The group of inventions consists of metal heat treating methods, method for the liquid or gaseous fuel and hot air combustion in a direct or indirect fired furnace, a heating device and regeneration nozzles for carrying out said method. The inventions relate to metallurgy and mechanical engineering and can be used for metal heat processing (melting, heating for deformation, heat treatment) and for sintering, drying and other types of heat treatment of non-metallic products, for example ceramics. The essence of each invention discloses novel technical features which make it possible, while implementing the inventions, to attain the air excess factor (E) values in the fuel and hot air mixture which are greater than 2.0 and mainly set in a range equal to or less than 6.0. Said inventions make it possible to reduce a waste of metal during the processing thereof in a direct or indirect fired furnace and to decrease the metal hydrogenation levels including aluminium, titanium and iron alloys. The use of the invention in the indirect fired furnaces makes it possible to extend the service life of radiant tubes and melting pots. Experimental data items obtained by the inventors prove that the technical result is attained by the respective composition of the atmosphere (gaseous phase) of combustion products of the liquid or gaseous fuel and hot air mixture with the air excess factor (E) values greater than 2.0.