Efficiency of heat exchangers
Thermodynamically thermal resistance of the wall, heat leaving the exchanger with POC influences the thermal efficiency of the heat exchanger.

According to the definition of overall thermal efficiency, it appears that the air can be preheated to the temperature above the flue gas temperature since no upper limit is assigned to the temperature of the preheated air temperature. Thermodynamically, in heat exchange between hot flue gas and cold air, air can not be preheated to the temperature above the flue gas temperature.

Reference: R. Schumann: Metallurgical Engineering Ptinciples