A frame is provided and is adapted to be installed in an opening and between the external unit and the roof or wall and having locating elements on each of a first side and an opposite second side of the frame, the locating elements or projections being configured together to locate the frame in the opening and to locate the frame in the external unit. The locating elements in the frame are positioned so that if the opening is the first type, the frame is adapted to be installed with the first side facing the roof or wall, and if the opening is the second type, the frame is adapted to be installed with the first side facing the external unit.