My ceiling is unfinished and I have a 8"x12" rectangular supply duct and a 8"x20" rectangular return duct running side by side below the joists along most of the length of the hallway in my basement. This will make the finished ceiling in the hallway pretty low, something around 7 feet. The hallway is 4 feet wide, so I'm wondering if I can replace these ducts with a 6"x16" supply duct and a 6"x27" return duct and get back a couple of inches of headroom. I can't seem to find any rectangular duct that's smaller than 8", so I'm wondering if it's a airflow problem for rectangular duct to that much longer on one side than the other, or if it's fine for me to just fashion my own ducts in the correct size.
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