Chimney Repairs
What is a firebox?    This is the interior  of your fireplace  where the  fire  is built.
Hopefully,  it is also the  ONLY  place in your chimney's  structure  that  you
experience a fire. The firebox is built to hold wood fires unless it was
constructed to burn only gas appliances. (Gas fireplace fireboxes do not
always meet the standard necessary for wood burning.)  Masonry fireboxes
use "firebrick" that is supposed to be held together with refractory mortar.
This is a mortar that can withstand higher temperatures than regular mortar.  
Unfortunately, it is quite rare in the Miami Valley, to find a firebox that has
refractory mortar.

That is why the joints between the bricks tend to deteriorate. This  is
particularly  evident in what we term as the "facing joints". The face, or facade
of a fireplace  is the brick, or stone, or various other material that is seen in the  
room around the opening of the fireplace. The facade expands and contracts
when heated, at a different rate than the firebrick. The stress put on the joint
between these two materials causes small cracks to appear. Over time, the
continual heating and cooling expands the cracks to the point that the mortar
will often fall out of the joint and leave a rather large gap. Heat and smoke can
easily be pulled into this area, which can become a fire hazard. I have looked
into these openings and seen building lumber within four to five inches from the
inside wall of the firebox!  Does that help you to understand the importance of
the integrity of these joints?

That leads to a statement that is very often repeated in our industry.  "There
are codes for building chimneys, and then there is the real world chimneys."
Which ones do you think we see almost ninety nine percent of the time?

The firebox is one of the most visible parts of your system.  This is an area that
the homeowner can check regularly to determine when mortar joints need to
be repaired, which is known as tuck pointing. If too many joints need attention
and numerous bricks have become loose, or cracked, the entire firebox may
need to be taken down and rebuilt. This is a major repair.

Another problem that weaken the joints in a firebox is water. Not having a rain
cover on your chimney for years can cause deterioration of the joints. You may
not see this effect for many years. The water runs down the liners,
accumulates on the smoke shelf (the flat area behind the damper) and slowly
works through the mortar of the smoke shelf until it gets behind the firebrick
and attacks the regular mason mortar. Years of this process will often show up
in weakened joints. A good rain cover costs a lot less than a firebox rebuild.

Prefabricated fireboxes often have about 1"-2" refractory panels on the back
and side walls that are part of the UL listing. It is better to take care of these
than to replace them. If your unit and parts for it is no longer made, you must
replace the entire unit. If you use after market parts, it will not meet the UL
listing. The original unit, with all parts intact, was UL tested to standards that
allowed it to have the UL listing. After market parts were not part of the test.  
Thus the unit would no longer meet the standard of the test. It will no longer be
UL listed.  

If you find cracks in the walls or the floor, they can be repaired if  they have not
become too wide. What causes the cracks? Since pre fab boxes are so small, it
is often much easier to just toss a log onto the fire instead of gently placing it
on the fire. I understand that it takes more time and care to do this, but if you
don't you will eventually crack the back wall. Very hot panels and a hard object
striking at that time, and of course numerous other times, will crack the panels.
Also some grates scrape or ram the back wall and this will start to gouge spots
in the panels. This creates weaker spots that can crack more easily.