The article presents the results of experimental and numerical study of the initial stage of free convective flow and hart transfer development near a vertical heated plate with a stepwise change in heat flux for a smooth surface and a surface with periodic steps. The dynamics of the velocity fields were studied by the method of track visualization. Ra number varied from 7 X 10(10) to 1.8 X 10(10) within the time interval from 3 to 20 s from the onset of plate heating. Considerable enhancement of heat transfer near a vertical surface with a stepwise change in a heat flux (Nu grows by more than 8 times) compared to a similar problem with constant heat flux density was observed.