Effect of using different obstacles on thermal stratification in a cylindrical hot water tank is analyzed numerically. The numerical method is validated using both experimental and numerical results. Temperature distributions within the tank for 12 different obstacles are obtained. The results indicate that placing obstacle in the tank provides better thermal stratification compared to the no obstacle case. The obstacle types having gap in the center appear to have better thermal stratification than those having gap near the tank wall. The obstacles numbered 7 and 11 represent better obstacle shapes and configurations for thermal stratification among the considered cases and consequently the tanks with these obstacles can supply hot water at higher temperatures. Other obstacle types have little effect on improving thermal stratification in the tank. Further comparisons of obstacles 7 and 11 in terms of the temperatures of hot water supply indicate that the obstacle 11 provides the best thermal stratification in the tank among all the considered cases. (c) 2005 Published by Elsevier Ltd.