The objective of this study is to investigate the effect of placing phase change material inside the horizontal mantled hot water tank on thermal energy storage performance. Six paraffin tubes of 1-litre were placed inside the tank side by side, respectively. At the end of this study, it was found that using paraffin inside the tank increased the amount of hot water obtained from the tank. The amount of obtained hot water did not increase proportionally with the increase of the amount of paraffin, as the volume of the tank was constant because the amount of hot water in the tank decreased with increasing the amount of paraffin. The stored energy in the paraffin was not transferred to the water efficiently. In order to obtain the highest volume of hot water, four paraffin tubes should be placed inside the tank. Discharging period duration could be increased 17 minutes with four tubes paraffin. Each added 1-litre of paraffin reduced the temperature in the tank by nearly 1 degrees C.