Heat Pump Water Heater (HPWH) is classified as one renewable energy efficient technology, which could be explained by their environmentally friendly nature with low energy consumption. The Coefficient of Performance (COP) of the HPWH can be influenced by multiple factors: ambient temperature, relative humidity, hot water usage profile, the size and shape of heat exchangers (evaporator and condenser), type and shape of compressor, refrigerant, filling capacity of refrigerant, etc. Therefore, in this study, the performance of HPWH is critically examine to ascertain its optimal operation condition. Three main factors are considered here: use pattern of water in tank, drop-in refrigerant R1234yf and condenser configuration. This study examines two different models, HPWH-66 and HPWH-50. Simulation is employed and suggestions are made towards a better performance. The performance of HPWH system is found to strongly depend on the stratification of water in the storage tank, then 3D CFD simulation has been used to explore the entrance effects on the thermal tank. In the drop-in experiments, it is shown that R1234yf can be a desirable alternative for the baseline R134a without any special changes to the heat pump facility. By closing the valves at the inlet and outlet of one condenser circuit, a one-circuit condenser system could be achieved. For both refrigerant system, one-circuit condenser only brings the COP down within 5.5%. Finally, a linked EES-Fluent model has been developed to simulate the transient heating-up performance of the HPWH system with the validation by experimental data
