Abstract: The invention and its group of variants consists of metal heat treating methods, a method for the combustion of liquid or gaseous fuel and hot air in a directly or indirectly fired furnace, and a heating device and regeneration nozzles for the carrying out of said method. The invention, and its variants, relate to the field of metallurgy and mechanical engineering, and can be used for metal heat processing (e.g., melting, heating for deformation, heat treatment) and for the sintering, drying (and other types of heat treatment) of non-metallic products such as ceramics. The essence of the invention are the novel technical features that make it possible to attain, while implementing the invention, the air excess factor (?) values in fuel and hot air mixture that are greater than 2.0 and mainly set within a range equal or less than 6.0.