Duct Systems - Pressure Classifications

Duct systems are often divided into three pressure classifications:

• Low Pressure Systems - where fan static pressures are less than 3 in WC (750 Pa) and duct velocities in general less than 1500 fpm (7.5 m/s)
• Medium Pressure Systems - where fan static pressures are in the range 3 to 6 in WC (750 - 1500 Pa) and duct velocities in general less than 2500 fpm (12.5 m/s)
• High Pressure Systems - where fan static pressures are in the range 6 to 10 in WC (1500 - 2500 Pa) and duct velocities in general less than 4000 fpm (20 m/s)

Medium pressure classification is commonly used for primary main ductwork like fan connections, risers and main distribution.

Low pressure classification should be used for secondary ductwork like runouts and branches from main ducts to end terminals.