The Seebeck coefficient anomalies in the so-called Kondo-insulator compound FeSi have been studied at liquid helium and intermediate temperatures. It is shown that the most complete and consistent interpretation of the iron monosilicide magnetic, transport and optical properties can be achieved in the framework of the Hubbard model. The estimation of microscopic parameters provides additional arguments in favour of the spin polarons formation and dramatic renormalization of the intragap electronic density in FeSi at low temperatures.