The boiler contains a furnace with a burner, a combustion chamber of the furnace, which is connected to the gas duct, and a part the walls of the gas duct have a zone of heat transfer from gaseous combustion products to the liquid. The above-mentioned zone of heat transfer of the gas duct has at least one section of the main one heat exchange and at least one area of ​​additional heat exchange. Additional plot heat exchange contains at least one additional heat exchange pipe crossing a gas pipe, the one whose internal space is connected to the internal space of the above belt channel for the passage of liquid. There are peripheral flow distribution screens coaxial with the gas duct and have the form of transverse partitions, which are made in such a way that have an outline similar to the outline of the cross-section of the gas duct, and form slits holes along the inner surface of the gas duct, and the specified central distribution screen the flow of gaseous combustion products is coaxial with the gas duct and has the form of a transverse one partition, which along the periphery has a continuous connection with the inner surface of the gas duct and contains an opening in the central part for the passage of gaseous combustion products. The channel formed in the gas duct for the flow of gaseous combustion products has an area cross-section not less than the area of ​​the exit opening of the gas duct located after the latter in the direction of the flow of combustion products of the peripheral flow distribution screen or the last of the above-mentioned at least one pipe of additional heat exchange.