I just had my licensed chimney sweep in to do a final cleaning before I install a pellet insert -- I've decided on a Harman P35I. But he noted that I have an unusually small flue -- a bit less than 8" square. Now, I'm set on installing an OAK even though my installer (really, all installers I've talked to) suggested I don't need one. Quick question: it appears the 4" vent and the 3" oak will just fit in my flue. Is it OK for them to be butting against each other all the way to the top? I'll run it by the installer but I thought I'd throw it out there. Thanks in advance.