I have had two granite faces surrounding my zero-clearence fire place crack. It is my assertion that the installers applied adhesive to both the concrete board and the steel firebox insert. The different expansion rates caused the steel box to apply pressure to the granite. Both surrounds cracked the very first time we lit a fire.
The granite installer claims the firebox installation is defective with no specific claim. The face of the granite gets very warm when the fireplace is used. But that is why you use granite and you don't use wood in this application.
I observed the second installation and witnessed the adhesive on the metal firebox. The fireplace insert installation instructions clearly state that the metal firebox must float behind the facing material. Only a flexible heat resistant caulk seal should be used around the opening.
So I feel the installer was in error by adhearing the metal firebox to the granite. The installer refuses to take care of this problem and I need to file a claim against them.
Any info on the expansion properties of granite under heat would be a great help. I intend to take a thermal reading of the granite face with a roaring fire to present in my case.