The increase in fossil fuel prices and the negative environmental impacts of fossil fuel use force people to use renewable energy more widely. Recovery of waste heat also ranks among the renewable energy sources. It is proposed mainly in passive houses and energy-efficient buildings. Heat from wastewater can be optimally used for heating, cooling and hot water preparation in low-energy houses. Thermal energy can be extracted from the drain outside the housing and also in the building. For heating or cooling systems in buildings sources of wastewater with higher flow, for example the administrative buildings, the swimming pools or industrial produc- tion are suitable. For preheating domestic hot water locally regenerative systems inside buildings can be used.