The integration of a flue gas condensing unit in a district heating (DH) system source increases the efficiency of the DH system due to the partial recovery of flue gas heat. A multi-regression equation obtained linking the gas condensing unit efficiency indicator with several statistically significant independent variable parameters based on the statistical analysis of the DH system data in Ludza, Latvia, is discussed in this paper. Parameters related to the interaction between the gas condensing unit and the DH system, boiler capacity and the influence of the DH network return temperature on the efficiency of the gas condensing unit is discussed in more detail. To assess the efficiency of the gas condensing unit and to show the distribution of the boiler house heat load between the boiler and the gas condensing unit during the heating season, a diagram is proposed. As a result of the installation of a gas condensing unit after the boiler, the primary energy savings during the heating season is 11.8%.