Abstract Low global warming potential refrigerants R1234ze(E) and R1234ze(Z) are anticipated to be the refrigerants of choice for high-temperature heat pump systems in industrial applications. Their thermodynamic attributes are thermodynamically, experimentally, and numerically assessed in this study. The thermodynamic assessment indicates that the theoretical coefficients of performances (COP) are maximized at a condensation temperature approximately 20 K below the critical temperatures for each refrigerant. However, when the volumetric capacity is inadequate, the actual COP differs from the theoretical COP because of the large pressure drop. The breakdown of irreversible losses, which are experimentally quantified at a condensation temperature of 75 °C, results in the largest portion of the total pressure drop. The simulation results obtained at condensation temperatures of 105 and 125 °C indicate higher COPs than that at 75 °C for R1234ze(Z). The major factor is the reduction in the irreversible loss caused by the pressure drop. The above assessments demonstrate that R1234ze(Z) is suitable for high-temperature applications rather than in typical air conditioners.