I'm somewhat a newbie. Installed 2 Englanders in 2 previous homes with no issues. Now, I recently purchased this 24-IC to be installed in my 12 year old home. I have no manuals with this unit. This unit has an 8" opening for venting. Is it an atrocious idea to reduce this down to 6"? I've seen in other threads that a 14' flue is the minimum. Is that correct? This install will require starting anew. There are no openings or an existing flue. I am open to any and all suggestions.