Under our buildings or streets we find a source of energy that we have so far ignored. The sewer contains thermal energy; its temperature is generally between 10 and 20 °C throughout the year. This temperature allows economical operation of heat pumps for heating nearby buildings (eg nurseries, schools, gymnasiums, swimming pools). The only challenge is the best way to extract the heat from the sewer. Our solution for recovering this energy with above-ground heat exchangers is practical and efficient.