Each winter, we can see the heat, warm air, coming up the sewer shafts in city streets and we should realize the enormous heat potential that is down there and just flows away, untapped and unused. The hidden potential is bigger than the heating potential of wood, solar, geothermal energy and biomass together. It is very important, that this vast potential is discovered by politicians and made public. The extraction of heat from waste water has become economical and is funded by states, since we are able and allowed to use the waste water in public sewers to heat and cool buildings, with up to 25% lower cost. Heat recovered from waste water is economical with a return on investment of 2 to 6 years. It is ecological; saving up to 70% of CO2 emissions and primary energy, and it is in compliance with public energy policies.