ACCA Manual D is the industry standard for residential duct system design. It provides the engineering methodology for sizing ductwork to deliver the correct amount of air to each room as determined by the Manual J load calculation.
Manual D calculates the total external static pressure available in the system, then sizes each duct section to deliver the required CFM (cubic feet per minute) to each supply register and from each return grille. It accounts for duct length, number of fittings, duct material, and system operating conditions.
Poorly designed ductwork is responsible for a significant portion of HVAC comfort complaints and efficiency losses. Common duct design errors include: undersized return ducts (the most common issue), excessive duct length, too many sharp bends, flex duct installed kinked or compressed, and improperly sized supply registers. Manual D calculations should be performed by every HVAC contractor designing a duct system.