Second Nature® low-temperature (SNLT2), secondary coolant refrigeration systems utilize CO2 as the heat transfer fluid. Since CO2 has excellent thermodynamic and transport properties, the CO2 secondary refrigerant removes heat from display cases and walk-in freezers via copper piping that is significantly smaller than what is typically required in traditional, direct expansion (DX) systems. And, since HFC refrigerant is confined to the primary system located in the machine room, the total refrigerant charge and the potential for leaks are greatly reduced. SNLT2 systems allow for significant reductions in the amounts of copper and HFC refrigerants required, providing sustainability benefits that will pay dividends throughout the life of the system.