Mixed convection in a horizontal channel loaded with discrete material and equipped with a top exhaust was studied experimentally and analytically. The steady how was two-dimensional 2D, while observed unsteady convection, triggered at a critical Richardson value, was highly three-dimensional 3D. Measured temperatures and those predicted with a 2D numerical model are in good agreement. The model, however, underestimates the onset of the thermally-induced transition to unsteady convection.