A ventilation system D is usually placed in the ceiling or in the wall, there is also a possibility to fix this in the floor. With this type there is both a mechanical exhaust and supply of air. The heat of the discharged air is transferred to the incoming air by means of a heat exchanger. The new air is blown into the dry rooms, such as the living room or bedroom, via the valves. Since the supplied air is previously heated, there is a limitation of heat loss. With this type of ventilation, the supplied air is first filtered for pollen, pollen and dust.