A water heating system in thermosyphonic flow using polypropylene finned tubes was built and used to perform experiments. The system consisted of 36 parallel polypropylene finned tubes each with 19-mm inside diameter and 1 m long made especially for this study. Experiments were performed at Sharif University of Technology, Tehran, Iran. Experiments were usually started at 9:00 a.m. and ended at 3:00 p.m. during the months of September, October, and November when the ambient temperatures varied from 20 to 30°C. During each experiment, ambient, inlet, and outlet water temperature and the slope of the collector relative to the horizon were measured. It was found that the best collector slope was 36°.