The invention concerns an air guide casing with a fresh air inlet (1), an ambient air inlet (2) and an air outlet, wherein is provided a shell-valve (10) capable of selectively connecting, in extreme positions, the air outlet to the fresh air inlet (1) or to the ambient air inlet (2). The invention is characterised in that it includes at least a fin-valve or a butterfly valve which is actuated on the basis of the position of the shell-valve and which does not rise in any position above the possible positions of the shell-valve in the fresh air inlet. The various types of valves can thus be optimally and selectively used in accordance with the operating state and mode.