A chimney fire is the burning of residue deposits referred to as creosote, on the inner surfaces of chimney tiles, flue liners and stove pipes.
Causes:
It all begins with incomplete combustion of fuel in the attached appliance, usually a coal stove, wood, or open fire. The unburned volatiles are heated to the vapor state but not consumed due to a lack of adequate heat and oxygen within the appliance. The volatiles distillates escape into the chimney, where they contact cooler surfaces and condense into tar-like deposits and soot.
Layers will accumulate until either the chimney plugs completely, or the chimney reaches a temperature and oxygen level at which the deposit will ignite. These fires tend to burn very hot, due to the level of volatile material present.
Usually, a chimney fire is cause by an old bird’s nest that has fallen into the chimney and lodged there. The fire can be just as serious as when ember ignites the nests as one ignited of soot. In some cases, an old house with a large and very thick chimney can withstand a chimney fire.
Extremely high temperatures can affect the strength of the chimney causing distortion of metal structures, and potential failure of ceramic structures.
Deposits that lead to chimney fires includes using wet/green fuels, the operation of appliances with insufficient air intake, and low operating temperatures for prolonged periods followed by hot fires. This occurs typically when mild weather periods are followed by cold snaps.
There are several major issues that are at risk when a chimney fire occurred. The danger of burnt debris being exposed from the top of the chimney that could ignite other parts of the structure. The main cause of damage is where the heat of the chimney fire will pass through the masonry materials and overheat nearby combustibles.
Prevention and Suppression:
To prevent this build-up of deposits, run your appliances hot during the initial ignition phase regularly, only building short and hotter fires, regular cleaning of flues using a chimney sweep and only using internal chimney structures where possible versus a chimney attached ton an external wall.
What tends to contribute to the problem is the latter being cooler, as well as creating downdrafts which tend to introduce smoke into the structure as the fire subsides. To prevent the bird’s nests, a wire grid can be attached to the chimney.