AUGUST 26, 2015
When combustible objects are placed too close to a heat source at temperatures of 212°F or more for a long period of time, a chemical breakdown or decomposition called pyrolysis can occur. Pyrolysis is a major cause of fires throughout North America. Every year many tragedies and fires are caused by pyrolysis and yet most people don’t know what it is or the importance of preventing it. Understanding the potential presence of the threat is where prevention of this little-known danger begins.
Materials break down chemically as they dry out due to exposure to extreme heat. Essentially the ignition temperature of the material is lowered so that they ignite much more easily than it was possible before. Severely pyrolyzed wood can ignite at only 212 degrees F, while it would normally have a catch-fire temperature of about 500 degrees F, before it had any exposure to intense heat. Eventually the pyrolyzed wood will ignite, and a direct flame is not required for the fire to start. All that is required for pyrolysis to occur is heat and oxygen.
METROPOLITAN’s fire investigators say that of all the solid fuel-related fires they have investigated, they are caused by pyrolysis over 86% of the time. About 10% of the fires they have investigated were caused by improper installation or failure of the seals, connectors, appliance, piping, valves, or chimney. Another 4% of the fires have been caused by careless use or improper maintenance of heating systems.
What Causes Pyrolysis?
Pyrolysis is practically an inevitable event when fireplaces, fireplace inserts, solid-fuel stoves and furnaces are installed improperly or the system is not maintained to prevent the escape of hot gases into the combustible components. The tragedies resulting from pyrolysis are completely preventable.
All that needs to be done is to ensure that the amount of space between a wood-burning stove, stovepipe, and other heating appliances is at least the minimum recommended by the manufacturer. 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.
It is common for unprotected furnishings, walls, and other combustible structural components to be placed too closely to heat sources. Because everything seems fine for months or even years, homeowners fail to realize that the process of pyrolysis is occurring and could unexpectedly result in a fire at any time.
Pyrolysis is also caused by a damaged chimney liner. Even the smallest amount of deterioration in the chimney liner or the deterioration of seals and joints or the lack of sealants can cause nearby combustibles to catch fire from the escaping gases.
Insufficient Clearances between Fireplaces and Combustible Materials.
Insufficient clearances between fireplaces and combustible materials can be a serious construction defect. The construction, placement, chosen material, and other aspects of fireplaces have to adhere to certain regulations and building codes. These regulations and building codes are in place to look out for the safety of the inhabitants of a building. They can be strict but they do serve a public purpose. Not being compliant with these restrictions can result in a hazardous situation. Insufficient clearances between fireplaces and combustible materials are fire hazards.
Clearances to combustibles is something they didn’t worry about in the old days. It is quite common to see wood beams or 2x4s right against the masonry of a chimney. When there are fires in old homes, it’s also common to find that some of this wood ignited. Oddly enough, the process of pyrolization takes place over many, many years. The unscientific definition of that is that the ignition temperature of wood gets lower over time. In other words, it takes less heat to catch it on fire 50 or 100 years later than when it was new.
Use of Non-Approved Decorative Chimney Terminations.
Use of non-approved decorative chimney terminations is a common problem that many homeowners may have, unbeknownst to them. A construction defect like this may only become evident to a homeowner when there are some obvious issues, such as excessive smoke collection or when products of combustion are venting into the dwelling. If you are having issues with the functioning of your fireplace and chimney you may need to verify that use of non-approved decorative chimney terminations is not the issue.
Non-manufacturer approved accessories installed into fireplace.
Due to the strict requirements and testing process, all manufacturers require that only manufacturer approved parts be used in conjunction with their fireplaces. By using aftermarket glass doors, refractories and other fireplace components that aren’t approved by the original manufacturer, there is a risk of voiding the UL listing and the warranty, creating a liability issue for the person or person(s) providing the services or even for the homeowner.
The chimney structure or flue or connections to the fireplace have become settle, separated or disconnected
Based on our forensic investigations, we see that gaps have been created around the chimney structure or around the liner or the firebox. This resulted in hot gases escaping the fireplace and drying out the wood around it. As we stated earlier, the process of pyrolization takes place over many, many years. And one fatal year, the wood finally burns and causes extensive damage and loss of life.
There are many reasons this shifting or disconnection can happen. At times, it could be a construction defect, or installer error. Many times we observe inadequate mortar or missing mortar due to lack of maintenance or installation of incorrect type of mortar. It is not uncommon to observe lack of seal between the firebox and the lintel at the top of the firebox – this is a common construction defect leading to fires as the escaping gases will slowly pyrolize the wood framing.
Other times, the flue may have shifted in an earthquake, or a metal strap supporting a bend in the system could have failed. At other times, the chimney moves due to severe weather (excessive freeze-thaw cycles, heavy winds, erosion, flooding, and so on). There are also instances where a chimney sweep unfamiliar with prefabricated metal fireplace systems may have disconnected the flue in the process of cleaning the flue, or removing the cap.
Leaking Gas Connections had led to Fires
After gas line is connected, it is a code requirement that each appliance connection, valve, valve train, shall be checked while under normal operating pressure with either a liquid solution, or leak detection device, to locate any source of leak. Tighten any areas where bubbling appears or leak is detected until bubbling stops completely. DO NOT use a flame of any kind to test for leaks.
Leak test with a soap solution after installing or servicing with main burner on. Coat pipe and tubing joints, gasket etc. with soap solution. Bubbles indicate leaks. Tighten any areas where the bubbles appear until the bubbling stops completely.
Before performing any service on the appliance, ensure the gas has been completely shut off, the unit cooled, and the electricity shut off to the appliance. The burner and valve control compartments must be cleaned annually. A vacuum with a brush attachment works well. The logs should be cleaned gently with a soft bristle brush. The logs are fragile and are easily damaged.
Miscellaneous Fireplace Construction Defects
Several times we have observed missing or blocked out cleanout pits. Oftentimes during renovation of old homes, we see that the fireplace ash pits are covered to create a finished room. This has created fire hazards as the ash inside the pit will ignite when enough of it accumulates inside the ash pit.
At other times, the valves inside the fireplace will get clogged with debris (pet hair, ash, dust, etc.). Sometimes these valves are improperly installed and they leak, creating fire hazards.
Finally, we often see that manufacturer’s instructions are not followed during fireplace installation. The key point to make here is that all equipment and appliances and instruments have a certain operating range that if exceeded due to improper construction or installation or maintenance, these components will fail causing property damage and loss of life.
As we noted earlier, if subrogation is contemplated, it is important to notify potential responsible parties to inspect the scene, prior to evidence removal and storage, in order to avoid spoliation claims.