A numerical investigation of the effects of plate separation, plate length and temperature difference on free convection between asymmetrically heated horizontal parallel plates has been performed. The lower isothermally heated plate is facing upwards, the upper plate is adiabatic. Three-dimensional laminar numerical simulations are obtained by solving the full elliptic governing equations using a finite volume based computational fluid dynamics (CFD) code. Comparisons of computational results with data from the literature are made in terms of relevant dimensionless numbers. It was observed that plate spacing, plate length and temperature difference influence the overall heat transfer rate.