The article describes the influence of additional installation of the combustion air humidifier for a biomass hot water boiler, which is equipped with a flue gas spray condenser, on its operating parameters and the heat supplied to the district heating system. Humidification of the air is carried out by spraying with water heated in the flue gas condenser, which is pre-cooled in the exchanger for preheating the return water from the district heating system. The case of the additional installation of the combustion air humidifier to the biomass hot water boiler USTSEN type JUE-FVB 7.2 with a heat output of 8 MW equipped with flue gas spray condenser is analyzed. Humidification and preheating of the combustion air leads to fuel savings and an increase in the dew point temperature of the flue gas, which improves the conditions for utilizing the waste heat from the flue gas in the condenser, as the heat is available at a higher temperature level. Humidification of the combustion air also affects the operating parameters of the boiler. In particular, the flue gas outlet temperature and flow rate increase and the efficiency of the boiler decreases slightly. Fuel consumption, however, decreases due to the heat input in the preheated and humidified combustion air. The application of the proposed solution would increase the heat supplied to the district heating system, by about 7 %-9 % of the boiler nominal heat output, depending on the heating of the air in the humidifier. A summer drop in biomass moisture would be reflected in a reduction in the energy benefit of the flue gas condenser by more than 20 %. However, humidification of the combustion air can limit this negative effect.