1. Size by Room Size the duct system room by room. (Note: Oversizing the central unit is a no-no, but up-sizing your ducts by 1 inch or 2 inches can reduce friction.)
2. Provide Return Paths Provide easy ways for air to flow back to the central return grille: under-cut the doors, or install return vents in the walls.
3. Measure the Flows “Commission” the system: Measure all register airflows and room air pressures after drywall, with all doors closed.