The development is based on the residual energy from hot water, using it before it flows into the sewer. On the plumbing, the cold water comes with a temperature between 5 and 15 ° C (seasonally adjusted), “HTEeconomizer” heats this cold water by using the residual energy of the warming we use for everyday life. When returning this already warmed water back to the boiler, the corresponding heater in the water tank starts to operate more economically and consumes less energy. The system achieves levels of 50-70% efficiency of the process. “HTEconomizer” can be adapted and used in newly built houses, passive buildings, residential complexes, hotels, restaurant kitchens, business buildings, shopping centers and more. The product can be combined with flow systems and Renewable Energy Sources. It is mounted under the siphon of the sink, and the operation and cleaning of the sink is convenient and easy.