A fan assembly for use in a sidewall of an animal house. The fan assembly includes an impeller, a motive force configured to rotate the impeller about a fan assembly axis, and a butterfly shutter. A shutter operation mechanism moves the butterfly doors of the shutter between the closed position and the open position in synchrony with an operating speed of the impeller. The shutter operating mechanism includes pushrods connected to the butterfly doors, and a linkage between the impeller and the second pushrods. The linkage includes at least one weight configured to rotate with the impeller about the fan assembly axis, wherein rotation of the at least one weight causes the linkage to exert a force on the pushrods to push the butterfly doors toward the open position when the impeller is rotating.