Existing energy recovery systems are used in the fields of air conditioning and ventilation for recovering energy from the outgoing air or external air. Subsequent to an energy recovery system (10), additional thermal energy is extracted from a used air volume flow AB, which is coming from an air-conditioned room, by another system, which consists of a heat pump (3), a heat exchanger (1), another heat exchanger (2), an accumulator circuit (9), an energy accumulator (9.1), and of a mixing valve (6). The associated supply air volume flow ZU is, when heating, additionally heated by the heat pump (3) by means of thermal energy from the used air volume flow AB. This results in increasing the energy yield from the used air volume flow AB. The temperature of the supply air is regulated. When cooling, the supply air volume flow ZU is cooled to the desired supply air temperature.