A refrigerant vapor compression system comprising: a refrigerant compression device having a refrigerant discharge outlet and a refrigerant suction inlet, a refrigerant heat rejection heat exchanger for passing refrigerant received from said compression device at a high pressure in heat exchange relationship with a cooling medium, and a refrigerant heat absorption heat exchanger for passing refrigerant at a low pressure refrigerant in heat exchange relationship with a heating medium disposed in serial refrigerant flow communication in a primary refrigerant circuit; an expansion device disposed in the primary refrigerant circuit downstream of said refrigerant heat rejection heat exchanger and upstream of said refrigerant heating heat exchanger; a first flow control device disposed in the primary refrigerant circuit downstream with respect to refrigerant flow of the discharge outlet of said compression device and upstream with respect to refrigerant flow of the refrigerant heat rejection heat exchanger; an unload circuit operatively associated with said compression device including an unload refrigerant line having an inlet in refrigerant flow communication with the primary refrigerant circuit at a first location downstream with respect to refrigerant flow of the discharge outlet of said compression device and upstream with respect to refrigerant flow of said first flow control device and at a second location downstream with respect to refrigerant flow of said refrigerant heat absorption heat exchanger and upstream with respect to refrigerant flow of the suction inlet to said compression device, and a unload circuit flow control device disposed in said unload refrigerant line; and a controller operatively associated with said first flow control device and said unload circuit flow control device, said controller operative to switch said refrigerant vapor compression system between a first operating mode wherein said compression device operates in a loaded cycle and a second operating mode wherein said compression device operates in an unloaded cycle; characterised in that: the controller positions said unload circuit flow control device in an open position and positions said first flow control device in a closed position to operate said refrigerant vapor compression system in the second operating mode; and the controller positions said expansion device in a closed position in the second operating mode. a refrigerant compression device having a refrigerant discharge outlet and a refrigerant suction inlet, a refrigerant heat rejection heat exchanger for passing refrigerant received from said compression device at a high pressure in heat exchange relationship with a cooling medium, and a refrigerant heat absorption heat exchanger for passing refrigerant at a low pressure refrigerant in heat exchange relationship with a heating medium disposed in serial refrigerant flow communication in a primary refrigerant circuit; an expansion device disposed in the primary refrigerant circuit downstream of said refrigerant heat rejection heat exchanger and upstream of said refrigerant heating heat exchanger; a first flow control device disposed in the primary refrigerant circuit downstream with respect to refrigerant flow of the discharge outlet of said compression device and upstream with respect to refrigerant flow of the refrigerant heat rejection heat exchanger; an unload circuit operatively associated with said compression device including an unload refrigerant line having an inlet in refrigerant flow communication with the primary refrigerant circuit at a first location downstream with respect to refrigerant flow of the discharge outlet of said compression device and upstream with respect to refrigerant flow of said first flow control device and at a second location downstream with respect to refrigerant flow of said refrigerant heat absorption heat exchanger and upstream with respect to refrigerant flow of the suction inlet to said compression device, and a unload circuit flow control device disposed in said unload refrigerant line; and a controller operatively associated with said first flow control device and said unload circuit flow control device, said controller operative to switch said refrigerant vapor compression system between a first operating mode wherein said compression device operates in a loaded cycle and a second operating mode wherein said compression device operates in an unloaded cycle; characterised in that: the controller positions said unload circuit flow control device in an open position and positions said first flow control device in a closed position to operate said refrigerant vapor compression system in the second operating mode; and the controller positions said expansion device in a closed position in the second operating mode. the controller positions said unload circuit flow control device in an open position and positions said first flow control device in a closed position to operate said refrigerant vapor compression system in the second operating mode; and the controller positions said expansion device in a closed position in the second operating mode.