A ventilation unit is arranged to effect flow reversal, according to two opposite directions, of a gas flow generated by a fan without reversing the rotation direction of the fan. The ventilation unit includes a fan, at least a first piping arrangement through which a gas flow is intended to pass in one of the two possible directions depending on whether it is blown or sucked by the fan, and a switching assembly arranged between the fan and the at least a first piping arrangement, at least a pair of switching devices being provided in the switching assembly and being movable between two positions for selectively and alternately connecting the at least a first piping arrangement to a delivery duct and to a suction duct of the fan, so as to allow the passage of a gas flow in a direction or in the opposite direction within the at least a first piping arrangement.