An air conditioning unit having a flush-mounted self-contained evaporator assembly adapted for mounting into the front or rear bulkhead of a bus or similar type vehicle. The evaporator assembly includes a specially designed housing to provide a compact self-contained unit that can fit in a small space, such as the bulkhead above a passenger compartment, while the associated compressor and condenser units are distally located. The housing has chamfered top surfaces so as to provide additional clearance within the bulkhead during installation, and a floor pan with two sloping surfaces forming an inverted apex. Lateral drains located at both ends of the apex facilitate drainage of condensate from the unit when the vehicle is in motion or in a non-level stationary position.