I'm getting an OAK for my Harman Accentra insert this week. It is an open box item, so it doesn't have the instructions, so bear with me if this is addressed in them. I know I need a 3" pipe to run up the chimney, but I have some left over 4" pipe and was wondering if it would be problematic to use that? I'm going to get an exhaust adapter or some other metal fitting to retrofit the 4" pipe onto the 3" adapter. I'm assuming a bigger intake pipe won't have any negative effects? Please correct me if I'm wrong. Also, my chimney is probably 15 feet in length. My exhaust runs all the way up to the top (4" flexible pipe). Would it be okay if I ran the intake pipe 5 feet up? Or should it go higher? I'm not sure of the length I need for the OAK to perform as intended. Any insight and shared knowledge is greatly appreciated!