Improving the efficiency of heating systems can give a tremendous contribution towards the EU energy efficiency target for 2030. Significant heat losses are one of the main disadvantages of the dominating high temperature district heating (DH). Even in summer, the high retention time of water in the network leads to thermal losses from domestic hot water reaching up to about 30 %. An empirical model based on experimental data of heat energy consumption in multi-apartment buildings is created for economical optimization of the operation of DH systems. A methodology has been developed that allows estimating the total energy cost savings (including losses) of buildings associated with the reduction of heat loss in the system, by reducing the supply flow temperature, and increasing electricity consumption by increasing the mass flow rate of water in the network.