Air source heat pump (ASHP) water heater is a renewable electro-mechanical system that possesses a potential to produce sanitary hot water with a coefficient of performance (COP) ranging from 2 to 4 depending on the heating up modes and climatic conditions. A split type ASHP water heater comprises of a heat pump, water circulation pump and hot water storage tank and it operates by the principle of vapour compression refrigerant cycle (VCRC). The research focused on the influence of coefficient of performance of heat pump, ambient temperature and relative humidity to the average coefficient of performance of ASHP water heater with heat pump rated 1.2 kW retrofitting 400kPa, 200L storage tank under three distinct heating up modes namely; normal operation with hot water drawn off, first hour rating and stand by losses heating up cycles. A data acquisition system (DAS) was designed and built to accommodate the power meters, flow meters, temperature sensors, ambient and relative humidity sensor and data loggers to measure accurately the input parameters. The flow meters measured volume of hot water heated up by heat pump and hot water drawn off to the building. Temperature sensors measured the temperature of inlets cold water and out lets hot water of heat pump and storage tank. In addition power meters also measured the power consumption of heat pump and the ASHP water heater. A multiple comparison test was conducted to investigate the existence of mean significant difference of COP of the system obtained from the various modes. The optimal experimental results showed average COP of 3.16, 4.19 and 3.07 of system respectively, at specific average ambient temperature and relative humidity for each of the modes, with hot water set point temperature of 55°C. Finally, a surface fitting model in MATLAB was used to evaluate the performance.