In a typical design process, ductwork size is minimized. In an integrated system design process that uses the concept of "right sizing", ductwork "oversizing" can be justified. Relatively small duct sizes require larger pressure drops and more fan energy than larger duct sizes. Therefore, for instance, extending a duct plenum makes a larger but effective lower-pressure distribution system. In addition, the end of a critical duct path run should be slightly oversized to keep velocity and pressure losses lower. It is usually more cost effective to maintain a duct size to the next branch or take-off rather than installing a fitting that would only reduce the trunk by 50 mm (2 inches). Another place where oversizing comes into play is noise elimination. In general, lower velocities prevent rumbling or whistles that would otherwise require energy-consuming sound attenuators to eliminate.
