Outlet gases of the humid air turbine (having a temperature of about 125 oC and great content of steam) can be used for the preheating of feed water of the steam power plant fueled with coal. So the efficiency of the plant can be increased and its ecological indices can be improved. The attainable incremental efficiency of the humid air turbine and the increased efficiency of the combined plant has been determined for three variants of the repowering of an existing steam power plant. The variant presented in Figure 4 is recommended for practical application.