The invention solves the problem of pneumatic locking the actuator piston rod in a selected intermediate position during ventilation. The control unit is characterized by the features that it is constituted by a four-way controlling distribution valve (10) with a directional slide (17) moved by a lever mechanism (11) connected with the smoke flap cover (2), that in the cylindrical bore of the body (12) of the controlling distribution valve (10) there is, beside the directional slide (17), a second slide, the smoke removal slide (22), that in the stroke zone of the directional slide (17) are located consecutively towards the smoke-removal slide (22) the following passages: the piston passage (14), the directional opening passage (16), and the piston-rod passage (13), and that the directional slide (17) has an external piston (19) and an internal piston (2) connected by a necking (21) of a length (b) longer than the spacing (a) between the piston passage (14) and the piston rod passage (13), and the length (c) of the external piston (19) is greater than the spacing (d) between the piston passage (14) and the directional opening passage (16).