To limit the probability that masonry of insufficient thickness will lead to an inability to dissipate excess heat that is conducted or radiated from the fire chamber through the back or sides of the fireplace, which could lead to the ignition of combustible building components, which could lead to damage to the building.
To exempt certain fireplaces from the application of Sentence 9.22.3.1.(1), where the risk of excessive radiated heat is reduced through installation of a steel fireplace liner with an air circulating chamber surrounding the firebox.