The boiler has a furnace with a burner, a combustion chamber of the furnace, which is connected to the flue, and part of the wall of the flue has a zone of heat transfer from gaseous combustion products to the liquid, where these gaseous combustion products are in contact with the inner surface of the wall, and the liquid is in contact with the outer surface of the wall. That is, the specified part of the gas duct wall, which forms the heat transfer zone, separates the channel for the passage of the flow of gaseous combustion products and the belting channel for the passage of the heating liquid. In the above-mentioned heat transfer zone of the gas duct, at least two distribution screens are installed, which are located coaxially with the gas duct and have the form of transverse partitions, which are made in such a way that they have an outline similar to the outline of the cross section of the gas duct and form slit-like openings along the inner surface of the gas duct. At the same time, the area of ​​the openings formed by the distribution screens is 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 distribution screen.