Natural convection in a cubical enclosure with a hot source centered on a vertical wall and with an adjacent, fully cooled vertical wall was numerically investigated for Rayleigh numbers (Ras) of 10 3 -4 £ 10 6 . At Ra < 10 5 the heat transfer strongly depends on the hot sector side and its dependence on Ra is weak. Opposite characteristics occur at very high Ra, and there is a long transition between these two regimes. This behavior is caused by a mixing pattern, which, at low Ra, is similar to the one found in side heated cavities, whereas at high Rayleigh numbers lateral mixing is dominant.