A hot water storage device is one of the most common household appliances yet it is also one of the biggest sources of energy consumption. With natural resources fading, it is imperative that typical high-energy users such as hot water systems are made as energy efficient as possible. Research has shown that the thermal performance of a hot water system can be increased by maximising the level of thermal stratification within the storage tank, which could lead to huge energy saving. To analyse the effects of tank geometry and operating conditions on the thermal stratification within a storage tank. seven three-dimensional models have been numerically simulated by using the computational fluid dynamics program Fluent with realistic boundary and initial conditions applied. The level of thermal stratification in each model has been quantified using exergy analyses. The results show that increasing the tanks height/diameter aspect ratio, decreasing inlet/outlet flow rates and moving the inlet/outlet to the outer extremities of the tank all result in increasing levels of thermal stratification. (C) 2009 Elsevier Ltd. All rights reserved.