Horizontal mantled hot water tanks are widely used in solar domestic hot water systems. Increasing thermal performance of the hot water tank is significant issue for thermodynamic efficiencies of the system and user satisfaction. This study presents an experimental study for determining the effect of obstacle placing on thermal performance in horizontal mantled hot water tank. Obstacles were positioned perpendicular to the flow direction inside the tank in different positions. First, one obstacle was placed inside the tank in different positions and then two obstacles were placed inside the tank. Results were presented over the temperature distribution inside the tank, mantle outlet temperature, main outlet temperature, energy efficiency and exergy efficiency. At the end of the study, it was found that obstacle placing in horizontal mantled hot water tank increased the thermal performance of tank. Temperature distribution results showed that placing obstacle inside the tank increased stored hot water temperature and volume. Mantle outlet temperature could be decreased 1.5 degrees C, and the main outlet temperature could be increased 3.6 degrees C by placing obstacles inside the tank. Consequently, when all performance criteria considered, the best thermal performances were seen in a = 150 mm, a = 150 mm b = 100 mm and a = 350 mm b = 100 mm. (C) 2019 Elsevier Ltd. All rights reserved.