Abstract These systems are the most common man-made devices that utilise solar energy. 1 Their development and commercialisation during the last seventy years are described in the contexts of the relevant economic and social constraints. The characteristics of thermosyphonic flow and the various analytical models describing n.c.s.e.w.h. behaviours are reviewed. Problems have been encountered due to inadequate system performance, the need to elevate the storage tank above the collector and damage resulting from corrosion and frost. The methods that have been suggested for overcoming these are discussed. The significant features of different systems which allow them to satisfy particular requirements are outlined.