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Wood and Pellet Heating Basics

August 16, 2013 - 3:02pm

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Wood-burning and pellet fuel appliances use biomass or waste resources to heat homes or buildings.

Types of Wood- and Pellet-Burning Appliances

The following is a brief overview of the different types of wood and pellet fuel 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, fireplaces are still energy losers.

Only high-efficiency fireplace inserts have proven effective in increasing the heating efficiency of older fireplaces. Essentially, the inserts function like woodstoves, fitting into the masonry fireplace or on its hearth, and use the existing chimney. A flue collar must be installed that extends from the insert to the top of the chimney. A well-fitted fireplace insert can function nearly as efficiently as a woodstove.

Wood Stoves and Wood-Burning Boilers

Wood stoves are the most common appliance for burning wood. New catalytic stoves and inserts have advertised efficiencies of 70%-80%.

Advanced combustion woodstoves provide a lot of heat but only work efficiently when the fire burns at full throttle. Also known as secondary burn stoves, they can reach temperatures of 1100°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 combustion stoves have advertised efficiencies of 60%-72%.

Like wood stoves, centralized wood-burning boilers have been improved over the last several years. Modern, centralized wood heaters use wood gasification technology that burns both the wood fuel and the associated combustible gases, rendering them efficient up to 80%. In addition, systems are available that can switch to oil or gas if the fire goes out.

Masonry Heaters

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 12–20 hours. Masonry heaters commonly reach a combustion efficiency of 90%.

Pellet Fuel Appliances

Pellet fuel appliances burn small, 3/8-1 inch pellets that look like rabbit feed. Pellets are made from compacted sawdust, wood chips, bark, agricultural crop waste, waste paper, and other organic materials. Some models can also burn nutshells, corn kernels, and small wood chips. 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.

Pellet fuel appliances are available as freestanding stoves or fireplace inserts. Freestanding units resemble conventional cordwood heaters in that they generally heat a single room well, but not adjacent rooms unless you use a fan to force the warm air into those other spaces. There are also fireplace inserts that 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 35 and 130 pounds (16 and 60 kilograms) 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 side wall or upwards through the roof. More advanced models have a small computer and thermostat to govern the pellet feed rate.

More Information

Visit the Energy Saver website for more information about wood and pellet heating in homes, including system selection, installation, chimney placement and sizing, maintenance, and fuels.

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