With the dawn of solar energy utilization and the escalating economic effectiveness of heat recovery systems, the need for efficient and reliable hardware capable of increasing the availability of low grade heat is evident. In a heat pump system, a narrow difference between the sink and source energy level translates to a low pressure ratio operating condition. Performance maps for hermetic refrigerant compressors in this regime have not been previously examined. Three types of off-the-shelf compressors were experimentally tested on a specially designed calorimeter. Comparative results are presented for compressors when operated over an evaporating temperature range of 6.3 to 38.1°C (20 to 100°F) with condensing temperatures ranging from 32.6 to 49.2°C (90 to 120°F). With this set of conditions, the performance evaluation of the various compressors operating over pressure ratios ranging from 1.30 to 4.76 was accomplished. Results indicate that a significant improvement in performance and reliability can be garnered through proper selection of the refrigerant compressor.