The control volume method was used to solve numerically the equations related to mass, momentum and energy balance. Air flows trough a channel formed by two vertical, flat, parallel and adiabatic walls. The natural convection was caused by a retangular Isothermal element attached to one of the walls. The innovation of this work lies in the utilization of the momentum equations in its elliptical form. It was achieved using the pressure boundary condition to solve an equation that corrects the boundary velocity. The variation of the medium Nusselt number was insvestigated from Ra equal 153 to Ra equal 1223, attaching the element in three diferent positions: at the beginning, in the middle, at the end of the channel and varyin the distance between the walls. The local Nusselt number and the velocity Field around the element were also studied. An optimum distance between the walls for each Rayleign number was that the geometry, of the element has a decisive effect on the heat transfer process, and so it must not be neglected.