By Ed Brouwer
For Firefighting In Canada

Chimney fires should never be considered routine. A chimney fire can burn with such explosive intensity that it can be – and often is – detected by neighbours or passersby. Flames may shoot several feet from the top of the chimney. People in the house report being startled by a low, rumbling sound that reminds them of a freight train. Do not respond thinking, “It is only a chimney fire.” Think structure fire and respond accordingly.

Traditionally, chimneys were solid structures that were constructed of heavy stone, block or brick. The chimney either was an integral part of the overall building structure, or was connected to the structure by a series of ties between the chimney and the outside wall of the structure. The flue was constructed of firebrick, clay, slate or tile. Few chimneys are built this way today because they are extremely expensive and there is a shortage of experienced, capable masons.

Most modern chimneys are prefabricated from steel tubing or pipe. This double- or triple-walled pipe is extended from the heating device through an insulated thimble in the wall and up the exterior of the structure. The chimney is fastened to the exterior wall by a series of metal brackets. The inner wall of the pipe serves as the actual flue. All chimneys should be equipped with a spark-arrester screen and a cap on the top.

A common cause of chimney fires is the ignition of residue within the flue. This residue is formed when fires are routinely burned at less than free-burning levels. This may be due to an insufficient oxygen flow to the firebox, or when large logs are added to a small fire. Incomplete combustion produces low heat levels and large amounts of fire gases. As these gases travel up the flue, they begin to cool and behave much the same way petroleum does as it is separated at a refinery. When the gases reach their condensation temperature, the liquid clings to the inside surface of the flue. This substance is called creosote. Creosote is black or brown and has a crusty or flaky consistency. It can be tarry, drippy, sticky or shiny. When it is hardened it is highly combustible. If the buildup of the volume of creosote is sufficient, a chimney fire is possible.

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