Problem #1: Water Damage
ASTM C199 refractory mortar has a sodium silicate base, which makes
it, water-soluble. A deteriorated chimney crown and/or lack of a rain
cover allow water to run down the chimney inside the tile. This water
may mix with chimney soot (this makes it slightly acidic) before it
settles on the smoke shelf. Here it begins to dissolve the
water-soluble mortar in the firebox.
Problem #2: The Firebox Deteriorates
NFPA 211 requires a minimum of 8” inches total fire wall thickness
to effectively block heat transfer to combustible materials next to the
chimney, for example, 2 x 4 wall studs, and siding. When the firebox
deteriorates and the refractory mortar joints are loose or missing, the
ability of the firebox to block heat transfer can be reduced to just
one half the original requirement.
Problem #3: Pyrolysis
Defined as “chemical decomposition caused by heat”, pyrolysis is the
process by which a burnable material exposed to temperatures of
approximately 220° degrees Fahrenheit or more for a prolonged period of
time (and that is not really very hot) will dry out, break down and
burn. It doesn’t need the presence of a direct flame to ignite either;
it simply needs enough heat and oxygen.
Time Is the Enemy
Because your fireplace “has worked just fine” for a number of years
is of little comfort, because time is working against you. Pyrolysis
works its’ destruction in a matter of years, months, weeks, days, or
even hours. In a test conducted by the NFPA, a 22” inch high stack of
wood fiberboards (1/8” inch thickness) was exposed to a heat source of
only 228° degree Fahrenheit. The pile of wood, self ignited in only 96
hours.