Decentralised ventilation systems with continuous operation are also called permanent fans. They are a complete ventilation unit with a built-in cross-countercurrent heat exchanger. The units have two fans that convey air simultaneously. This results in two air flows. The outdoor and supply air flow brings fresh air into the room, while the extract and exhaust air flow removes stale air from the room to the outside. The cross-countercurrent heat exchanger is positioned between the two air flows. In it, the heat is recovered from the indoor air and transferred to the supply air. This means that a large part of the heat remains in the room despite continuous ventilation.
Due to integral filters and the corresponding design of the unit, neither dirt particles nor noise from outside can penetrate through to the rooms. Permanent fans can supply air to and vent individual rooms and can be installed in the external wall both in supply air areas (living room, bedrooms or children's rooms) and in extract air areas (kitchen, bathroom or WC). One appliance that uses this operating mode is the Vitovent 200-D decentralised ventilation system.