The guideline applies to air-conditioning systems. The recovery of heat from exhaust air is an important step towards reducing the primary-energy demand for heating. The guidline specifies the basic terminology for heat recovery systems and describes the required equipment, systems, techniques and their operation. To this end, it provides criteria for decisions and characteristics for selection and calculation. The guideline also enables statements regarding suitability and economic efficiency, as well as regarding the reduction of energy expenditure and carbon dioxide emissions.