In this study, the effect of the excess air ratio on the efficiency of the boiler system was investigated in a water tube boiler that has capacity of producing 55-ton steam rate per unit time at 245 degreesC and 20 bar pressure. The boiler combustion air is supplied from a fan driven by a 30 kW at 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. In that stage, boiler efficiency was low due to thermal energy losses and also consequently leads to electrical energy losses. In order to increase the boiler efficiency, the fan motor speed is decreased to 400 rpm from 1450 by using variable speed drive (VSD). After implementation of the VSD, stack temperature was reduced from 195 to 145 degreesC. This caused an increase of the boiler efficiency 2.5% and 8000 kW h electrical energy was saved in a month. The paper describes a method of energy saving in a boiler house by using a VSD. It is evaluated and quantified the energy losses associated with excess air rate and its cost to manufacturers. An overview and analysis of such conservation measures, along with, the potential energy and cost saving, the implementation cost and simple payback period are also given. (C) 2003 Elsevier Ltd. All rights reserved.