The main goal of this paper is to develop the method of leaving heat recovering in industrial furnaces, steam and heat water boilers. For direct increasing of the equipment thermal efficiency it is possible to use the leaving heat for heating of air going into the burning. For this purpose the effective heat exchangers "gas-gas" type on the basis of fining thermo-siphons (TS) can be used. For approach the high thermal parameters of TS it is offered to use bimetallic tubes. The internal tube working under high pressure is from steel. The external aluminum part of the TS tube has spiral aluminum fins. Working medium in TS is distilled and decontaminated water. The procedure of numerical calculation of air heaters is created and applied for industrial device design.