How Wood Pellet Stoves Work


A wood pellet stove
A wood pellet stove
AP Photo/Pat Wellenbach

­Everyone loves a fire. But let's face it -- the chopping and storing and constant stoking of firewood can be a drag. Not to mention the worry about whether or not you put it out all the way befo­re you go to sleep. Plus, wood smoke contains hundreds of chemical compounds that can affect your health, and in many areas, wood smoke is a major contributor to pollution. So what's a fire lover to do? Rest assured there's an alternative that's not only convenient, but also cost-effective and environmentally friendly. It's called a wood pellet stove.

Leading up to the 20th century, 90 percent of Americans used wood to heat their homes. Once fossil fuels came on the scene, using wood for fuel fell out of fashion, and by 1970, it's estimated that about 1 percent of the American population was burnin­g wood to heat their homes. However, the energy crisis of the 1970s prompted people to start using wood again as a renewable energy alternative, and since then, fireplace use has been on the rise.

­Wood pellet stoves resemble wood burning stoves and fireplaces, but the similarities end there. These stoves are electronically sophisticated appliances that offer an environmentally friendly and low-cost heating option for your home. In a time when environmental awarenes­s is at the forefront and families are keeping a close eye on their finances, wood pellet stoves have become all the rage. ­

All wood pellet stoves require the same kind of fuel - the wood pellet. These pellets are the byproduct of sawmills and are made from recycled sawdust and wood shavings. The wood pellets look a lot like rabbit food and are considered eco-friendly because they're cheap and easy to manufacture. Plus, they have a very low pollution rate. You can also buy pellets made of grass and corn, but they're not intended for use in stoves specifically designated for wood pellets.

In our next section, we'll examine the inner workings of a wood pellet stove.

The Mechanics of Wood Pellet Stoves

Wood pellet stoves operate with electricity. The pellets are loaded into the hopper, which is located either on the top or the bottom of the unit. The auger, which is like a long screw, is a motorized device that delivers the pellets from the hopper into the burn pot. The auger's speed determines the temperature of the stove.

The burn pot, which is housed in the combustion chamber, is then ignited. Pellets are heavily compressed, so they're dense and low in moisture, creating a hotter flame. The burn pot serves as the carburetor for the stove, mixing the air and fuel to create combustion, which simply put, is the process of burning. The ashes from the burnt pellets are captured in an ash pot, which needs to be cleaned periodically.

Unlike a standard fireplace, a pellet stove heats a room through convection. As you probably know, hot air rises. This is because as a gas rises in temperature, it becomes lighter and less dense, causing it to rise above the heavier cool air. Convection is the transmission of heat that occurs from this forced combination of cool and hot air currents. So, the convection blower pulls cool air in from the room, passing over the fire in the burn pot and making the flame hotter, which enables the pellets to burn evenly and efficiently.

This heated air then moves across a heat exchanger, which is designed to transfer clean air into your home through the room blower. The heat exchanger acts as a furnace when it's used in combustion, and it's located in the combustion chamber to prevent the outside of the stove from becoming hot.

The exhaust blower pushes the gases out of a narrow pipe in the back of the stove. This pipe can be vented into an existing chimney or connected to the outside through a small hole. It's important to note that even though a chimney isn't required, the exhaust blower is most effective when the pipe is installed at a vertical angle.

The stove is operated by a thermostat, which controls the number of pellets that the auger feeds into the combustion chamber. More pellets equals more heat. For example, pellets delivered at one pound (0.453 kilograms) per hour produces a gentle flame that will last a long time, but at five pounds (2.267 kilograms) per hour, your fire will blaze [source: hometips.com].

Next, we'll discuss the different types of pellet stoves.

Types of Wood Pellet Stoves

There are two different types of sta­ndard wood pellet stoves: free-standing and insert stoves. If you have an existing fireplace, you can buy an insert that fits into the firebox and vents up the chimney. If not, you can purchase a free-standing unit with it's own exhaust pipe. ­

Pellet stoves come in different sizes, styles and colors, tailored to suit the specific needs of your home. While there are many different models on the market with all kinds of bells and whistles, there are a few key characteristics that differentiate one wood-pellet stove from another.

The first is where the hopper is located. In top feed models, pellets are loaded into the auger from the top of the stove, and the pellets go down a tube into the fire. This design minimizes the chances of the fire burning up to the hopper but is also more likely to get clogged with ashes. For that reason, it requires high-grade pellets that are low-ash. However, the top-feed stoves have the advantage of better heating efficiency because pellets stay in the burn box until they're completely burned. Bottom feed hoppers deliver the pellet horizontally, from behind or beside the fire. This design allows you to use standard grade pellets because the horizontal movement inherently moves ash away from the burn area. This helps keep air inlets open and requires less cleaning of the burn box. However, bottom-feed models may not be as efficient.

Wood pellet stoves are available with different heat output levels to accommodate most room sizes. These levels are measured in Btu, or British thermal units, which is the standard unit of classification used in the heating and cooling industries. The options range from 8,000 to 90,000 Btu, but the majority of models are between 40,000 to 60,000 Btu.

Another stove option is manual or automatic ignition. Manual ignition requires a liquid or gel starter material that's lit with a flame, and the process is similar to starting a fire in a wood burning fireplace. Stoves with automatic ignitions have start buttons, so when you push the button, it feeds pellets into the burn pot and heats the self-igniter.

­ In our next section, we'll weigh the benefits against the disadvantages of pellet stoves.

Benefits and Disadvantages of Wood Pellet Stoves

Wood pellet stoves are generally small, and the bags of pellets are about the size of a mulch bag, making them easy to store. They're also easy to operate; they on­ly require loading pellets and igniting the flame. And depending on what size hopper your stove has, it may need to be loaded only once a day.

Because the fire is contained in a heat box inside the unit, there is a minimum of smoke, which lessens the smell in your home and prevents the outside of the unit from heating up. Pellets create considerably less ash than firewood, giving off less creosote, a flammable byproduct of combustion that can build up and cause chimney fires.

Wood pellets are made from recycled materials and are heavily compressed, which reduces the moisture content. Dry fuel creates more heat, causing the pellets to burn hotter and cleaner than their wood counterparts. Wood pellet stoves also emit fewer pollutants than traditional fireplaces. They're considered to be carbon neutral by many environmentalists, due to the fact that the pellets are made from trees that lived on carbon, so the two cancel each other out. The pellets also are inexpensive, and you can purchase them in small quantities as needed, versus the upfront costs of purchasing a cord of firewood.

Given the number of benefits a wood pellet stove offers, it's hard to believe there are any disadvantages. But indeed there are a few. Prices of wood pellet stoves range from $1,700 to $3,000 and usually require installation, which is an additional cost to consider.

Stoves are primarily manufactured in colder climates, such as the Pacific Northwest, so purchasing a stove from an authorized dealer close to where you live can mean paying more for freight costs. Also, if you have limited storage at your house, it could require multiple trips to the store to keep enough pellets on hand. And there's a chance that pellets aren't available at a store near you, which means paying shipping costs to get them delivered.

Stoves run on electricity, so if your power goes out, so does your stove. You can purchase a back-up generator to have on hand -- at another additional cost. And, the cost of pellets and electricity is generally less than what it would cost to run a gas heater per year, but slightly more expensive than a fireplace.

What to Look for in a Wood Pellet Stove

Woody Keeney pours wood pellets from a standard 40 pound bag into a wood pellet burning stove. ­
Woody Keeney pours wood pellets from a standard 40 pound bag into a wood pellet burning stove. ­
AP Photo/Lee Marriner

­There are many makes and models of pellet stoves, s­o it may be overwhelming trying to choose the best one for your home. Before purchasing your stove, make sure pellets are readily available in your area to avoid having to pay shipping costs.

­Consider the square footage of the room that you want to install it in. To keep those tootsies warm, a room needs an average of 5,000 Btu output from the stove per 200 square feet of space [source: Consumer Reports]. If you get a stove that's too big, you'll end up burning the pellets on low to avoid overheating, which is not only a waste of fuel, but also a big cause of pollution. So be sure to do the math before committing to a particular model.

Purchase the stove only from an authorized dealer, especially if you're shopping online. If your dealer doesn't have­ the stamp of approval, your manufacturer's warranty may be void. Look for a label from the U.S. ­ Environmental Protection Agency (EPA) stating that your stove has passed all inspections. Also check to make sure there's a label stating the overall efficiency range as well as the heat output in Btu. High-quality wood pellet stoves typically have an overall efficiency range of 75 to 90 percent.

Wood pellet stoves are only safe to sit on certain materials. Be sure to check with your dealer to see if your floors are OK, or if you need to purchase a different flooring material to put under your stove. If you want to be able to light the stove and walk away for the day, be sure to get one with a large hopper. And if you live in a cold area where power outages are likely to happen, consider getting a stove with a battery backup.

If you like the look of fire, look for a stove with a good flame pattern and a large viewing glass. Some models even have ceramic logs to give you the look of a traditional fireplace. Finishes include enamel, tile and marble, and if you're feeling particularly fancy, you can even get one with 24-karat gold trim.

For more information on keeping yourself warm on a budget, cozy up to the links on the following page.

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Sources

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