Wood Heating For The Home
Before the 20th century, 90 percent of Americans burned wood to heat their homes. As fossil fuel use rose, the percentage of Americans using wood for fuel dropped, falling as low as one percent by 1970. Then during the energy crises of the 1970s, interest in wood heating resurfaced as a renewable energy alternative. Air pollution, however, grew along with the renewed interest since wood burning (combustion) emits air pollutants, including nitrogen oxides, carbon monoxide, and particulate matter. This led the U.S. Environmental Protection Agency (EPA) and many states to develop low emissions requirements for wood-burning appliances. The wood-burning appliance industry rallied to meet the strict new emission levels. As a result, today you can choose from a new generation of wood-burning appliances that are cleaner burning, more efficient, and powerful enough to heat many average-sized, modern homes. Altogether, the wood-burning appliance you choose, your chimney, how you maintain the system, the fire you build, the wood you use, and even the safety precautions you take will determine the overall energy efficiency of wood heating.
In areas where wood is readily available, wood-burning stoves are a possible alternative or supplement to furnaces. Although wood-burning stoves may save on utility bills, they can also require a significant amount of work on the part of the homeowner. Modern wood stoves are designed to minimize smoke and other pollutants, but wood stoves still produce much greater air pollution than a modern high-efficiency furnace.
Fireplaces, though appreciated for their aesthetic appeal, should not be thought of as a heating device. Although fireplaces will heat the immediately surrounding area, they will also draw cold air in through the rest of your house and draw much of the heat in your house up the chimney. Fireplaces also produce significant air pollution. When burning a fire, you should turn your heat down or off and open a window near the fireplace. Glass doors for the fireplace opening and fresh-air intakes will help reduce energy losses.
Wood-Burning Appliances: A wood-burning appliance's efficiency is a product of its combustion efficiency (how completely fuel is burned) and heat transfer efficiency (how well the device transfers heat to the home). When it comes to heat transfer, wood-burning appliances have either radiant or convective attributes-or a combination of both. Radiant devices radiate heat onto nearby objects directly. When we rub our hands in front of a fireplace, we experience radiant heat. A convective device, on the other hand, includes a heat-transfer system that circulates air through the appliance and distributes it, thus heating a home's air supply.
The location of the appliance (and chimney) will influence how well heat is distributed and conserved in your home. Wood-burning appliances are essentially space heaters. Therefore, it's best to put one in the room where you spend most of your time, but it should include a way for heat to circulate to the rest of the house.
It's also important to use a properly sized appliance for the space to be heated. You don't want to make the mistake of purchasing one that is too big. 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. A reputable dealer should talk with you about size requirements, but here's a good rule-of-thumb: 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. (A Btu is the amount of heat needed to raise the temperature of 1 pound of water 1° F.)
Because most air pollution from wood-burning appliances results from incomplete burning of the wood and smoke, both of which waste energy, strategies for increasing energy efficiency often help reduce air pollution as well. Although industry is working towards improving technology so that appliances burn 100 percent of all wood and smoke (including particulates), even the most energy efficient ones have not yet reached this ideal. However, if you have an older wood-burning appliance, consider upgrading to one of the newer EPA-certified appliances. You will decrease emissions and choose a more energy efficient way to burn wood. All woodstoves sold after July 1, 1992, should bear an EPA certification sticker.
High-efficiency, wood-burning appliances not only have lower emissions but they are also often safer. For example, in addition to saving energy, complete combustion helps to prevent a buildup of flammable chimney deposits called creosote. A wood-burning appliance that has been tested for safety will bear a label from a recognized authority. However, avoid buying a secondhand appliance even if it has a safety certification label. A minute flaw resulting from its previous use could result in a hazard.
For safety, and to maximize efficiency, you should consider having a professional install your wood-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 wood-burning appliance to heat your home. The following is a brief overview of the different types of appliances available.
High-efficiency fireplace inserts: Designed more for show, traditional open masonry fireplaces fall at the bottom of the list in energy efficiency. On average, they are only about 10 percent efficient. In fact, conventional fireplaces can cause net heat loss because they can draw so much warm air out of a home while contributing little heat.
A few devices have been designed to improve the energy efficiency of conventional fireplaces. Of these, only high-efficiency fireplace inserts have proven effective in increasing the heating efficiency of older fireplaces. Essentially, the inserts function like woodstoves. They fit into the masonry fireplace or on its hearth, and use the existing chimney. You must install a flue collar that continues from the insert to the top of the chimney. A well-fitted fireplace insert can function nearly as efficiently as a woodstove.
High-efficiency fireplaces: Some modern fireplaces heat at efficiencies near those of woodstoves 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.
Masonry heaters: Masonry heaters produce more heat and less pollution than any other wood-burning appliance. They are common in Europe, but you won't find many in the United States, perhaps because of their high installation costs. Masonry heaters include a firebox, a large masonry mass (such as bricks), and long twisting smoke channels that run through the masonry mass. 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 percent.
Advanced combustion woodstoves: 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 to 72 percent. 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 their catalytic cousins described in the next section.