I'm looking for a source of info on sizing plenums.... We have a situation in a job where because of the location of the furnace we may have to limit the length of the supply plenum. It will end up being 10 or 12 inches long (from a horizontally set Carrier unit) with a single 14" duct coming directly out of the end. Ends up being an adapter more or less to attach duct system to furnace. Any thoughts on problems this might cause?