Besides the hygienic aspect, also the aspect of energy saving of heating residential buildings is very important. This is only possible by mechanical ventilation with heat recovery. This paper describes a part of the large variety of systems, which are nowadays available on themarket. The main difference of these systems are: * single room unit/decentral unit and * central unit for one dwelling or a single family building. For the heat recovery are used: * only a plate heat exchanger (return air/supply air) * only a heat pump (return air/supply air or return air/heating water) or* a plate heat exchanger combined with a heat pump (return air/supply air). As heat source for the heat recovery are used: * only the return air * the return air mixed with the flue gas by using a gas heating system.