Your chimney’s flue system is used to vent the products of combustion. This fact alone makes it prone to cracks.
Your chimney’s clay flue tiles will crack if the flue system becomes too hot. This would occur if a chimney fire occurred. A chimney fire occurs when the fire in your fireplace is no longer contained strictly in your fireplace.
If the fire is able to move up the chimney into the flue system the extreme heat will most certainly crack your chimney flue tiles.
In some cases if only one flue tile is broken near the top of the chimney and it is accessible enough for replacement, then it can be removed and replaced. Using a masonry hammer you would remove the ENTIRE flue tile. Strike it only a few times to crack it further and remove the pieces by hand for best results.
Stainless steel liners are the most expensive repair for the flue system. These are insulated and dropped into the flue, creating a whole new flue system for the fireplace to vent through. This method is a surefire way to repair your cracked chimney flue tile. Always consult with a professional about what repair is most appropriate for your chimney flue liner. Follow any manufacturer instructions and follow all fire and building codes in your area. Consulting with a professional before and after you complete the repair will ensure that the work you’ve done is up to code and safe for use