The heat transfer by convection from the wall downstream a backward facing step is analysed for various air flow conditions upstream the step: uniform how outside a laminar boundary layer, uniform flow outside a turbulent boundary layer and, at last, a fully developped flow. The local heat flux is obtained from temperature measurement by infrared thermography on the surface of a thermally thick wall used as boundary conditions of a numerical model. The analysis concerns the maximum heat flux point positions and the influence of the boundary layer thickness on the maximum Nusselt number.