Abstract: A device for a variable-flow ventilation system has a filter (10) associa with a ventilation air intake and a shutter element (12) made of an impermeable material, designed to be applied against filter (10) to cover it wholly or partially. The shutter is associated with a mechanism for controlling shutter movement which is connected to a computer (18) which receives information constituted by the pressure value or flow value at the outlet from the fan housing (2). Depending on the information supplied to the computer, the computer acts to hold the pressure at a constant value by affecting the position of the shutter relative to the filter and/or causing the filter to advance.