In this paper, the process of thermal heat storage within a water heating system utilizing phase change materials was numerically investigated using a CFD simulation. A two-dimensional model of the water-PCM energy storage system was designed and modelled using ANSYS Fluent. Two cases of without and with PCM were studied to investigate the possibility of using PCM materials in water storage heating systems. Paraffin wax with a melting temperature of 52 °C is used as the PCM material. Results showed that PCM (can maintain the water temperature at a higher level than that of without using PCM that meets the thermal requirements of water heating storage systems for domestic application. Also, results showed that the water temperature at the end of the simulation reaches 39.7 and 48.2 °C for the cases of without and with using PCM respectively. This means the final temperature of with PCM case is 8.5 °C higher than that of without PCM case and a temperature increase of about 21.46% can be achieved. This study highlighted the advantage of using PCM materials in water storage heating systems.