Increase Efficiency - Decrease Air Pollution and Creosote
Wood is abundant in many parts of the Great Land, and using a renewable resource for energy makes good sense. The U.S. Department of Energy anticipates three times as much energy will come from wood in the future.
Woodstoves seem to be the coziest, most independent way to stay warm. Unfortunately, woodstoves emit significantly more pollutants than oil-fired furnaces, and complete combustion of wood (complete efficiency) is nearly impossible.
A great deal is involved in getting the most heat possible from your wood. To accomplish this, and avoid unnecessary pollution and the risk of chimney fires, you should operate your stove properly. These pages describe some simple easy steps for all woodstove users to follow.
There are several steps to getting the fullest benefit from your stove, such as sizing and selection, installation, operation, and maintenance. The brochure: Buying an EPA-Certified Woodstove (PDF 530K) provides a convenient method for determining what size stove is best for your heating needs.
For more information on choosing a clean-burning wood stove or fireplace, and for a current list of EPA-certified woodstoves, visit EPA's woodstove website.
Woodstove efficiency simply means heating the room instead of allowing the heat to escape up the chimney.
When wood is burned, the heat of combustion drives combustible (burnable) gases and water vapor from the log's interior. If an insufficient amount of air exists in the firebox, these gases and other fuel particles will remain unburned as they pass up the stack to the outside. These particles and gases (except water vapor, carbon dioxide and ash) are a source of additional heat energy. If sufficient air is present and the flue gas is hot enough, burning will continue in the air space above the fire and in the lower portion of the chimney pipe. The heat of combustion is transferred to the stove surface and then to the room. At some point along the chimney pipe the flue gas temperature drops below that required to support additional burning. Hence, if all of the fuel is not burned when reaching this point it is either emitted to the outside air or deposited on the inside stack surface as creosote. In either case, a loss of energy has occurred.
Complete combustion of wood to carbon dioxide and water vapor does not produce any health-related air pollutants.
Unfortunately, even when operated at optimum conditions, woodstoves do not achieve complete combustion throughout the entire burning cycle. Generally, stoves emit 30 to 250 times more solid particles and up to 1,000 times more carbon monoxide than oil-fired furnaces (on a heat-equivalent basis). These compounds do have damaging health effects and are considered to be air pollutants.
Breathable particles (smoke, creosote fumes, ash)
Carbon monoxide (odorless, colorless gas, typical pollutant of automobiles)
Usually, for pollution to reach levels affecting the public health, two factors must occur simultaneously. First, there must be a significant quantity of pollutant and second, there is little wind to dissipate and remove the pollutant from an area. Valleys or locations surrounded by hills on three sides are especially susceptible to stagnant air conditions.
Although wood has been used almost exclusively for home heating in years past, population concentrations at that time were not high enough to create significant air pollution problems. Today, however, air pollution from wood burning is a problem in several of our communities and cities.
Creosote is a black substance composed of the unburned combustibles deposited on the inside of the chimney or stack. This material represents wasted fuel that has been gradually cooling as it travels up the chimney. Upon reaching a temperature near the condensing point of water vapor (212ºF), this wasted fuel is deposited along with water droplets on the walls of the stack as a black tar-like substance. Since creosote is a fuel, it is extremely flammable at temperatures around 100ºF and could result in a dangerous chimney fire. It is impossible to entirely prevent creosote formation from a wood burning stove, but the quantity can be greatly reduced if you minimize the conditions under which it is formed.
Wood seasoning is the process of storing fresh-cut wood so that it may dry. Fresh cut or downed wood is usually very high in water content (45-55%), especially in southeast Alaska where high rainfall prevents in-field drying. In the interior parts of the state, where annual precipitation is quite low, downed wood may be significantly dried while lying in the woods or stored openly in a yard.
The heat efficiency from burning wood is dependent on the wood moisture content since a proportion of the fire's heat is needed to boil off the water in the wood. Thus, the more water in the wood, the greater the heat loss. Additionally, the extra steam in the firebox inhibits the complete burning of other gases liberated from the wood. If only wet wood is burned, the total wood volume used for an entire winter may be as much as 30% more than if seasoned wood were used.
Because of added water, each log you split and carry into the stove can weigh up to twice as much as a dry log.
When buying a new stove you could literally spend months looking at different brands and designs. However, there are basically two distinct classes: non-airtight and airtight.
Non-airtight stoves have been used since before the colonial days. These stoves are a simple design such as the common potbellied and barrel stoves. Today, they still enjoy a high popularity due to their simplicity and the lower initial costs compared to airtight stoves. However, since the rate of intake air is not critically controlled, usually an overdraft of air occurs. Because the hot air moves quickly through the stove and out the stack, much of the heat is lost to the outside air.
Airtight stoves are devices that are literally capable of suffocating the fire by closing the dampering system. Consequently, by having a very finite control of the air supply, the heated gases stay in the stove longer and more of the fire's heat is transferred to the air inside the house. Hence, a greater overall efficiency is achievable for this class of wood burning stoves. However, to get high efficiency the wood must be dry, the air damper set correctly and the wood loaded to provide good air circulation. Many stoves incorporate design characteristics not readily visible to the eye that improve or diminish the efficiency rating. Such beneficial design characteristics as chambers for preheating intake air, internal baffles to mix air and unburned gases, and secondary combustion chambers are beneficial if the fire is maintained near optimum conditions.
Some new stoves incorporate a catalyst. These airtight stoves can achieve 15 to 20% greater heat efficiency while also reducing creosote formation and air pollution.