Research on the combustion of wood on a fixed grate with a separate supply of combustion air is described in this paper. From the layer of the wood on the grate, only fixed carbon was burned in the primary combustion chamber in the presence of primary air, whereas the volatiles were burned subsequently in the secondary combustion chamber in the presence of secondary air. A one-dimensional mathematical model describing the combustion in the overfeed layer of wood chips is given. Theoretical results were confirmed by the experiments carried out in the experimental furnace. The scheme of the experimental device and the results of experiments and calculations are presented. It was confirmed that the conditions in the layer and the quality of combustion were highly effected by the ratio of the fuel mass flow and mass flows of primary and secondary air. It was found that the ratio between primary and secondary air could be determined on the basis of the measured concentration of CO2 in flue gases leaving the burning layer. The optimum ratio of primary and secondary combustion air at any excess air was determined.