Is there any reason that the main duct has to run down the middle of the house? I want to move it to one side of the basement. That way as I finish off the ceiling I can box it in and it is out of the way of headroom. If I keep it down the middle using narrower duct will that reduce the airflow? The duct work going to the rooms is that old fiberglass flextubing, it is history, so will be replacing that with good old metal.
I have also finished framing in the furnace room, planning to use a louvered door there. That space is about 8x10. I want to move the hot water heater in there as well.