Chimney Fires
Solid-fuel-burning appliances—if improperly maintained or installed—are the cause of many residential fires each year. The majority of the solid-fuel-burning devices in residences are wood-burning units. A consideration for the investigator is how the unit is utilized. Many fireplaces are used sporadically and for aesthetic value. Other wood-burning units are used as primary heat sources for the structure. The maintenance of the unit and its initial installation are considerations that need thorough investigation.
Donan Engineering is routinely involved in assessment of the condition of wood-burning chimney systems. In recent years, homeowners have become much more aware of the need for proper cleaning and maintenance of wood-burning chimney systems. Certified chimney sweeps can assess the functionality of chimney systems. As more and more homeowners use chimney cleaning services, more chimneys are deemed “damaged by fire,” meaning that most insurance companies cover damage to any portion of the structure that is the result of a “hostile fire.”
Many different types of approved wood-burning chimney systems are used. Masonry-constructed clay-tile-lined chimneys are very common units. When masonry clay-tile-lined chimneys are constructed, many times the expansion of the liner due to subjection to elevated temperatures is ignored, though this liner will expand each time it is heated. If insufficient clearance for liner expansion exists, the liner will most likely fracture when it expands.
Numerous types of metal pipe are used for wood-burning fuel devices. Triple wall air-separated pipe, double wall air-separated pipe, and double wall-insulated pipe are frequently used. The biggest fire hazard with the use of metal chimney pipes is maintaining the proper clearance to combustible materials. Discoloration and severe oxidation of the interior of metal chimney pipe is the most easily recognizable evidence of excessive temperature.
If a chimney fire has occurred, some very distinct and recognizable evidence will remain. Chimney fires are the result of creosote build-up inside the chimney that is ignited by the heat of the exhaust gases; this causes the temperature inside the pipe to exceed the safe operating temperature rating of the pipe. During the chimney fire event, the burning creosote will be carried out of the top of the chimney. As the creosote exits the chimney and is cooled by atmospheric air, it will condense, and evidence of a brownish liquid will be present on the exterior of the chimney and inside the vent cap. If a chimney fire occurs, and fire is involving 15 to 20 feet of metal piping, the air influx at the wood burning device will be so intense that a roaring sound will be heard. Any person that has witnessed a chimney fire is quick to mention the noise associated with the event. Before damage to a chimney can be categorized as the direct result of a “hostile fire,” indicators must be evaluated. If no evidence of creosote deposits, report of a roaring sound, excessive evidence of oxidation on metal surfaces, or large fractures in clay tile liners is present, a hostile fire did not occur. If it cannot be determined that a hostile fire occurred, it is unlikely that any significant damage found in the chimney system is an insured loss.
