Incoming air passage (101, 201, 401) comprising a connecting piece for connection to an air supply system and a housing which delimits an air distribution chamber, wherein the housing comprises a horizontally disposed air inlet surface through which air can be introduced into the air distribution chamber and which can be connected to the connecting piece, and wherein the air inlet surface is arranged perpendicularly to a central axis of the connecting piece, wherein the housing further comprises an air outlet surface arranged parallel to the air inlet surface, wherein the air outlet surface is formed by an air-permeable plate through which the air leaves the air distribution chamber and the incoming air passage (101, 201, 401) has adjustment means (112, 212, 412) which can be changed from a first position into a second position or in the reverse direction, wherein the adjustment means (112, 212, 412) comprise at least one conductive element (113, 213, 413) and at least one bearing element and in the first position of the adjustment means the at least one conductive element is in the form of a closed frame in the air distribution chamber, characterized in that a) the housing is provided with walls which are arranged at an acute angle (α) to the air outlet surface and b) the incoming air passage (101, 201, 401) has adjustment means (112, 212, 412) for changing a flow direction of the air leaving the air outlet surface, c) wherein the at least one conductive element (113, 213, 413) is arranged in the first position in the air distribution chamber on the air outlet surface or is arranged at a small distance from the air outlet surface and in the second position abuts the transitional piece or walls of the housing, so that the space between the at least one conductive element (113, 213, 413) and the air outlet surface is greater than in the first position and d) wherein the air flows out of the air outlet surface perpendicularly in the first position of the adjustment means (112, 212, 412) and in the second position approximately horizontally and e) part of the housing is a transitional piece which is arranged between the walls and the connecting piece, wherein a cross-sectional surface of the transitional piece measured perpendicular to the flow direction is greater than the cross-sectional surface of the connecting piece, wherein the transition from the connecting piece to the transitional piece exhibits a sharp change in cross section. a) the housing is provided with walls which are arranged at an acute angle (α) to the air outlet surface and b) the incoming air passage (101, 201, 401) has adjustment means (112, 212, 412) for changing a flow direction of the air leaving the air outlet surface, c) wherein the at least one conductive element (113, 213, 413) is arranged in the first position in the air distribution chamber on the air outlet surface or is arranged at a small distance from the air outlet surface and in the second position abuts the transitional piece or walls of the housing, so that the space between the at least one conductive element (113, 213, 413) and the air outlet surface is greater than in the first position and d) wherein the air flows out of the air outlet surface perpendicularly in the first position of the adjustment means (112, 212, 412) and in the second position approximately horizontally and e) part of the housing is a transitional piece which is arranged between the walls and the connecting piece, wherein a cross-sectional surface of the transitional piece measured perpendicular to the flow direction is greater than the cross-sectional surface of the connecting piece, wherein the transition from the connecting piece to the transitional piece exhibits a sharp change in cross section.