In a masonry fireplace, firebrick is used to construct the firebox (the place where you burn the wood). Refractory brick panels line the firebox of a prefabricated metal fireplace. In both cases, the bricks and fireclay mortar are designed to withstand extreme temperatures. However, over time, the brick, mortar, or panels can crack and crumble, creating a serious fire hazard.
If the brick is crumbling, it should be replaced with a new firebrick embedded in refractory mortar.
Crumbling mortar joints in a firebox should be chiseled out and replaced with new refractory mortar. (Refractory mortar is specially designed to withstand high temperatures.) This process, known as tuckpointing, is the same as replacing or repairing mortar joints in any brick structure. The only difference is that in a firebox, the mortar must withstand extreme temperatures.
If the integrity of the majority of the firebrick and mortar in the firebox is in question, a qualified chimney sweep or masonry contractor should inspect it. If replacement is in order, this is a job that is best left to a pro.
Repairing minor cracks and mortar joints in prefabricated metal fireplaces is essentially the same as with a brick firebox with one exception. If a panel develops extensive cracks or is beginning to crumble, it should be replaced with a new panel.
To make the job of finding a replacement easy, you need to jot down the fireplace make and model number. It can be found on a metal plate just inside the opening of the firebox. Give this information to the manufacturer or an installing dealer. Although replacement panels are often a stock item, a special order may be required. This can take from a few days to a couple of weeks. Don't use the fireplace until a full and final repair has been made.
This is a job that most do-it-yourselfers can handle. Remove the old panel by unscrewing the screws that hold it in place. The new panel should fit snuggly against the adjoining panels. When replacing a rear panel, the side panels must first be removed. Refractory mortar is not generally needed as the corners are designed to fit snuggly against one another.