Bellingham Home Inspector (King of the House): Pyrolysis and Your Safety

"Pyrolysis" The word shares a root word with the term "pyromaniac".  That is not a coincidence as both involve fire. Pyrolysis is sometimes present when hot things, like metal chimneys, are too close to wood -- such as the rafters in the attic. Or it might be present if a wood stove, a fireplace insert or even a B-vent is too close to a combustible. Pyrolysis is a known scientific phenomenon, that has been witnessed and defined by the scientific community. The facts:

Pyrolysis

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

If a wood burning device, or a flue or a B-vent from a gas appliance is too close too, or contacts wood, that wood will eventually deteriorate (pyrolysis) and at some point it can 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 will be no warning.

Some critical temperatures are provided here, taken from a chart prepared for fire investigators. 

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.

The photos below, when compared to charts above, show that dangers exist in these homes.

Hot B-vent

 Electric heater on hardwood floor

• Steven Smith's professional profile
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