an indoor unit of an air conditioning system includes front and rear cases joined together to form a space in which a heat exchanger , fan , and fan motor are housed . a grill member hingedly mounted on the front case has a front air intake opening formed therein . a top air intake opening is formed in an upper side of the rear case . a filter is mounted on a rear side of the grill member to extend across the front air intake opening . an upper portion of the filter underlies the top air intake opening . edges of the front and rear case overlap one another , and one of those edges forms a slot in which the other edge is received . boards for carrying electrical components are mounted in an out - of - way manner to minimize the space taken up .