inches an overall side to side dimension of approximately 68 69 inches and an overall front to rear dimension of approximately 21 22 inches by using a standard supporting framework with a standard supporting base and supporting upright structure a single evaporator condenser and compressor may be used or dual evaporators condensers and compressors may be used with it being understood that an appropriate expansion valve or valves may be employed along with a reversing valve for use when switching between air conditioning mode and heat pump mode and other conventional components and controls used in refrigeration and air conditioning systems may be employed installation of the hvac unit does not require holes being formed in the roof 14 of the bus 12 which reduces the possibility of the condenser unit becoming clogged by snow or ice the modular structure of the hvac unit of this invention enables a single system unit or a dual system unit to be installed in the same available space thereby providing simplified design and easier installation the air distribution system to circulate air from the passenger compartment through the evaporator assembly when in a cooling mode and back to the interior of the bus may utilize two three.