combustible gases, as well as particulates, before they can exit the chimney. Components include a metal channel that heats secondary air and feeds it into the stove above the fire. This heated oxygen helps burn the volatile gases above the flames without slowing down combustion. While many older stoves only have an air source below the wood, the secondary air source in advanced combustion stoves offers oxygen to the volatile gases escaping above the fire. With enough oxygen, the heated gases burn as well. In addition, the firebox is insulated, which reflects heat back to it, ensuring that the turbulent gases stay hot enough to burn. New advanced non-catalytic combustion stoves often have efficiencies of 65 t0 75%, HHV.
Another benefit is that the secondary channels funnel hot air toward the glass doors, keeping them clean for viewing the fire. They can also be slightly less expensive than conventional woodstoves fitted with catalytic combustors. Like wood stoves, centralized wood-burning boilers have been improved over the years. Some modern, centralized wood heaters use wood gasification technology that burns both the wood fuel and the associated combustible gases, rendering them up to 80% efficient. In addition, systems are available that can switch to oil or gas if the fire goes out.
Masonry heaters are also known as "Russian," "Siberian," and "Finnish" fireplaces. They produce more heat and less pollution than any other wood- or pellet-burning appliance. Masonry heaters include a firebox, a large masonry mass (such as bricks), and long twisting smoke channels that run through the masonry mass. Their fireboxes are lined with firebrick, refractory concrete, or similar materials that can handle temperatures of over 2,000°F (1,093°C).
A small hot fire built once or twice a day releases heated gases into the long masonry heat tunnels. The masonry absorbs the heat and then slowly releases it into the house over a period of 12 to 20 hours. Masonry heaters commonly reach a combustion efficiency of 90%.
Most are intended for burning wood, but they were historically designed to burn almost any type of solid fuel. The relatively small but intense fire also results in very little air pollution and very little creosote buildup in the chimney. Because most of the heat from the fuel is transferred to the masonry and slowly released into the room over the day, this type of heater does not need to be loaded with fuel as often as other types of wood heating appliances. In addition, if the masonry heater is built where sunlight can directly shine on it in the winter, the heater will absorb the sun's heat and release it slowly into the room.
A wide variety of masonry heater designs and styles are available. Larger models resemble conventional fireplaces and may cover an entire wall. Smaller models take up about as much space as a wood or pellet stove. They can be custom-built or purchased as prefabricated units. Some large designs may cost $5,000 or more. Plans and kits are available, but they are not easy do-it-yourself projects and require masonry expertise.
In addition to their expense, masonry heaters have one significant disadvantage when compared to conventional wood stoves and fireplaces—they cannot provide heat quickly from a cold start.
Pellet Fuel Appliances
Pellet fuel appliances burn compacted pellets usually made of wood, but they can also be derived from other organic materials. Some models can burn nutshells, corn kernels, and small wood chips.
Pellet fuel appliances are more convenient to operate than ordinary wood stoves or fireplaces, and some have much higher combustion and heating efficiencies. As a consequence of this, they produce very little air pollution. In fact, pellet stoves are the cleanest solid fuel, residential heating appliance. Pellet stoves that are certified by the EPA are likely to be in the 70% to 83% efficiency range. Pellet stoves have heating capacities that range between 8,000 and 90,000 Btu per hour. They are suitable for homes as well as apartments or condominiums.
Most pellet stoves cost between $1,700 and $3,000. However, a pellet stove is often cheaper to install than a conventional wood-burning heater. Many can be direct-vented to the room and do not need an expensive chimney or flue. As a result, the installed cost of the entire system may be less than that of a conventional wood stove.
Pellet fuel appliances are available as freestanding stoves or fireplace inserts. Freestanding units resemble conventional wood heaters in that they generally heat a single room well, but not adjacent rooms unless they have a fan to force the warm air into those other spaces. Pellet-fireplace inserts fit into existing fireplaces. Several companies now make pellet-fired furnaces and boilers for replacement of, or a supplement to, gas- or oil-fired furnaces and boilers in residential space heating systems.
All pellet fuel appliances have a fuel hopper to store the pellets until they are needed for burning. Most hoppers hold between 35 and 130 pounds (16 and 60 kilograms [kg]) of fuel, which will last a day or more under normal operating conditions. A feeder device, like a large screw, drops a few pellets at a time into the combustion chamber for burning. How quickly pellets are fed to the burner determines the heat output. The exhaust gases are vented by way of a small flue pipe that can be directed out a sidewall or upwards through the roof. More advanced models have a small computer and thermostat to govern the pellet feed rate.
Pellet appliances usually require refueling only once a day. However, because the fuel is compressed, the bagged pellets can be difficult to lift. Some models use bulk-filled storage systems and are fully automatic.
Most pellet appliance exteriors (except glass doors) stay relatively cool while operating, reducing the risk of accidental burns. Pellet stoves burn fuel so completely that very little creosote builds up in the flue, posing less of a fire hazard.
Unfortunately, pellet appliances are also more complex and have expensive components that can break down. Moreover, they need to be cleaned by the homeowner on a weekly basis and by a professional on an annual basis. They also require electricity to run fans, controls, and pellet feeders. Under normal usage, they consume about 100 kilowatt-hours (kWh) or about $9 worth of electricity per month. Unless the stove has a back-up power supply, the loss of electric power results in no heat and possibly some smoke in the house.
Chimney Placement and Sizing
Chimneys harness the heat of the fire to create what's called a stack effect. As the warm air from the fire rises, cooler house air rushes into the wood-burning appliance through vents, providing the oxygen the fire needs to burn. Starting a fire with a good hot burn will encourage this healthy draft to flow. Also, between the higher and lower pressure zones of the home lies a neutral pressure zone. The neutral pressure zone tends to move toward the largest air leak. When the top of the chimney is located above the home ceiling (as it should be), the chimney's neutral pressure zone is above the neutral pressure zone of the house. Such proper chimney placement creates a gentle flow of air into the appliance and out the chimney even when no fire burns.
If you are designing or building a new home, consider placing the chimney inside your home. A more traditional chimney, constructed along the outside of a home, will lose valuable heat to the cold, outside air. If the chimney air temperature falls below that of the inside air, the cold, smelly chimney air will be pulled into the house by the low pressure of the stack effect. In such a scenario, the house has become a better chimney than the chimney. So when a fire is lit, smoke fills the room.
Chimneys must match the size of the appliance, meaning the flue size should match the stove outlet. If the chimney is bigger than the stove or fireplace outlet, exiting exhaust slows, increasing creosote buildup and decreasing efficiency. High-performance chimneys are also insulated. Older masonry chimneys can be relined to safely and efficiently connect them to newer high-efficiency, wood-burning appliances. Again, the chimney liner should be continuous from the appliance outlet to the chimney top. It is not uncommon to pay as much for the chimney as for your appliance.
Free-standing woodstoves exhaust into a connecting pipe, which then connects into the chimney. If the connecting pipe is longer than 8 feet (as in a vaulted ceiling), you should consider investing in double-layer pipe with 1-inch airspace between pipe layers. Efficient modern stoves produce large amounts of heat. Much of this heat can radiate from a longer length of single-layer pipe, slowing down the draft, which can impact the overall efficiency of your wood-burning system.
To keep your wood- or pellet-burning system operating efficiently and safely, you'll need to maintain it on a regular basis.
Every year, preferably before each heating season, have a chimney sweep certified by the Chimney Safety Institute of America inspect your wood-burning system. In addition to cleaning the chimney, a certified chimney sweep should have the knowledge to help make sure your appliance, hearth, connecting pipe, air inlets, chimney, and all other components are functioning efficiently and safely.
Catalytic combustors need to be inspected after every two cords of wood burned, and replaced according to the manufacturer's recommendations. Most catalytic stoves or inserts have a place for a thermometer to help you check the combustor. The catalytic cell is removable and replaceable and costs between $75 and $160. Catalysts in new models, especially those made since 2005, should only need changing once every 5–8 years if burning seasoned wood. Older catalytic stoves made prior to 2000 need to be checked more often and catalysts need to be replaced more often.
Cleaning out the inside of a wood stove with a wire brush periodically will also help the wood-burning appliance heat more efficiently. Even one-tenth of an inch of soot can drop the heat transfer efficiency of the metal by 50%.
For pellet-fuel appliances, it is very important to follow the manufacturer's instructions for operation and maintenance. Inspect fans and motors regularly, and maintain them properly. Manufacturers advise removing unused pellets from the stove hopper and feed system at the end of the heating season. This reduces the chance of rusting, which can cause expensive damage to the appliance. It also minimizes difficulties in lighting the appliance at the start of the next heating season. Clean the flue vent on a regular basis to prevent soot buildup.
Wood and Pellet Fuels
Selecting and Storing Wood
All species of wood have similar heat (Btu) content on a per pound basis when completely dry. Therefore, denser woods will generally cost more and burn longer. Woods like oak, hickory, and pine will burn overnight.
Because a lot of energy can be wasted burning wet wood, you should use wood that has been properly seasoned. Properly seasoned wood is harvested in the spring and allowed to dry throughout the summer. Look for wood that is of even color, without any green. It should have a moisture content of 20% or less by weight.
Store your wood away from the house in case termites discover the woodpile. The top of the pile should be covered, but leave the sides open so air can circulate. If possible, store the wood a foot off the ground (on concrete blocks, for example) to keep it dry.
Pellet fuel is normally sold in 40-pound (18-kg) bags at about $3 to $4 each or about $180 to $250 a ton. Most homeowners who use a pellet appliance as a main source of heat use two to three tons of pellet fuel per year. Pellet fuel appliances are almost always less expensive to operate than electric resistance heating, oil, and propane-fueled appliances.
Most pellet fuels have a moisture content of 5% to 10%. Well-seasoned firewood is usually around 20%. A few pellet manufacturers contain either petroleum or non-petroleum lignin used as a lubricant in the pellet production process, though most contain no additives.
The Pellet Fuels Institute launched the PFI Standards Program, a third-party accreditation program providing specifications for residential and commercial-grade fuel. This standard assures the consumer of the highest quality pellet when certified pellets are purchased.
You can also check pellet fuel quality by inspecting the bag for excessive dirt and dust, which can form clinkers in the stove. There should be less than one half of a cup of dust at the bottom of a 40-pound (18-kg) bag. Pellet stoves designed for low-ash content (typically top-fed stoves) tend to operate poorly when used with pellets of a higher ash content. Many pellet appliance manufacturers are redesigning their products to burn pellets with varying ash contents.
Most pellet fuel appliance dealers either maintain a supply of pellets or recommend a supplier. You may also check the local telephone listings under "Fuel" or "Pellet Fuel," or inquire at a local tree nursery or at home and garden supply stores.