Smart control of a multi-storage solar collector assisted hot water system is essential to achieve the best performance possible. This is the element of the system, where small extra care and investment paid back far beyond the extra cost. Heat loss of hot water circulation can be significant, especially if the daily consumption is confined to some peaks, and no consumption in-between. Usually circulation is only led through the last storage that is heated by the auxiliary energy. So the majority of the circulation heat loss is covered by the auxiliary heater. The system monitored applies a special arrangement and control system to maximize solar energy utilization including the circulation loss to be covered by solar energy. However there is a special characteristic of the arrangement that can cause efficiency reduction rather than gain, and it happened as revealed. The main part of the control system worked without serious failure.