Recent full scale experiments has detected the presence of low Reynolds number effects in the flow in a ventilated room. This means that one are unable to predict the flow patterns in some geometries for air change rates - or Reynolds numbers - which are relevant for ventilation engineering by a standard model of turbulence. In this paper it is investigated if it is possible to simulate and capture some of the low Reynolds number effects numerically using time averaged momentum equations and low Reynolds number k-e model. The test case is the laminar to turbulent transitional flow over a backward facing step with expansion ratio (h/H eq. 1/6). The results are evaluated and held up against experimental LDA data and simulation with a Reynolds stress model (RSM).