A refrigerant system includes a first, substantially outdoor, two phase heat transfer fluid vapor compression circulation loop including a compressor, a heat exchanger condenser, an expansion device, and the heat absorption side of a heat exchanger evaporator condenser, connected by conduit in a closed loop and having disposed therein a first heat transfer fluid having a critical temperature of greater than or equal to 31.2° C. The system also includes a second, at least partially indoor, two phase heat transfer fluid circulation loop that transfers heat to the first loop through the heat exchanger evaporator condenser. The second loop includes the heat rejection side of the heat exchanger evaporator condenser, a liquid pump, and a heat exchanger evaporator, connected by conduit in a closed loop and having disposed therein a second heat transfer fluid that has an ASHRAE Class A toxicity rating and an ASHRAE Class 1 or 2L flammability rating.