The experimental investigations as well as the simulation study show that the solar gains under low-flow and high-flow operation are quite similar. On clear and cloudless days the solar system gain under high-flow operation exceeded the solar gain under low-flow opera­tion by up to 5.7 %, while for unsettled irradiation the thermal performance of the low-flow system was up to 4.6 % better. Under intermittent sunshine the low-flow operation benefits from a reduced number of switching cycles and from longer operation intervals. Periods of low irradiance are better utilized under low-flow operation. If the electric power consump-

tion of the collector pump is taken into account and the savings of primary energy are re­garded, then the low-flow operation comes generally out on top.

The analysis of the temperature distribution in the domestic hot water stores showed that the better stratification caused by low flow rates is responsible for the lower return tem­peratures in the collector loop. Thus the mean collector loop temperatures under low-flow and high-flow operation are nearly equal and the similar solar gain of both systems is ex­plainable.

This investigation shows that for small solar domestic hot water systems with an internal heat exchanger a low-flow rate in the collector loop could be advisable, at least if a uniform flow distribution inside the collectors is guaranteed by the construction. Furthermore the development of pumps that are optimized for low flow rates would make low-flow operation even more advantageous than it is already now.