The domestic air source heat pump is a reliable and energy efficient system in the production of sanitary hot water. It generates sanitary hot water by harnessing the low grade aero-thermal energy during the process of vapour compression refrigerant cycles. The primary refrigerant in the domestic air source heat pump water heater is responsible for the transportation of the useful high grade thermal energy from the evaporator to the condenser where is dissipated to heat up water to the desired set point temperature (55°C) during the vapour compression refrigerant cycles. The study focused on the critical monitoring of the thermodynamic properties of the distinctive refrigerants (R407C and R417A) used by two identical split type air source heat pump water heaters and under dynamic heating modes. The experimental data that displayed the temperature profiles from the installed temperature sensors at the compressor suction and discharge lines as well as the condenser inlet and outlet ends in conjunction with the electrical energy consumption were fully analysed to evaluate the refrigerant impact on the performance of the air source heat pump water heater. The preliminary results from the two split air source heat pumps retrofitting 150 liters high pressure geyser under the first hour heating rating and controlled simulated hot water drawn off demonstrated that the system with refrigerant R417A performed better than the other with R407C although there was no significant mean difference in their coefficient of performance. The coefficient of performance of the both systems under the different heating up scenarios was above 2. The REFPROP simulation software was used to give an analytical comparison of the two refrigerants. The results obtained from the study can be applied in the selection of an excellent thermo-physical refrigerant to be utilized as a primary fluid in a domestic air source heat pump water heater.