i currently have a 7" liner in my oil furnace. I think the installers only used 304 stainless and it has put pinholes in my liner . My furnace only needs a 5.5" My old liner is pour insulated. I was gonna rip out the liner and inso but now I'm thinking about just ripping the snout off the 7" and dropping my 5.5 through the 7" and just insulating In between the liners. What do you guys think?