A valve assembly for a blower driven ventilator is disclosed. A first conduit (1) comprises an inlet (2) for receiving fluid from the blower (209) and a first orifice (4). A second conduit (5) comprises an outlet (6) for forwarding fluid to a patient (203). A membrane (8) comprises a front side (9) and a back side (10), wherein at least part of the membrane (8) is movable between a closed position and an open position. A switching circuit (12) enables to selectively open and close a fluid connection between the cavity (11) and the blower (209) via the fluid inlet (2), a fluid connection between the cavity (11) and the patient (203) via the fluid outlet (6), and a fluid connection between the cavity (11) and ambient air (13). A control system (17) controls the switching circuit (12) during a ventilation operation of the blower (209).
