It is revealed that the humidifying of blown air in complex heat recovery systems of gas-fired boiler plants provides the significant reduction in the concentration of nitrogen oxides in exhaust-gases due to the suppression of their formation in the boiler furnace when moisture is introduced with this air.  Problems of environmental protection and energy-saving became priority in world practice. The main directions of deciding these pressing problems in municipal heat-power engineering is to improve the environmental indicators of heating boiler plants and increase the efficiency of using fuel in them through the use of technologies for deep recovery of the exhaust-gases heat. The relevance of scientific problems in these directions is increasing due to the steady increase in the fuel-energy costs and the strengthening of requirements to reducing environmental pollution. When using these technologies of deep heat-recovery of exhaust-gases, the condensation mode of the heat-recovery equipment is realized, when, apart from to the so-called clear heat of these gases, the latent heat of condensation of the water vapor contained in them is also used. The condensation mode implementation also improves the ecological indicators of the boiler due to the reduction of fuel consumption and the dissolution in the resulting condensate of a part of harmful emissions formed during its combustion. The use of modern heat-recovery technologies for the gasfired boiler plants with complex use of recovered heat for the preheating of boiler water, water of the chemical waterpurification system and blowing air makes it possible to reduce fuel consumption in the boiler and, accordingly, its harmful emissions by 8...12 %. Humidification of the blowing air through the use of the recovery heat also provides a reduction of nitrogen oxides emissions to 60 % by suppressing their formation in the boiler combustion chamber.