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 is found almost anywhere in a wood heating system.
At temperatures below 250 degrees F creosote will condense on the surfaces of stove pipes or chimney flues. When the temperature gets below 150 degrees F 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 flaky 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), the temperature of the surface on which it is condensing (higher temperatures reduce chance of creosote condensation) and the type and dryness of wood being burned. (Vis. 1) Creosote may build up to a considerable thickness on the interior surface of the chimney and the draft opening may subsequently be reduced. 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.
Draft can be increased by having as few bends as possible between the appliance and the chimney, having the proper height and diameter, keeping the chimney in good repair and by having a separate flue for each appliance. Also use proper sized stove pipe. In a large chimney, draft can be increased by decreasing the flue size. This can be done by installing a new smaller flue
or a stainless steel stove pipe liner.
No wood burning system is 100% safe and fire-proof. Extra care help prevent fire. 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 fire.
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. After you once have cleaned the chimney, you may want to check it after 2 weeks, 1 month, 2 months, etc., to determine how often your chimney needs to be cleaned.
Chemical chimney cleaners are commercially available. These 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.
Chemical cleaners are intended to be used after chimneys are cleaned or new.
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