In this study, the stack gas temperature of a biomass-fueled boiler was estimated from material and energy balance equations written for a boiler system and by using data collected from an operating boiler. A parametric study was subsequently carried out in order to gauge the effects of changes in the moisture content of the biomass, the efficiency of the boiler, and the air flow rate to the boiler furnace on the stack gas temperature and the stability of the boiler operation. The results indicate that there exist values of the moisture content and the air flow rate above which the operation of the boiler becomes unstable. The results further confirm the long-held views that regular maintenance of a boiler and drying of biomass fuel prior to feeding to the boiler furnace have favorable impacts on the operation of biomass-fueled boilers.