By renovating residential buildings - by insulating their envelope and replacing transparent constructions, it is possible to significantly reduce the heat demand for heating. However, insufficient attention is paid to the hydraulics of the primary heat networks of district heating systems in connection with this renovation. This results in an uneven heat supply to the dwellings connected to the system. The content of the paper is a comparison of hydraulic conditions in primary heat network of district heating system before and after renovation of residential buildings connected to this system. The aim of the paper is to use hydraulic calculations and pressure diagrams to show the disproportions that exist in the primary heat network.