Overtime everyone with a fireplace will need to do some type of repairs and it is extremely important that the repairs are done right. For my own peace of mind I would not attempt any major repair to a Fireplace unless I had been properly trained by working with someone who was a professional. I have worked on wood burning fireplace, Oil Heat Chimneys, and Gas Heat Vent systems as a contractor but as easy as some of the work is I understand my limits and I would always suggest that a home owner hire a contractor who specializes in this work.
Fireboxes are the area of a Fireplace where you light your fire. They contain the burning wood and heat within the fireplace, they also reflect that heat back into the room and they contain the gasses and direct them up the chimney and out of your home.
They are an extremely important area within your fireplace because they see dramatic shifts in heat and must contain the fire so it will not lead to a house fire. In the matter of a few minutes this area will go from a cold room temperature of about 70f to thousands of degrees hot. The brick or slabs used in this area must be durable but also able to withstand these heat changes over many years.
Why do Fireboxes Crack or Deteriorate?
Like we said the extreme temperature changes in a Firebox will put high stress on firebrick used for your firebox. Overtime the brick will deteriorate however there are some things that can expedite this.
Most important is that your chimney has a firecap which will limit rain or moisture from entering the chimney and getting into the brick. Water present in a brick or mortar joint when exposed to high heat will turn to steam and will crack the brick.
Next is over firing your fireplace. There doesn’t seem to be much training in how to properly stack a fire and many people will simply place another log in their fireplace just because it fits. If the fireplace is overfired it will cause the firebrick to fail. This is more common than you think and inspections and training in the proper use of your specific fireplace will limit this.
I strongly suggest the first year you own your home that you have a competent chimney company clean your fireplace and provide you training in how you should use your fireplace including the proper stacking of wood.
Common Damage and Repair
The most common damage would be loose mortar joints. If the refractory brick is loose it may need to be completely reset. This can be done by repairing a single brick but then often requires repointing of all the bricks in the fireplace.
Repointing of firebrick is a normal maintenance problem and not actual damage. You will need to repoint your joints as needed throughout the life of the fireplace depending on the amount of use it sees. Repointing requires removing about a half inch of the mortar joint and replacing it with fresh refactory mortar or a specific joint clay compound.
Actual cracks in brick and refactory panels are the hardest problems to cure. If you have a single brick that is damaged then you may be able to replace that single brick and redo its mortar joint.
Refractory panels that crack need to be replaced and there is no fixing them with mortar compound. Some people will try to fix these cracks but it is a really bad way to deal with this problem and it will not cure it.
If you have a cracked refractory panel it is possible to replace them yourself but the material normally comes in a larger piece that must be cut to fit. It may be just as easy to find a local fireplace company to replace this piece for you or at minimum cut the panel for you so you can replace it. You will need to remove the old piece and measure it or provide it to them for cutting.
Final Note
As said above Fireplace work can be performed by the home owner but only if you have been trained to do this work. It does not take a lot of time but it does require that you do things correctly or you will run into some serious problems.
My suggestion is that you hire this work out if you are unsure of your skill level.
There are usually ways of repairing a fireplace and chimney before it requires full tare down and rebuilding but even if you are at that point you can look at manufactured fireboxes and stainless steel chimney liners for retrofit.