This paper compares the performance of four load side heat exchange units that could be used with storage tanks in solar heating systems to make domestic hot water (DHW). The unit consists of a flat plate heat exchanger and a controller that regulates the primary (tank side) flow in a predetermined way. Several tests were carried out in order to determine the static and dynamic performance of the units. The two best units were connected in turn to a standard solar heating system and the thermal performance of the whole system was measured using a six-day test sequence. In addition simulation models of these two units were constructed and their parameters identified. The results show that not all of the units are suitable for use in solar heating systems. Two completely different control strategies achieve reasonable thermal performance. When used in a standard high flow Swedish solar system instead of the usual internal load side heat exchangers, the solar fraction of the whole system is improved by over 10%.