It doesn't matter much unless you burn a fire in that Fp. Then, heat can get drawn up that gap in what is termed "second chimney effect" and ignite combustibles up in the chase. The listing should address how to seal this gap. I know Heatilator, Heat&Glo brands state to use a high temp silicone rated 300F on their gas units and Rutland fireplace mortar or equivalent on their woodburners. The problem with cementitious sealants is they crack over time.

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disgusted with some people on this forum. Out of here!