This paper presents preliminary results of a CFD analysis of passive solar air collector. The geometry used in simulation is a representation of a prototype solar air collector which has been investigated experimentally in laboratory of Heat Engineering and Refrigeration Department of Koszalin University of Technology. The external dimensions of collector are 2 x 1 m (heigth x width). The absorber plate is flat. The set of experimental results is used in validation the numerical approximations of the flow field. Most notably, the mean velocity at inlet and temperatures can be easily compared between these two data sets, leading to a comprehensive conclusions. The numerical results are in agreement with experiment. The results revealed a behavior of fluid in the discussed geometry that promotes backflows and may be the cause of momentum loss.