•A thermal solar system for space heating was designed and built.•A TRNSYS model is developed and validated through experimental measurement.•A sensitivity approach is proposed to quantify the influence of the design parameters on solar fraction.•Pump control impacts considerably the solar fraction in large solar systems.•The proposed approach can be used for the optimization of solar space heating systems design. In this paper, a solar water heating system (SWHS) based on 4 m2 flat plate collectors is integrated to an office building to ensure the heat demand. The SWHS is equipped with Labview software and a multichannel digital card that makes the system control and data acquisition automatic. A TRNSYS model is developed and validated against experimental data and then used for optimization purposes. A sensitivity approach based on the Design of Experiments method and dynamic simulations is proposed to optimize the solar fraction. For this purpose, 134 simulations have been carried out. Metamodels of the solar fraction have been then established for three ranges of the collector area; namely, 2–10 m2; 10–20 m2 and 20–30 m2. The results have shown that the solar fraction is highly dependent on the collectors’ area and the storage volume. Furthermore, the impact of the pump control strategy on the solar fraction has increased with larger systems. Finally, optimum design parameters have been obtained using the fitted models to achieve the targeted solar fraction of 60%. The proposed approach could be used for the optimization of solar heating systems design.