This work presents a theoretical study of fouling in flat plate solar collectors. The thermal performance of solar collectors is determined by operating variables such as surface area, network configuration, and flow distribution, as well as external factors such as solar radiation and weather conditions. However, in most flat plate solar collectors, where water is the working fluid, it is common for scaling to occur, reducing heat transfer and increasing pressure drop. Fouling in pipes increases resistance to heat transfer and increases pressure drop. Consequently, the thermal performance of solar collectors decreases while increasing pumping power. An inadequate design of a network of solar collectors causes poor fluid distribution, which affects thermal performance. In channels with low water flow rates, the fluid reaches higher temperatures, which increases the rate of scaling deposition on the surfaces of the pipes. There are few studies on fouling in solar collectors and its effect on thermal and hydraulic performance. Therefore, the use of a model to predict fouling and the development of a mathematical model to determine the flow distribution in the network will allow for the identification of improved network structure designs. It is shown that in retrofit, reduction of free flow area by 40 %, reduces fouling deposition and pressure drop due to scaling but only reduces the maximum outlet temperature by 0.55 °C between the clean and fouled conditions.