Abstract: The invention relates to a ventilation device for ventilation room by room and/or zone by zone and comprises: a) a device casing (12), which is provided in or adjacent an outer wall of a building or the like and which connects room air to outside air, b) a heat-exchange (22) of counter flow type, provided in the casing (12) and transferring heat from the outgoing and in-going airflows, respectively, c) two fans (18,20) arranged in said casing (12), of which one is connected to the ingoing air duct (14) of the heat-exchanger (22) and the other to the outgoing air duct (16) of the heat-exchanger (22), d) at least one sensor (30) for determination of the airflow's relative deviation in velocity from a neutral value of the airflows passing through each respective duct (14, 16), and e) a control means (32), which, depending on the values given by the sensor (30), is arranged to regulate the rotation speed of the fans (18, 20), for the purpose of substantially balancing the airflow, so that it is substantially equa