The combustion process when wood is burned is never complete. The smoke from a wood fire usually contains a dark brown or black substance, which has an unpleasant odor. This tar-like substance is called creosote, and it is found almost anywhere in a wood heating system.
At temperatures below 250 degrees, creosote will condense on the surfaces of stove pipes or chimney flues. When the temperature gets below 150 degrees, the creosote deposit will be thick, sticky and similar to tar. This tends to trap carbon from smoke which dries and bakes inside pipes and flues. This flakey substance is very flammable.
Creosote is more of a problem with wood stoves than fireplaces, since the exhaust gases from stoves are cooler than those from the fireplaces. The amount of creosote condensing on the surfaces of the system varies according to the density of the smoke and vapor from the fire – less smoke means less creosote. A serious fire may be ignited if creosote is allowed to build up. Most problems with creosote are due to poor chimneys with a low draft and cold walls. You can reduce the creosote problem several ways.
Vapor in the flue gases may be controlled by using the driest wood possible and using only small pieces of wood during mild weather. The stack temperature can be raised by insulating the stove pipe connection so that it cools as little as possible before reaching the chimney. Using an insulated pipe also aids in increasing the stack temperature.
In many stoves, a sealed overnight fire will deposit creosote even with dry hardwood. To dry the creosote always open the draft caps and let the fire burn hot for at least five minutes every morning and before bedtime. This should only be done in a new or clean chimney and should be done daily or every time you use the wood stove. Allowing hot flame in the chimney at intermittent times can result in a small chimney fire.
No wood-burning system is 100 percent safe and fire-proof. A safe installation and extra care help prevent fire, but accept the idea there could be a fire, and be prepared to handle it. Chimney fires are most likely to occur during a very hot fire, as when cardboard or Christmas tree branches are burned, or even when a stove burns normal wood but at a higher than normal rate. Make certain everyone in the house is familiar with the warning signs of a chimney fire-sucking sounds, a loud roar and shaking pipes. Instruct everyone on what to do in case of a fire. Instruct all adults on how and when to use a fire extinguisher.
Follow these steps if you have chimney fire.
• Call the fire department immediately. Close all openings and draft controls, if you have an air-tight stove; close the damper, if possible. Get everyone out of the house, and put them to work watching for sparks or sign of fire on the roof or nearby. One adult should stay in the house to check the attic and upper floors for signs of fire. Discharge a class ABC dry chemical fire extinguisher or throw baking soda into the stove or fireplace if the chimney is not sound or there is a danger of the house or surroundings catching on fire. The chemical travels up the chimney and often extinguishes the flame. Check the chimney after a fire. A chimney fire can range from 2000 to 3000 degrees which is hot enough to cause deterioration of metal or cause masonry to weaken.
• If a chimney fire occurs once, chances are that it will occur again. A problem with frequent chimney fires is the possibility of the framing catching fire. The ignition temperature of new house framing is about 500 degrees. Over a period of years, as this wood is repeatedly heated by chimney fires, the wood will ignite at a much lower temperature.
• Chimneys need to be cleaned to remove creosote and soot deposits. This will prevent chimney fires and improve the draft as well. How often the chimney is cleaned depends on how frequently the wood burning appliance is used, how it is operated and the type of installation. Some authorities recommend cleaning the chimney after every third cord of wood is burned, and most recommend at least once a year. Any time you observe excessive soot and creosote, the chimney should be cleaned.
• Chimneys are normally cleaned by mechanical means to scrape off any loose creosote buildup. Mechanical sweeping of chimneys not only removes layers of creosote from the chimney surface, it removes the resulting loose soot and creosote from the chimney, fireplace, or wood stove.
• Chemical chimney cleaners are commercially available. They are not intended for use in chimneys already containing heavy deposits of soot and creosote. Chemicals such as sodium chloride, or table salt, are sometimes used as a chimney cleaner. These chemicals combine with water released from a hot fire to form a weak acid that dissolves small amounts of creosote.
• Sodium chloride is corrosive to metal and is not recommended for metal chimneys. Cleaners that contain copper sulfate will coat any soot in the chimney and act as a catalyst to allow soot to burn away at lower than normal temperatures. A substantial percentage of fireplace and wood stove chimneys do not provide a straight path from the firebox to the outside. If chemical chimney cleaning products perform as claimed and cause of debris in the chimney to fall, that debris still needs to be removed from the smoke shelf, baffle, catalytic combustor, or offset in order to ensure a properly functions chimney. Chemical cleaners are intended to be used after chimneys are cleaned or new.
The only efficient and effective method of cleaning is to use a chimney brush, since the brush scrubs the entire surface uniformly. A product called the “Chimney Sweeping Log” has many wondering whether an annual mechanical cleaning remains necessary. The manufacturer of the Chimney Sweeping Log claims that the product contains “specially developed minerals” that act to reduce deposits of tar and creosote, thus reducing the risk of chimney fires. To use the products, you simply place the log in your fireplace or woodstove and allow it to burn for roughly an hour and a half.
A federal court found that the claims by the manufacture of the “Supersweep” product that the logs removed creosote were false and that the name “Supersweep” was misleading. The court’s order expressly bars to manufacturer from future use and any of the flowing claims in connection with their fire log products.
Roger Williams is an agriculture educator for the Oklahoma Cooperative Extension Service in Cherokee County.