A rack system (100) comprises a plurality of racks (105) arranged to form at least one aisle (120) between them. The aisle (120) is sealed such that essentially all of a cooling medium supplied to the aisle (120) passes through the racks (105). A bleeding opening is provided for enabling a bleeding of the cooling medium out of the aisle (120) and an entering of an ambient medium into the aisle (120). A sensor (such as a temperature sensor) is arranged to determine a medium flow direction through the bleeding opening. In one embodiment, a signal generated by the sensor is used to control at least one parameter of the cooling medium supplied to the aisle (120).