Three dimensional turbulent free convection in a parallelepiped with heating of two side walls with variation of the height to width ratio over a wide range Ax = L/H = 0.1–5 is investigated numerically. The Rayleigh number was changed over the range Ra = 109–1011. The influences of volume geometry on 3D flow structure and heat transfer are studied. It is shown that the 3D character of the flow has a profound effect on heat transmission at a small aspect ratio parameter (Ax <1). For the longer length volumes, the whole coefficient of heat emission does not depend on the height to width ratio of the interior layer and can be determined from the 2D approach.