SUBROGATION OF FIRE LOSSES
It is also very common to see fires caused by pyrolysis in the latter years of a fireplace, when the system has deteriorated with time, allowing gases to escape through small gaps in the liner, failing mortar joints, etc. The numerous freeze-thaw cycles that all chimneys in the north America have been subjected to, will eventually cause the chimney structure to fail, and the liner to be cracked. A cracked liner puts the home at risk for a dangerous housed fire due to the effects of the pyrolysis. It may not happen in one year or ten years – but eventually the wood will be catching fire from the escaping gases through the small cracks in the liner or other places of the fireplace.
In summary, ventless fireplaces, while attractive and portable, suffer from a design flaw that may allow dangerous gases to enter the living space.