This paper describes the steady-state modeling and simulation of a wood log fired boiler. It is designed and manufactured by THERMODYNAMIKI S.A. (KOMBI), a company based on Northern Greece. Its nominal fuel power is around 32 kW and its efficiency is close to 75%. A specific operating case, selected and experimentally investigated by the manufacturer, is examined. Due to the highly transient and complex nature of wood log combustion, an ad-hoc model has been developed. Its main characteristic is the fuel division into three components: water vapor, volatiles and fixed char. The latter is considered to form a layer, which is treated as a porous medium thus forming a volumetric source zone adjacent to the firebed area. Volatiles and water vapor are modeled as volumetric mass sources, whereas a two-step reaction mechanism of the former is considered. The numerical results are compared against corresponding experimental data for the nominal load as far as flue gas characteristics (temperature and species concentration) at the boiler exit, are concerned. Based on the validated model, a proposed optimization pattern of the boiler is undertaken and examined by means of CFD.