Computations of natural convection are usually done using the Boussinesq approximation which limits their applicability to small temperature differences. Avoiding such restrictions requires the solution of the Navier-Stokes equations using a density-based method. However, density-based methods usually perform poorly at low Mach number, near the incompressibility limit. In this paper we use an implicit procedure based on the all-speed SLAU method to solve the full set of Navier Stokes equations on unstructured hybrid grids. This method solves for primitive variables using the thermally perfect gas assumption. The stiffness in the equation in low speed regime is handled without time-derivative preconditioning. Although the algorithm is formulated in generalised form for arbitrary fluids, in this paper we limit ourselves to ideal gases.