This study presents the thermohydraulic principles for retrofitting existing flat plate solar collector networks with the aim of increasing energy capture using the installed capacity. The arrangement of a solar collector field influences its thermohydraulic performance and pumping costs. In this study, factors such as scaling-induced fouling, solar radiation, and flow distribution are considered. The case of an existing plant consisting of a total of 40 collectors is examined. It is concluded that the optimal arrangement should include 8 collectors per line connected in series as a limit to enhance thermohydraulic performance and reduce overall operating costs. The annual savings due to reduced natural gas consumption in the original plant structure (4Lx5px2s) amount to $17,522.72 USD per year; however, modifying the structure to 4Lx10s, the savings increase to $19,818.64 USD per year. Under fouling conditions, the new structure reaches savings of $19,512.41 USD per year. This suggests that, in design, the network configuration should prioritise the number of collectors in series per line, only limited by the increase in pressure drop.