as shown in fig5 thus the fan housing can be mounted on either on of the mounting rings 22 and is attached simply by aligning the male lobes 33 with the female receptacles 32 and by rotating the housing so that the male lobes move behind shoulders 34 of the ring 24 to lock the lobes 33 in place on the ring 24 the fan 23 includes a housing 35 formed by a front piece 36 and a rear piece 37 which are clamped together by screw fasteners 38 the front portion 36 forms a domed convex shape facing forwardly of the front portion 36 so that it can closely follow the curvature of the concave surface of the mounting ring behind the domed front face of the fan housing is provided a fan rotor 40 mounted on a central shaft 41 of a motor 42 the fan rotor extends across the housing inside the front face so as to drive air through the housing outwardly through the front face to pass through the openings 29 of the mounting ring the front face of the fan housing therefore has slot shaped openings 44 shaped to match and align with the openings 29 in the mounting ring the motor 42 is mounted within a motor mount 45 carried within the interior of the housing 35 the motor is arranged along a central axis of the fan housing behind the rotor 40 and in front of a central closed area of the rear portion 37 of the fan housing the rear portion is also domed and includes a series of radially extending slots 47 extending outwardly from the central area to an outer periphery of the rear portion 37 a timer t and an on off switch s are shown schematically in.