Pyrolysis. That is a word you should take note of. If you analyze it and think "gee, might have the same root as pyromaniac", then you are correct. The catch is, the pyromaniac is out trying to cause trouble. Pyrolysis, and how it would affect you or a friend, is purely the result of an error or mistake in workmanship and the rules of physics. What is it all about? It involves things like metal chimneys that are too close to wood, such as the rafters in the attic. Or it might involve a wood stove, an insert or a fireplace with an improper base or that is located too close to wood. Pyrolysis is a scientific phenomenon, not something made up by trouble-making home inspectors despite what some people think. Inspectors are taught to call out a number of similar hazardous conditions, including a B-vent that is touching combustibles, such as wood or insulation. The facts:

Pyrolysis

When the temperature of the wood reaches 248 degrees Fahrenheit the pyrolysis reactions can start, first with the hemicellulose breaking down, and as the temperature rises, this is followed by the cellulose and lignin. As the temperature goes up, the wood dries faster and the problem is accelerated.

If a fireplace, a wood stove, an insert, or the flue from a device that gets hot, is improperly built -- contact with wood --that wood will eventually deteriorate (pyrolysis) and at some point it is able to ignite at a much lower burning point than is typical of wood. It can take five or more years for the wood to dry out, and for this chemical process to occur, but when it happens, there is no turning back and the homeowner will get no warning.

I have some basic temperatures here, provided by a website for fire investigators. I have converted the temperatures, for the common materials found in homes, to Fahrenheit for the convenience of those of us in the USA. Be sure to look at the photos I posted below, to compare to these temperatures.

248 to 302 degrees F -- Wood slowly chars (pyrolysis is working)

302 degrees F --           Decayed wood (found in attics and homes) bursts into flames, ignites

374 to 500 degrees F -- Ignition temp of various common woods

302 degrees F --            Paper yellows

424 degrees F --            Paper ignites

The full chart, in Celsius, is below.

Hopefully, this information, will allow realtors and others to better realize why a home inspector worries about combustible clearances. Another interesting article, with actual photos of charred wood, can be found at Charles Buell's blog.

This is a simple gas furnace B-vent. The reading was

taken five minutes after the furnace came on.

42 degrees below pyrolysis. These vents can reach 300 degrees F.

Electric heater, in direct contact with hardwood floor. 15 degrees

shy of pyrolysis one minute after heater was turned on.

Thanks for stopping by and don't burn the house down,

www.kingofthehouse.com

Bellingham WA home inspector