In this study, an experimental study has been performed for a natural gas fueled boiler operating at 42,000-kPa pressure and 713.15-K temperature with a nominal capacity of 33.33 kg/s to find improvement in the boiler efficiency. Temperature, pressure, velocity, and gas emissions measurements have been made and energy, mass balances, and exergy analysis have been formed. Then, efficiency of boiler, potential energy saving options and saving quantities, investment costs, and payback period for normal operating conditions have been calculated. From the measured data, the boiler and exergetic efficiency were calculated as 88.28 and 36.7%, respectively. It is seen that the main efficiency losses are leakage of air in the rotary type air heaters, operation of the boiler at high excess air/fuel ratios, operation of the boiler under the rated load, surface thermal losses, and high flue gas temperatures. The largest part of the boiler efficiency losses is due to the air leakage in the rotary type air heater. Due to air leakages, the load of the induced-draft fan increases and prevents the boiler to reach its normal boiler capacity. It has been calculated that if air leakage in rotary type air heater is reduced to acceptable limits (10%) in practice, boiler steam production capacity will be increased by 34.2% from 24.44 to 32.8 kg/s. Implementation cost is required only for reducing air leakages in the rotary type air heater and surface thermal losses. The investment cost for reducing air leakage in the rotary type air heater to the acceptable limits (10%) is between $600,000 and $700,000. Under these circumstances, the payback period of this investment cost is about 15 months.