Vortex Powerfans VTX400P duct booster fan, for 4 in diameter duct, is ideal for solving the problems caused by long duct runs. The VTX400P has a built-in automatic pressure switch that detects when air is flowing through the ventilation duct. The VTX400P automatically turns on for a period of 10 minutes and shuts off for a period of 1 minute and turns back on if there is still positive pressure in the duct. The pressure switch is equipped with an adjustment screw that can increase or decrease its sensibility to facilitate the unit’s installation and solve diaphragm orientation issues.