When to replace your firebricks
There is really no need to replace a cracked or broken firebrick unless the damage exposes the stove's bodywork to direct heat and flames. Such exposure would adversely affect the stove's tested safe distances to combustible materials and could warp the bodywork, either of which could potentially render the stove unsafe to use. Cracks in clay firebricks can usually be repaired with fire cement. Some minor surface crumbling on vermiculite firebricks is also acceptable. However, like firebrick cracks, crumbling firebricks should be monitored, to ensure that the bricks do not become so thin that this impairs the insulation properties which then potentially allows the bodywork to overheat and become damaged. Firebricks that allow the bodywork to be exposed should always be replaced.