This paper investigates the potentional for energy saving in a boiler house that can be achieved through installing a variable speed drive (VSD) for the fan motor by including an oxygen trim control cycle. This investigation is carried out through surveys; energy saving calculations are based on experimental data. In particular, some unique energy saving measures and energy efficiency improvements in the boiler associated with the VSD system are analytically and experimentally studied. In this study, the relation between excess air ratio and boiler efficiency was investigated in a water tube boiler that has the capacity of producing 55 tons of steam per unit time at 245 degrees C and 20 bar pressure. The boiler combustion air is supplied from a fan driven by a 30 kW 1450 rpm induction motor. Since the fan motor running at nominal speed has supplied the same airflow rate, excess air amount and exhaust gas temperature were very high at low loads. The boiler efficiency was low due to thermal energy losses and also consequently led to electrical energy losses. After implementation of the VSD, stack temperature was reduced from 200 degrees C to 142 degrees C, so the boiler efficiency was increased about 2.5% and 8000 kWh of electrical energy were saved in a month. The energy losses associated with excess air rate and its cost to manufacturers are evaluated and quantified. An overview and analysis of such conservation measures, along with the potential energy and cost savings, the implementation cost, and simple payback period are also given.