Free convection flow of water induced by a small prismatic heat source in a confined space is studied in the present paper. The forcing of the flow is in the range of Rayleigh numbers where a spatial transition from laminar to turbulent flow can be observed. Velocity measurements were performed by means of Particle Tracking Velocimetry (PTV). From the velocity data a resolved and a subgrid field is obtained by means of spatial filtering. These data are analysed with respect to modelling consequences in Large-Eddy Simulation (LES), constituting an a priori test. A statistically steady flow has been obtained with a converged temporal mean and standard deviation as function of space. A low correlation of time mean model stresses with exact stresses is found. In the transitional region the inter-scale kinetic energy transfer, taken over a wavenumber corresponding to the laminar plume width, is found to possess a relative large standard deviation. In this region backscatter and forward scatter of kinetic energy, with respect to the mentioned wavenumber, are of equal importance. Application of a dynamic model to the filtered data yields a qualitative good representation of the exact inter-scale kinetic energy transfer.