This article introduces the design of an indirect thermosyphon solar water heating system coupled with a parallel circular tube rings type heat exchanger. The proposed heat exchange coil set is able to provide similarly of heat exchange areas of a comparable size helical coil but reduces the friction loss of the thermosyphon fluid flow. The design can also maintain the thermal stratification along the vertical height of the storage water tank. Hence, the thermal efficiency of the solar water heater is improved as compared with a same system but coupled with a comparable size helical coil and shell type storage water tank. Detailed mathematical model development from individual components to the integration of whole system are described. The transient system model is simulated by a self-developed FORTRAN program. The experimental and simulations results of the solar thermal system are fully presented in this paper. Comparison of both results shows good agreements and convinces the accuracy of the entire system models as well as the suitability of the computational method. The effectiveness of the proposed heat exchange coil set is evaluated by its NTU value which is computed by program simulation. The NTU value reveals that the overall efficiency of the solar thermal system can be improved by further physical design optimization. Simulation results analyses also consolidate the enhancement of the thermosyphon flow rate (in other words, the efficiency) of the proposed design over the helical coil design as expected. (C) 2015 Elsevier Ltd. All rights reserved.