The current financial and economic programme in the Russian Federation is aimed at increasing energy efficiency in all areas of society. The most effective way of increasing the efficiency of boiler plant operation is to improve the technology of deep heat recovery from the heated flue gas from the chimney system of hot-water boilers. The physical essence of this phenomenon consists in cooling the escaping high-temperature combustion products of gaseous fuel, through contact with the cold surface of the heat exchanger, to a temperature below the dew point under the pressure in the convective bundle of the boiler. In this regard, we set a goal to develop the most efficient design of condensing economizer, which would allow us to obtain fuel economy of gas boiler at the level of operation of condensing heat generators. A practical model of a condensing economizer for increasing the efficiency of heat generators based on convection-type gas boilers has been created. A study has been carried out, the results of which allow us to conclude on the effectiveness of its use for individual and local heating of residential and public buildings.