The present paper reports the results of the design optimization of an air manifold utilized for delivering the air necessary for the combustion process in a biomass-fired batch boiler. A CFD model is built and validated by using experimental data. The numerical model is used to improve the device performances by reducing the entropy generation, considered as a measure of the effectiveness of the design. Other parameters, namely outlet velocities, are also monitored in order to guarantee an efficient combustion process. Four different designs of the manifold, characterized by a progressive reduction in entropy generation, are proposed and then compared with the base case. In each design step possible sources of irreversibility, such as corners, sharp variation of the flow direction, etc. are smoothed, for example by rounding the corners or by accompanying the flow in its directional changes. For all the cases, entropy generation is monitored and a reduction in its value of similar to 10 times is observed if the first and last designs are compared. Values of velocities and pressure drops are monitored to confirm the acceptability of the improved designs. Finally, entropy generation and pressure drops are analysed for varied air flow controlled by the fan power. (c) 2020 Elsevier Ltd. All rights reserved.