The sole thing that one should make sure a wall-mounted or exterior mounted exhaust one must-have is a grill on the outside, or a motor guard, or an automatic shutter so that the functioning of the fan doesn’t get hampered by the things that can get stuck in the fan from the outside. These hindrances can be a bird, or dust, or for that matter anything that can fly and get in touch with the inside parts of the exhaust fan and hinder its functioning and create an unwanted mess.