Abstract One of the major barriers to a broader market penetration of solar thermal systems is the few information on their long-term performance and reliability. Several works have shown a significant mismatch between foreseen and monitored system performance, mainly due to the lack of an accurate numerical validation analysis or to installation issues. Thus, it is necessary to implement validation and verification processes in the design of solar thermal systems, pointing out quantitative indexes on which a comparison between monitored and simulated outputs has to be based. This work aims to contribute to the discussion, presenting a numerical validation procedure that can be applied and replicated to any monitored solar thermal system component. In particular, here it is tested to the validation of an immersed heat exchanger of a water storage tank.The final comparison between monitored and simulated quantities has shown a very good agreement in terms of storage fluid temperatures and heat transfer rate, with a difference of less 2% on the energy exchanged during one exemplary monitoring months.