Low pressure duct is leaking, disconnected or twisted. Install fan powered boxes. Not Operating Properly Not enough static pressure at the inlet. Too much static pressure at the inlet. Static pressure sensor is defective, clogged, or located incorrectly.
Static pressure setting on controller is incorrect.
Static pressure controller needs calibrating. Fan speed not correct. Inlet duct leaking or disconnected. Main duct work improperly designed. Not enough straight duct on the inlet of the box.