Chimney Fires and a Burning Question - Why Not Stainless Steel Liners?
During an unusually cold U.S. winter in 2015, many insurance companies saw an increase in the number of chimney fire claims reported when homeowners increased their use of wood stoves. Annually, an average of 22,700 fireplace and chimney fires cause about $112 million in residential damage. Fortunately, the noise a chimney fire makes usually alerts residents so fatalities from chimney fires are rare, with an annual average of approximately 30 deaths. Most chimney fires involve a terracotta (clay) flue liner and a wood stove. Clay tiles have been used to line chimneys since the 1880s and are still the predominant chimney liner used in new construction. In the early years of my career as a property adjuster in northern New England, I was busy from October to April handling chimney fire claims that resulted from woodstoves connected to a chimney with a clay-lined flue. After the fire, the clay liner was usually replaced with a stainless steel liner, some of which come with a lifetime warranty. Despite their widespread use, clay tiles are not nearly as durable as stainless steel liners.
• Clay tiles are susceptible to cracking from thermal shock when the liner heats up too quickly and cannot expand fast enough to dissipate the heat transfer. Small amounts of creosote may ignite and burn off in a stainless liner with no damage, but they will crack a clay liner. This phenomenon is demonstrated in a chimney safety instruction video. The owner of a hearth shop told me that if a wood stove is used as a primary heat source and connected to a chimney with a clay liner, the liner will develop cracks within five years.
• A special acid-resistant refractory mortar should be used between clay tile sections, but rarely is. The mortar used between tiles deteriorates over time, creating a condition that allows flue gases to escape creating a fire hazard.
• Stainless steel has greater corrosion resistance and can be used with gas appliances that produce corrosive gases, which will deteriorate a clay liner.
• Clay flues are the only chimney liner that do not meet the UL 1777 listing, but are still allowed by code for new construction, despite having also failed the National Bureau of Standards thermal testing in 1949.
• In colder climates, clay tiles can be damaged from spalling (chipping or splintering) as a result of freeze-thaw cycles.
Why are clay tile flues used on new construction rather than stainless steel flues? Generally, there are two reasons - tradition and cost. Masons prefer to work with clay - not stainless steel - and a clay liner costs less than half of what a stainless steel liner costs. Unless building codes are changed, clay tiles will likely continue to be the most popular chimney flue liner used in new construction despite their drawbacks. For underwriters, this means the issue will continue to be tricky, especially since applications are not likely to indicate the nature of flue construction. In fact, of the woodstove supplemental underwriting applications we have reviewed, less than 10% address whether or not a stainless steel liner has been installed.