... is not in use.during operation ,the air forces the shutter doors open.the butterfly fan uses ... semicircle shutter doors,wind resistence is small ,good air-tightness and high efficiency.for the cone part ... doors close ,the rubber sealing ring make good air -tightness .in different condition ,not run all ... type fan,could avoid this problem .ensure air pass through the evaporative cooling pads or airinlet ...