The effect of relative height between the bottom of the tank and the top of the collector on performance has been studied both theoreticaly and experimentally. According to the national testing standard methods, the efficiency of the system was tested. The results show that the simulation results are in good accordance with the experimental ones. The data indicate that the optimum value of relative height is 0.74m with the maximum efficiency of 67.7% for the system with a total area of 1.5m2 and storage water tank capacity of 120L. Increasing or decreasing the relative height within a range of 0.3m has no significant effect on thermal efficiency.