This-study presents the thermal analysis of a solar flat plate water heater working under real operating conditions in a hot sub-humid region, in the city of Merida, Yucatan, Mexico. This water heater was evaluated under thermosyphon conditions and, with the use of a submersible pump, under forced flow conditions. In both cases the thermal performance of the collector was studied in order to determine the impact of. flow on the working temperature and efficiency of the collector. A comparison of the temperature values registered for both thermosyphon and forced flow regimes showed that there were significant reductions both in the temperature registered at the outlet of the collector, and in the temperature differential between the inlet and the outlet of the collector. Moreover, greater efficiency values were found when the water heater was working under forced flow. These results suggest that the use of a submersible pump in a solar flat plate water heater could be an efficient way to control the maximum working temperature in a solar collector in the region, while also providing a positive effect on the daily efficiency. (C) 2016 Elsevier Ltd. All rights reserved.