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This article defines pyrolysis - the chemical change that occurs in wood and other materials, lowering the material combustion point in response to repeated heating at even relatively low temperatures such as around 200F.
An understanding of the meaning and process of pyrolysis forms the basis of standards & requirements for clearances from combustibles, such as the required distances between a flue vent connector or chimney or woodstove and nearby wood framing or other potentially flammable materials. In our page top photo our inspection client is pointing to inadequate clearance between a flue vent connector and wood framing.
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The reason that building codes specify a healthy distance between wood materials (or other combustibles) and flue vent connectors is not just that the heat from the flue will immediately set the wood on fire. Rather it is also that wood that has been heated over time, even to the relatively low temperature of 200 to 300F, will be chemically affected to become more readily combustible.
Our photo at left shows how wood-to-combustible clearance hazards can be tricky to spot - only when this wood door was left open was it in contact with a heating flue.
[Click to enlarge any image]
Definition of pyrolysis and How it Causes an Increase in the Risk of Building Fire
Pyrolysis is defined as the chemical or thermal decomposition of a material when it is exposed to heat.
Watch out: Actual fire also "decomposes" combustible material, but in a fire the material is burned or decomposed both visibly and rapidly - dramatically. In contrast, pyrolysis can decompose material also lowering its combustion or ignition point to begin an actual fire, with no visible change in the external appearance of the material, particularly in the case of wood framing.
Wood exposed to heat, such as wood framing too close to a metal chimney in a building, is chemically transformed in an important way: its ignition point or combustion point is lowered - the wood can actually catch fire at a lower temperature. This means that by pyrolysis, wood and some other combustibles found in buildings are chemically changed by exposure to even relatively low but warm temperatures over time.
The chemical change of pyrolysis lowers the temperature at which a substance will catch fire. What may be surprising is that wood exposed to temperatures as low as 200 degF. can over time be changed such that its ignition point or combustion point is significantly lowered.
In our photo (left) I'm hold ing a ruler showing that we have only 4 3/4" between an oil fired heating appliance flue vent connector and nearby wood framing. Considering that in at un-tuned or dirty oil fired boiler I've measured stack temperatures at a terrifying 1000 degF., it should not be a surprise that pyrolysis is a real hazard in cases such as this one.
Because that chemical change occurs over time as a function of the frequency and duration of exposure of the wood to high temperatures, we can understand that a building owner may use an improperly-installed fireplace, woodstove or other heating device (lacking adequate fire clearance from combustibles) for quite some time before a fire occurs.
Or a fire may not occur after prolonged pyrolysis until a new user happens to build a hotter fire in the woodstove or some other similar change in conditions occurs.
In sum, a fireplace, wood stove, other heating appliance, chimney, or a flue vent connector may have been too close to wood framing for decades before unexpectedly causing the nearby combustible building components to catch fire.
Other Definitions of Pyrolysis - in Creation of Bio Fuels or Charcoal at low oxygen levels
The process of pyrolysis discussed above occurs at relatively low temperatures (200 F) and without attempting to restrict the level of oxygen, in processes that are well-studied and well defined in sources we cite below in this article at References on the chemistry of pyrolysis particularly Shafizadeh (1984) who points out that pyrolysis of wood includes an initial low-temperature process:
That oil fired heating systems easily reach temperatures of 700F and higher is demonstrated by our thermometer measuring flue gas temperature at a modern oil fired heater that needed service. Soot in the heat exchanger slows heat transfer into the hot water (or warm air if it's a furnace) thus sending more heat up the chimney and subjecting nearby building surfaces to high temperatures as well.
But pyrolysis is often defined differently or perhaps more narrowly in special applications. The production of charcoal by heating wood to high temperatures under conditions of very low oxygen is a process that has been used by humans for many centuries. A portion of a Wikipedia definition of pyrolysis is of interest and needs further comment:
Key in understanding the relation between this definition and the changes in wood that we described earlier is the portion of this statement that includes "... does not involve reactions with oxygen ...". The absence of a reaction with oxygen does not have to be due only to the unavailability of oxygen (as in the case of production of charcoal or biofuels). The absence of a significant reaction with oxygen also may be due to the much lower temperature at which wood pyrolysis is occurring. It is also helpful to understand that low-temperature pyrolysis of wood is almost certainly an endothermic process - that is, a reaction in which the wood absorbs energy from its surroundings in the form of heat. 
More recently the U.S. Department of Agriculture's Agricultural Research Service, in discussing the production of biofuels from biomass, defines a higher-temperature use of pyrolysis in the absence of (much) oxygen as follows:
Pyrolysis as a Factor in the Development of Dark Indoor Stains on Building Walls, Ceilings, or other Surfaces
At THERMAL TRACKING BRIDGING GHOSTING we pointed out that some theorists suggest that some fabric fibers are burned by pyrolysis in clothes dryers. Indeed, pyrolysis is a chemical process that through exposure to heat causes organic material decomposition that in turn lowers the combustion point of a material. And as pyrolysis changes the properties of a material it can indeed release gases, liquids (tar) and char (solid residue).
This view can be a bit confusing since pyrolysis is not quite classic combustion as it is commonly understood to involve fuel and flame, for example in consuming of oil, gas, or wood fuels.
Pyrolysis of wood, for example, can occur when wood is exposed to temperatures in the 200-300 degF range [some forensic sources give a lower range of 120-150 degF ] . Pyrolysis of wood does not immediately consume that material nor even change its physical appearance (at those comparatively low temperatures) so much as change it chemically so that its combustion point is lowered. Wood (other than decayed wood) may ignite at 180-220 degF.
At these lower temperatures the ignition point of wood varies considerably by species, moisture level, exposure time to heat at different temperatures and other conditions.
But be sure to also read our notes about other types of carbon like or soot like deposits, discussed at THERMOPHORESIS.
References on the chemistry of pyrolysis of wood - repeated heating lowers the combustion point of wood and other materials
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