Interesting topic and many excellent contributions. At the back of my mind I recollect an old fire investigation manual that referred to the pyrolysis of timber in old cotton mills where the timber was adjacent to steam pipes. I guess that this was probably superheated steam at a temperature of 200 degrees C much hotter than pipes carrying hot water. The repeated exposure to heat was reported to have caused the eventual spontaneous smouldering of the timber which led to significant fires. (If my memory serves me well.)
I have been unable to find any current references to this phenomenon though. I have also suspected this may have occurred in several house fires where log burning stoves had uninsulated steel flue pipes and these were boxed in using timber studding. In these cases the burning of green logs and lack of regular cleaning had led to fires in the flue and it was impossible to say whether the studding had been ignited by radiated heat from the fire in the flue or whether this gradual pyrolysis of the timber could have been a contributory factor. The common factor in each case was that the studding had been about 50 mm from the flue. All speculation so I recorded radiated heat as the most likely cause..
