The energy crisis is a hot topic in the world today. Society is increasingly concerned with reducing energy consumption in buildings. In addition to saving the environment, the required energy savings are also due to the huge and unexpected rise in energy prices in recent years. In Slovakia, energy savings in general are mainly contributed to by the thermal-technical standard STN 73 0540-2+Z1+Z2 of 2019, which, by tightening the required values of thermal resistance and heat transfer coefficient through building structures, contributes to a significant insulation of building envelopes and a reduction in the need for heating. However, less emphasis is placed on the energy demand for hot water preparation, which is an integral part of any building structure. Huge quantities of waste water are discharged daily into the sewers, the energy of which can be reused through heat transfer. The energy potential of wastewater lies in its temperature. In some cases, wastewater in internal sewer pipes can reach up to 60 ℃. By transferring the thermal energy from wastewater that is normally discharged into the sewer, it is possible to make a significant contribution to energy savings, for example in the preparation of hot water. Cold water can take the heat energy and raise its temperature. The article deals with the preparation of hot water using only the thermal energy from wastewater.