"The air-pre heater (APH) is the last stage heat trap placed in the heat exchanging passage of a power plant boiler. Efficient performance of APH is responsible for adequate coal drying capacity of pulverisers and effective combustion control with sustainable hot secondary air staging and supply. The temperature of ambient air being pre heated has a significant impact and role in the performance of APH due to variability in relative humidity (RH), density and volumetric flow affecting the efficiency of heat transfer in the APH with reference to design configuration. The change in ambient air temperature (AAT) in turn changes heat capacity of air which is also dependant on RH and density. Simultaneously, an increase in AAT will reduce the waste heat recovery opportunity in APH, if sufficient reserve capacity of heat exchange is not available in APH. The increase in AAT will also affect the performance of primary air and secondary air or forced draft fans due to change in pressure ratio caused by more volumetric flow demand through APH resulting increased leakage of air through rotary APH seals. In this paper the effect of these factors due to change in AAT is evaluated to assess the performance of regenerative APH.