About 5% of the total delivered energy consumption in Germany is used for water heating. Referring to buildings, the percentage of the delivered energy consumption for domestic hot water is about 11%. Due to continuous efforts to reduce the heating demand of buildings the percentage of energy used for hot water supply will increase decisively during the upcoming years. This tremendous amount of energy in the waste water is usually discharged unused in the sewage system. In order to achieve the European objectives of climate protection waste water heat recovery provides an enormous and for the most part unexploited potential to develop resource-efficient heating devices in buildings.