The mass flow rate in the collector loop of a solar domestic hot water system (SDHW) affects its thermal performance. The initial investment and the operation cost are also affected by the selection of the primary flow. Common design rules state a standard specific flow rate of 501/hm(2) for most collector models. However, the emergence of electronically commuted motors recommends reviewing the interest of the low flow alternative (7-141/hm(2)) for solar thermal systems. In this paper, the thermal performance of a typical serpentine collector has been measured for different mass flow rates. The resulting performance curves and associated measurement uncertainty bands show only a moderate improvement in performance as the flow rate increases. Once the collector thermal behaviour has been characterized, the whole system is modelled, and the initial investment and operating cost are calculated for two design options: high flow (801/hm(2)) and low flow (201/hm(2)). As expected, the solar fraction is slightly higher for the high flow system (4.6% higher) and the number of hours of operation is lower for this system (4.4% less hours of operation). Considering the lower investment cost of the low flow system, the best thermal behaviour of the high flow system requires near 18 years to amortize the largest investment. If other considerations like embodied energy are accounted for, the common practice in designing SDHW systems should promote low flow.