Burnt creosote removed from a flue |
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.
Cracked flue tiles from a chimney fire |
· 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.
Break in a flue tile |
· 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.
Burnt creosote on a flue tile |
· 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.
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