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Break in tile flue liner - note that it goes all the way through to the exterior of the tile

Exterior of tile liner with breaks

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Blown out section of tile flue liner

What you should know about chimney fires

By Marge Padgitt

The following information is from studies conducted and researched by the Midwest Chimney Safety Council, HearthMasters Education, and the Chimney Safety Institute of America. Much of this information is from the book Chimney Fires, Causes, Effects and Evaluation by the CSIA.  A copy of this book may be obtained by calling the CSIA at 317-837-5362. 

These questions often come up when evaluating chimney damages: What is chimney fire damage?  And what other possible causes might there be for damages to a flue liner, chimney, smoke chamber, and chimney cover? The MCSC has put together these guidelines in order to help insurance adjusters, engineers, and chimney inspectors determine the causes for chimney damages. This is very brief outline, and we suggest that a copy of the book named above be obtained if further explanation is needed.

​- Most chimney fires occur without the homeowner’s knowledge—in fact, only very few fires are witnessed or reported to the fire department.

​- When a sudden temperature differential of 500 degrees occurs in a chimney, the clay tile flue liners will crack due to expansion. This differential cannot be obtained by the normal operation of a fireplace or wood stove, and has not been able to be duplicated in field study.  Studies show that a chimney fire is the most likely candidate for the cause of tile liners to break. 

- Tile liners will break longitudinally first, due to the nature of their construction, then horizontal and diagonal cracks will occur in more severe fires.

- A NON-creosote chimney fire can occur when flue gasses accumulate in the flue and will ignite when      temperatures reach 1000 degrees. Note: Creosote ignites at 1000 degrees.

- Burnt, ash creosote may found in the flue and smoke chamber after a chimney fire.  This is lightweight, expanded creosote that can only be created by a chimney fire. 

- Isolated scorched areas of the flue may be present (although not always) and are positive indications of a chimney fire, since accumulating creosote does not avoid particular areas.

- Tar glaze may have melted away from the fire. Some creosote may melt and flow away from the combustion zone and may be found in the smoke chamber or damper area, or around the thimble entrance of a stove pipe, or around a chimney cover.

- Fires of long duration may cause thermal expansion of the masonry such as the cement crown, facial wall, and exterior chimney, which will result in clean breaks in the masonry.

- Holes and mortar bond breaks may be found in the smoke chamber area and flue after a chimney fire due to expansion.

- The chimney cover may be warped, discolored, or damaged. 


Myths regarding tile flue liner damages

- Thermal fatigue (p 4-11) (years of expansion and contraction) cracking: no evidence is found to support this idea.

- Lightning: (p 4-9) lightning can damage flue liners, but there is usually other damage to the chimney such as blown out bricks at the top of the stack.

- Moisture– (page 4-12) Rain entering the chimney from the top of the flue and from condensing flue gasses: Washed-out mortar joints and spalling (flaking) flue liners are caused by moisture. No evidence has been found to support the suggestion that cracked tiles are the result of moisture damages, however, if the chimney was not constructed properly with air space between the flue and surrounding masonry, and water leaked into the chimney between the flue and masonry and froze, it is not inconceivable that the expansion might cause a liner to crack horizontally.

- Settlement: (4.3.3) “Settlement is an overly-used diagnosis of distress in masonry structures of all types.” However, it does occur.  Look for inadequate foundation or footing and uneven settling.  Also look for shifted or offset flue tiles, which shows movement.