I have an exisiting 20' vertical run of 6" double wall stove pipe for a propane freestanding stove that I am replacing with a Harman Advance Pellet Stove. I was thinking of lining the 6" existing stove pipe with 3" Bioflex dura vent. The inner wall of the exisiting 6" stove pipe is 4". Can or should I do this? The exisiting stove pipe just runs vertical through the 1st floor ceiling and through the attic out to the roof. All of the liner specs say it should only be used in masonry applications. Thanks!