We investigate here, the free convection flow induced by a line heat source in a confined geometry. The buoyancy forcing of this flow can be characterized by a Rayleigh number, Ra, which is chosen in the range where an intermittent spatial transition from laminar to turbulent flow takes place. The objective of the study is to explore this flow with help of numerical simulations. We restrict ourselves to the case of an air flow with Ra=1010. For the numerical simulation techniques, we employ Direct Numerical Simulation (DNS) and Large-Eddy Simulation (LES). With help of DNS we consider first, a 2D representation of this flow at a resolution of 1952 which is found to be sufficient to represent the heat source and its resulting flow. Next, we consider the 3D case at a resolution of 1953. The 3D simulation reveals a symmetrical time mean recirculation which covers the domain above the heat source. This large scale circulation is driven by the small scale laminar plume generated by the heat source and which breaks down into turbulence. The flow is found to be essentially 3D, especially near the top wall. No clear turbulent inertial range is present. A LES for the same flow has been carried out at a resolution of 453. The comparison of the les results with the DNS data has been used to investigate the performance of several sub-grid models. It turns out that simple equilibrium sub-grid models perform fairly well in estimating the statistics of the flow.