Ingress of air in boilers leads to drops in energy efficiency. This paper presents the effects of air ingress in the combustion zone, postcombustion zone and air pre-heater (APH) on the energy efficiency and loading capacity of a coal fired thermal power plant operating on fuel with high ash (35-45%). The optimal O-2 in the flue gas for a pulverized coal fired system is 3.5% (corresponding to 20% excess air). The operating values are in the range of 4.2-6.0% in membrane type boilers and up to 10% in refractory type boilers (after sustained periods of operation). The leakage rate of boilers (up to the entrance of the APH) is designed at 0.2% while the average operating values are 7.25% for membrane type enclosures and 33.6% for refractory enclosures. The leakage rate of the APH is designed at 5.0% while the operating values range from 13.66% to 20.13% for rotary and tubular APHs. When the O-2 in the combustion zone varies from 3.5% to 8.0%, efficiency drops of 2.0% points are experienced in the boiler and turbine separately, and the gross overall efficiency drop is similar to 3.0% points. The units do not experience any capacity drop up to an 0, in the flue gas of 6.0% before the APH. At an O-2 in the flue gas (before APH) of 7.2%, a mild limitation on the unit capacity of around 2-3% is experienced. When O-2 in the flue gas (before APH) reaches a level of 9.0%, 20% capacity drop of the unit is experienced due to which the plant load cannot be raised higher than 80%. Beyond the level of 9.0% (rare occurrence), the unit is quite difficult to operate and has to be taken off for overhaul. (C) 2007 Elsevier Ltd. All rights reserved.