To limit the probability that, under conditions of high temperature, protrusions into the flue will lead to turbulence and an inefficient flow of flue gases, which could lead to the deposition and ignition of particulates and creosote, which could lead to the ignition of combustible building components, which could lead to damage to the building.
To limit the probability that extruded mortar protruding into the required clearance between the liner and the surrounding masonry [see Sentence 9.21.3.8.(1)] will lead to excessive heat transfer from chimney liners to combustible building components, which could lead to the ignition of combustible building components, which could lead to damage to the building.