Around 40 percent of all CO2 emissions in Germany are attributed to the provision of hot water and space heating for buildings. This is mainly due to the fact that inefficient gas and oil heating systems, which are harmful to the climate, are still being used in many existing buildings. In order to achieve the ambitious climate protection goals, it is necessary to equip existing buildings with heat pump systems. Good efficiency values are possible in older buildings! For building modernization, it makes sense to take a holistic approach in the planning, to reduce the building’s total energy demand, to interlink the different retrofit measures, and to optimally design the heat pump based on the specific building requirements. However, our analyses have shown that heat pumps can operate ecologically and economically even in buildings that have undergone little renovation.