A powered, direct discharge ventilator comprises an extraction fan (K) having a fan duct (12) and a self weathering, non-return shutter plate (P) for closing the exit from the extraction fan duct (12) when the ventilator is not operating. The shutter plate (12) is guided by rods (21) to be opened by the airstream when the fan (K) is started, whilst maintaining a position generally parallel to that which it occupies in its fan duct closing position and to be closed by springs (23) when the fan (K) is stopped. The fan (K) mounts a centrifugally operable latch (30), co-incident with its rotational axis (C), and the shutter plate (P) carries a catch (31) to be engaged by the latch (30) when the shutter plate (P) is closed, the catch then preventing lifting of the shutter plate (P) due to "wind-over" effect when the fan (K) is stationary. When the fan (K) is started, the catch (30) is centrifugally operated to release the catch (31) to allow the shutter plate (P) to open.