When the air supply is controlled mechanically and the air exhaust takes place on a natural way by ventilation openings, windows or shafts there will be an over pressure in the building. To prevent drought, the air supply in the room has to be placed as high as possible and the air inlet grid must have a possibility to be regulated. By preheating the incoming air drought problems can also be decreased. An air filter can be used to clean the incoming air. A ventilator controls the air supply and the outdoor air is transported into the building by ducts. Because of the mechanically produced over pressure in the building, the system is less dependent on the weather conditions than a fully natural ventilation system.