The protection of adjacent combustible material must allow for the fact that wood and other
combustible materials suffer from decreased resistance to ignition after exposure to continual
heating at even moderate temperatures. Wood itself has a relatively high ignition temperature —
ranging from 400 to 480°F for most species. When exposed to sudden heating, fresh wood
must be raised at least to this temperature before a self-sustaining combustion reaction will begin to
take place.
When wood is exposed to heat over a period of time, however, it undergoes a gradual change in
its molecular structure through a process called pyrolysis. The complex organic molecules of
which wood is composed are slowly broken apart, and much of the original weight and structural
integrity of the wood is lost. As this process continues, the material left behind is charcoal,
which is also known by the more ominous sounding and technically correct term pyrophoric
carbon. Pyrophoric carbon is different from wood and has different properties. First, it has a
significantly lower ignition temperature than that of the original wood. Various studies have fixed
this temperature at 200 to 250°F, and there are suggestions that the figure could be even lower.
Secondly, pyrophoric carbon is known to adsorb oxygen from the air into its porous structure. The
adsorbed oxygen can combine with the carbon with sufficient rapidity to generate considerable
heat. In other words, not only will pyrophoric carbon ignite at a lower temperature, but when
exposed to air it can generate some of the heat of ignition itself.3
There are numerous documented cases of ignition of wood near low-pressure steam pipes which
cannot get hotter than about 250°F. Typically,these occur in a hidden area where air cannot
circulate freely to dissipate heat. Invariably, the fire starts after months or even years of exposure,
and there is evidence that the intermittent nature of the heating pattern contributes to the likelihood
of ignition. Fires related to the long-term pyrolysis of wood usually begin with slight glowing of the
exposed charcoal. This incipient combustion releases heat which accelerates the ignition of
adjacent material. Finally, when sufficient heat has built up or when a fresh supply of air becomes
available, flaming begins and the fire spreads rapidly.
Ignition of wood surrounding chimneys is also well documented. While some of these fires may
be due to a sudden rise in temperature into the range of the ignition temperature of fresh wood,
the majority are caused by the same scenario recognized for steam pipes: long term intermittent
exposure to moderate heating. Concealed areas such as floor/ceiling assemblies and wall penetrations are particularly vulnerable, and the on-off pattern of heating from chimneys probably contributes to the problem. Wood exposed under these circumstances is converted to pyrophoric carbon and is "primed and ready" to burn. Often an unusual incident such as accidental overfiring or, as we shall see, a chimney fire provides the occasion for ignition.
The temperature produced by the chimney need not become extremely high. A rise into the 200°F range, together with the self heating properties of the carbon, may be sufficient to initiate the combustion process.
It is generally agreed that exposure to temperatures ranging from 200 to 250°F on a long
term or intermittent basis can result in the ignition of wood. Therefore, the engineering and test
criteria for chimney design require that wood exposed to heating from a chimney not rise in
temperature more than 90°F over ambient temperature, i.e., about 170°F. Some test
standards allow brief periods of higher temperature rise during short-term abnormal
operation tests, but the conservative approach to temperature limitation is paramount.
