A refrigerant system capable of operating at multiple capacity modes, the system comprising: an evaporator; a multi-stage compressor assembly having a first stage and a second stage; a first fluid flow path extending between the evaporator and the first stage; a second fluid flow path connected to the first fluid flow path and connected to the multi-stage compressor assembly between the first stage and the second stage; a first valve disposed along the first fluid flow path at or between the connection between the first fluid flow path and the second fluid flow path and the first stage, the first valve responsive to control signals to selectively regulate flow of refrigerant to the first stage; a second valve disposed along the second fluid flow path and responsive to control signals to selectively regulate flow of refrigerant along the second fluid flow path; a heat exchanger, an economizer heat exchanger, and an expansion device; a main flow line extending from the multi-stage compressor assembly serially through the heat exchanger, economizer heat exchanger, and expansion device, and the evaporator; an economizer line connected to the main flow line and extending through the economizer heat exchanger to connect either directly to the second fluid flow path or to the multi-stage compressor assembly between the first stage and the second stage; and an economizer line valve disposed on the economizer line and responsive to control signals to selectively regulate flow of refrigerant along the economizer line; the refrigerant system is characterised in that the multiple capacity modes include a first capacity mode wherein the first valve is open to allow refrigerant to the first stage of the compressor assembly, the second valve is closed and the economizer valve is open to allow for vapor injection of refrigerant between the first stage and the second stage of the compressor assembly, and a second capacity mode wherein the first valve is open to allow refrigerant to the first stage of the compressor assembly, and the second valve and the economizer valve are closed, a capacity of the system in the first capacity mode being about 150% the capacity of the system in the second capacity mode. an evaporator; a multi-stage compressor assembly having a first stage and a second stage; a first fluid flow path extending between the evaporator and the first stage; a second fluid flow path connected to the first fluid flow path and connected to the multi-stage compressor assembly between the first stage and the second stage; a first valve disposed along the first fluid flow path at or between the connection between the first fluid flow path and the second fluid flow path and the first stage, the first valve responsive to control signals to selectively regulate flow of refrigerant to the first stage; a second valve disposed along the second fluid flow path and responsive to control signals to selectively regulate flow of refrigerant along the second fluid flow path; a heat exchanger, an economizer heat exchanger, and an expansion device; a main flow line extending from the multi-stage compressor assembly serially through the heat exchanger, economizer heat exchanger, and expansion device, and the evaporator; an economizer line connected to the main flow line and extending through the economizer heat exchanger to connect either directly to the second fluid flow path or to the multi-stage compressor assembly between the first stage and the second stage; and an economizer line valve disposed on the economizer line and responsive to control signals to selectively regulate flow of refrigerant along the economizer line; the refrigerant system is characterised in that the multiple capacity modes include a first capacity mode wherein the first valve is open to allow refrigerant to the first stage of the compressor assembly, the second valve is closed and the economizer valve is open to allow for vapor injection of refrigerant between the first stage and the second stage of the compressor assembly, and a second capacity mode wherein the first valve is open to allow refrigerant to the first stage of the compressor assembly, and the second valve and the economizer valve are closed, a capacity of the system in the first capacity mode being about 150% the capacity of the system in the second capacity mode.