We report here, the first order kinematics decay model for calculating chlorine concentration decay in water distribution system. Furthermore, age factor is also included in our model. A detailed analysis of bulk decay coefficient, wall reaction coefficient, ageing factor and temperature are compared. The results showed that chlorine decay strongly depends upon the temperature ageing factor of the distribution system, time and wall reaction coefficients. Experiments were carried out to determine the chlorine decay in water distribution system and decay pattern was analysed every hour.