Air ducts can come with high, medium and low pressure ratings. These ratings are determined by the amount of positive and negative air velocity the duct can withstand. A medium pressure duct has a specific pressure rating, which includes some standard elements that differentiate it from a high or low pressure system.
Pressure rating for a medium duct system is between 6 and 3 inches. The larger rating is for positive flows, while return pressure flows meet the lower pressure rating, or negative flow.
The velocity of air range a medium duct is designed for is between 2000 and 4000 fpm. Velocity does not measure the same in all parts of the duct, as bends in the duct, proximity to the fan, and duct connections can change the velocity measurement in those areas.
Medium pressure ducts require the use of a class II fan. High pressure ducts use class III fans and low pressure uses class I. The different classes refer to the speed of the motor and ability of the fan to move air at certain velocities.
Smaller spaces require a high pressure duct, while open spaces can use low pressure duct work. Medium pressure ductwork is efficient and economical.
Medium pressure duct must meet the same seal class A, required of high pressure ductwork but not low pressure systems. The A class is defined as ductwork with all seams, joints, fastener penetrations and connections sealed through welding.