A ventilation system is disclosed comprising a main duct (101) connected to at least one motorized fan (105), and further connected to at least one workplace (Wa; Wb; Wc) via a local duct arrangement (102a; 102b; 102c). The local duct arrangement comprises an asymmetrical bend duct section (106). A first and a second pressure sensor (120,122) are arranged on different positions on the outside of the bend duct section and in communication with the interior thereof. The pressure sensors are configured to communicate with a control computer (200) configured to determine a pressure difference between the positions of the sensors and to control the speed of the fan. Further, a method of operating a ventilation system is disclosed and also an asymmetric bend duct section and the use thereof.