Automatic pressure switch allows for fully automatic operation of an inline duct fan for supply or exhaust air ventilation systems. The switch senses positive, negative or differential air pressure and activates or deactivates the fan. Fan runs for 10 minutes when pressure is sensed in the duct line then shuts off and turns on again as needed.Automatic pressure switch allows for fully automatic operation of an inline duct fan for supply or exhaust air ventilation systems. The switch senses positive, negative or differential air pressure and activates or deactivates the fan. Fan runs for 10 minutes when pressure is sensed in the duct line then shuts off and turns on again as needed.