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Solar thermal energy plays a vital role in the transition toward sustainable and energy-efficient heating solutions. Among the available technologies, flat plate collectors (FPCs) remain one of the most widely adopted systems for low-temperature applications (≤120℃), including residential water heating, industrial process heat, and district heating. Enhancing the thermal performance of large-scale FPCs is essential to improve efficiency, reduce energy losses, and ensure adaptability across diverse climatic conditions. This study presents a computational fluid dynamics (CFD) thermo-hydraulic analysis of riser tubes in a large-scale flat plate collector (FPC) using ANSYS Fluent. Three riser lengths (4.45 m, 5.45 m and 6.45 m) are examined with regard to temperature distribution, hydraulic behavior, and energy and exergy performance. An aqueous solution of 50% (w/w) glycol enters the risers at 15 °C with a velocity of 0.4 m/s, while non-uniform heat fluxes of 750 and 100 W/m2 are considered on the sun-facing and shaded sides. For riser lengths of 4.45 m, 5.45 m and 6.45 m, the CFD simulations indicate outlet temperatures of 89.63 °C, 96.81 °C and 103.61 °C, corresponding to thermal efficiencies of 74.1%, 77.0% and 79.4% and exergy efficiencies of 8.0%, 9.6% and 11.1%, respectively. Fully developed laminar flow is observed, with pressure drops (2.7–3.56 kPa) and low pumping power demands. To ensure numerical reliability, a mesh-independence study was carried out for the 5.45 m riser. Refining the grid from 6.5 × 105 to 7.43 × 105 cells changed the outlet temperature by less than 0.2%; therefore, Mesh 2 was adopted for all simulations. Taken together, the mesh-independence analysis and the comparison with monitoring data support the reliability of the numerical model. Overall, the longest riser provides the highest energy and exergy performance but operates close to the stagnation temperature range (100–120 °C), reducing safety margins. The intermediate 5.45 m riser emerges as the most balanced configuration, combining high thermal and exergy efficiencies with more moderate outlet temperatures and hydraulic losses; it is therefore recommended for real-world deployment in large-scale FPC systems.