The building sector is responsible for around 40 percent of CO2 emissions in Germany. To reduce emissions in this sector more significantly than before, heat pumps are playing an increasingly important role in apartment buildings. However, the requirements for the design of the heat pump and the overall system are more complex in multi-family houses than in single-family houses. In addition to questions of hot water preparation and the heat transfer system, the use of environmental energy at the location (heat source) must be included in the considerations. In residential buildings, geothermal energy or outside air is usually used as a heat source; if available, waste heat from waste water is an interesting option. Photovoltaic-thermal collectors (PVT), which are installed on house roofs and simultaneously serve as a source of heat and electricity for heat pumps, can be a real alternative, especially in urban areas where space is at a premium.