[zver11]

Have similar problem on old house. Main duct very undersized and prior owner tried to patch over problem with half way fixes. While exact calculation using manual D is ideal, rough rule of thumb: need approx 200 Sq inch cross section area on supply side main duct for 2.5 ton unit. Since duct description sounds like duct board used, take width and height, subtract 1 inch; from each for thickness of ductboard and multiply two numbers. If circular main duct, same principal applies but math more complicated. Using flexible duct results in much more resistance to airflow. Main symptom of inadequate main duct is that rooms getting first tap on main backbone duct get most of the airflow and rooms at far end get little. If this is the case, should have very strong (and likely noisy) flow from first rooms fed by system. If entire backbone duct accessible, ideal choice is to replace main backbone with a proper sized metal duct wrapped in insulation. Metal better, but slightly more expensive than ductboard, but should have separate wrap of insulation applied. Insulating built into duct board and ductboard is cheaper, but has higher flow resistance and more prone to damage, varmints and bacteria/fungal growth issues.