The objective of this experimental work is to compare the convection effect of the flat plate solar water heater (FPSWH), both with and without thermal performance enhancer, and to find the optimum mode of convection in the FPSWH. Two different types of thermal performance enhancers have been used, viz., rod and tube, and they were frictionally contacted with a solar absorber tube. The water flow rate in the FPSWH was varied from 0.008 kg/s to 0.025 kg/s. The Grashof number, Rayleigh number and Richardson number, which are based on solar irradiance, have been considered in this study for evaluating the effects of different modes of convection in a FPSWH. Based on the analysis, which a mixed convection heat transfer phenomena occurred in the FPSWH with the free convection domination. The plain copper tube FPSWH has significantly higher Grashof number, Rayleigh number and Richardson number in flows were compared to rod and tube thermal performance enhancer fitted collector. © 2019 International Solar Energy Society