Each cold winter day it’s possible to see warm air coming up the sewer shafts. This hidden potential is bigger than the heating potential of wood, solar, geothermal energy and biomass together. The extraction of heat from waste water has become economical and is funded by states, since it is possibkle and allowed to use the waste water in public sewers. The Rabtherm system was invented and developped more than 10 years ago and has worldwide patents.