Burning Wood Better
by L. Walker - Colorado State University Cooperative Extension agricultural engineer and research associate, chemical and bioresource engineering. 9/92 - Reprinted with permission
* Inefficient burning of wood causes an air pollution problem as well as less efficient use of wood fuel.
* The operator of a woodburning device needs to maintain a hot fire with an adequate supply of air to burn the volatile gases as they are released from the wood.
* Buying a stove that is too large tempts owners to burn wood slowly.
* Control heat output primarily by fuel load size rather than by air control only.
* Burn only dry and properly seasoned wood in a stove. Never burn trash, coal, railroad ties, plastics or wrapping paper.
Author’s note: This fact sheet is not a comprehensive document about the operation of any specific woodburning device. Consult your owner’s manual or your local stove and fireplace dealer for specific instructions.
Over 80 percent of the timber harvested in Colorado is burned in fireplaces and wood stoves, according to one estimate. Unfortunately, the rediscovery of our oldest heating fuel is causing an increase in air pollution, which is especially apparent in large metropolitan areas and towns located in high mountain valleys. The reduced visibility and pungent odors created by the inefficient burning of wood represent an air pollution problem and a less effective use of wood fuel. The Causes of Pollution and Wood Waste Three common causes of wood smoke pollution are:
* Topography and weather conditions in high valleys and natural basins. Pollutants released during frequent temperature inversions are concentrated near the ground.
* The large number of woodburning devices (fireplaces, fireplace inserts and stoves) concentrated in certain areas.
* Improper operation (slow burning) of woodburning devices. Slow burning causes incomplete combustion that results in increased production of particulates, carbon monoxide and other harmful pollutants. These pollutants reduce visibility and produce odors that are objectionable to many people. Researchers are investigating possible health effects.
Woodburners cannot change the topography or weather conditions, but they can significantly improve their operating methods. This will increase the effectiveness of the woodburning device, resulting in cleaner air, increased heat output and greater safety by reducing creosote deposits inside the chimney. (Creosote is a highly flammable substance produced by the incomplete combustion of wood.) Proper operation also results in more economical use of wood, minimizing the woodburner’s investment of time and money.
Be aware of community air conditions. It is best not to burn wood at all when local air quality is poor.
In order to understand the effective operation of a woodburning device, we need to grasp a few easily understood principles. Volatile gases are released from wood as it burns. Fifty percent of wood’s heat value is contained in these gases and, if not completely burned, they contribute to air pollution. The gases are released from the wood at relatively low temperatures but will not burn until they reach much higher temperatures and are mixed with a proper amount of air. The operator of a woodburning device needs to maintain a hot fire with an adequate supply of air to burn the volatile gases as they are released from the wood. Getting the Worst from Your Stove
Controlled draft stoves and fireplace inserts generate excessive air pollutants if operated improperly. A common, but highly improper, operating procedure involves fully loading the stove or insert with wood and restricting the air flow through the unit by almost closing the draft controls. This creates a cooler, slower burning fire that produces a great deal of smoke. This smoke not only represents lower combustion efficiency and increased creosote deposits on chimney walls, but also adds to the air pollution problem.
A stove that is too large tempts owners to burn wood slowly. Some stoves have a large heating capacity. When homeowners purchase units that have too much capacity, and when those stoves are filled with wood, they tend to produce more heat than is needed to be comfortable. Since the units are “airtight,” the homeowner can reduce the heat output simply by restricting the air flow to the fire. This results in a comfortable heat output, but also in the slow burning of wood in an air-starved condition. Polluted air and dirty chimneys are inevitable results. Getting the Best from Your Stove
A better way to control heat output from a stove is to limit the amount of fuel burned at a given time. Always maintain a hot fire. Do this by adding modest amounts of fuel. When warming requirements are low, maintain a small hot fire. Greater heating needs call for a larger hot fire. Always control heat output primarily by fuel load size rather than by air control only. (Nighttime heating requirements can best be met by building a large brick or stone mass around your stove. The hot stove will transfer heat to the thermal mass and the mass will radiate heat to the room for several hours after the fire goes out.)
When adding wood to a stove, open the draft controls to maximum setting for 10 to 30 minutes to quickly raise the combustion temperature. Then partially close the draft controls to obtain a moderate burn rate. However, monitor the stove’s performance to ensure that adequate temperatures are maintained for more complete combustion. A simple way to monitor the stove’s operation is to observe the smoke coming from the chimney. Little or no smoke is visible with efficient combustion.
For cleaner air and better fuel economy, burn wood that is dry and properly seasoned. Green or wet wood lowers the burning temperature, causing incomplete combustion. Split wood and stack it loosely in a dry place (preferably for one full summer) before use.
Don’t burn trash, coal, railroad ties, plastics or wrapping paper in your stove. Burn only seasoned wood. Stove Sizing—Bigger Isn’t Better!
Before buying a stove or fireplace insert, carefully consider the size you need. Maintaining comfortable room temperature with an oversized unit usually is achieved only by decreasing the air supply, thus increasing air pollution due to incomplete combustion. Careful selection of a properly sized stove reduces the temptation to burn slowly and helps limit air pollution because the stove owner will find it easier to maintain high combustion temperatures.
Consult carefully with your stove dealer on this important aspect of stove selection. Remember: “Bigger isn’t necessarily better!”
Minimizing Heat Loss and Pollution
It does little good to use your fireplace for heating purposes. Typical fireplace efficiencies are from minus 10 percent to plus 15 percent, compared with wood stoves that can be as efficient as 70 percent or better. Under even the best conditions, very little heat can be gained from most fireplaces. They are primarily aesthetic devices.
However, a few techniques can be helpful in minimizing heat loss and limiting pollution from a fireplace:
* Do not use your open fireplace at all when the outside temperature is below 20 degrees F. The infiltration of cold air into your home more than offsets any heat gained.
* Build a very hot fire. What little heat that does come from a fireplace comes from heating the fireplace masonry. Radiant energy from the hot masonry warms whatever is directly in front of the fireplace. The best way to obtain a hot fire is to use a relatively large fuel load. Stack the wood so that the logs are close enough (1 to 2 inches) to keep each other hot, yet far enough apart to allow the free flow of air between them. Use enough logs and arrange them in such a way as to concentrate the heat in the central part of the fire.
* Use fireplace doors wisely. Glass doors on fireplaces help reduce the escape of heated air from the house and up the chimney, especially at night when a fire is burning out. Close the doors and damper when the fireplace is not in use. Open the glass doors when using the fireplace. This allows some radiant heat from the hot masonry to enter the room. Be sure to have the spark screen in place.
Dramatic strides have been made in woodburning technology. Starting in 1990, the EPA mandated “clean burn” standards which have resulted in stoves which burn 10X as clean as earlier models - AND, more efficiently. New combustion-controlled fireplaces, which achieve efficiencies comparable to most EPA Approved wood stoves, are also available.
These newer stoves and fireplaces (generally, those available from 1990 on) when installed and operated properly, will reduce woodburning pollution and increase stove efficiencies.
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