This paper discusses the efficiency of flue and smoke tube boilers used in a district heating and cooling plant. In a general flue and smoke tube boiler, air for boiler combustion is provided at the lower level. However, in the boiler we studied, air for boiler combustion was provided at a higher level to use the high temperature air from the upper part of the boiler room to save energy. In this study, methods for improving the boiler efficiency were investigated by optimizing the intake air temperature of the boiler on the basis of CFD simulation and field measurements.