All of our inserts are designed to function with a 6" liner. However, it is adequate to install a 5" or 5.5" liner when the masonry chimney is too tight for a 6" liner. Under certain conditions, the probability of a drafting issue is increased, which could favor smoke spillage and/or reduce combustion quality. But generally speaking, the vast majority of homeowners who properly install a 5" or 5.5" liner and use good heating techniques will be very satisfied with the performance of their wood insert.