A heat transfer system includes a first two-phase heat transfer fluid vapor/compression circulation loop including a compressor, a heat exchanger condenser, an expansion device, and a heat absorption side of a heat exchanger evaporator/condenser. A first conduit in a closed fluid circulation loop circulates a first heat transfer fluid therethrough. A second two-phase heat transfer fluid circulation loop transfers heat to the first heat transfer fluid circulation loop through the heat exchanger evaporator/condenser, including a heat rejection side of the heat exchanger evaporator/condenser, a liquid pump, a liquid refrigerant reservoir located upstream of the liquid pump and downstream of the heat exchanger evaporator/condenser, and a heat exchanger evaporator. A second conduit in a closed fluid circulation loop circulates a second heat transfer fluid therethrough having an ASHRAE Class A toxicity and a Class 1 or 2L flammability rating.