Abstract: A reversible mode heating and cooling system for an interior space of a vehicle having an Auxiliary Power Unit (APU) employs a heat pump arrangement, wherein a refrigerant compressor is driven by APU's diesel engine through pulley and belt arrangement. An air duct is provided to communicate engine exhaust air to an external heat exchanger during a heating mode. Openings and dampers are provided in the air duct to direct engine exhaust air flow to atmosphere and introduce ambient air into the external heat exchanger during cooling mode. Exhausted air from the external heat exchanger may be re-circulated back to the air inlet side of the engine. Provisions are also provided for operating the refrigerant compressor from the electrical power grid.