Two collector models were analyzed under thermosyphon solar thermal system conditions: an extension of the physical model described by Duffle and Beckman (1991) and a modified correlation model based on the test efficiency curve obtained from European Standards. Special attention was paid to the body forces term of the momentum equation, a key aspect for thermosyphon system calculations. The models were verified and validated using a virtual test that numerically reproduces efficiency curves according to EN12975 (2006). A virtual test generated to represent thermosyphon unsteady system conditions was used to analyze model response under transient conditions. The Extended Duffie-Beckman model was shown to perform well when submitted to strong unsteady boundary conditions such as inlet fluid temperature, irradiance and mass flow rate. The model based on the efficiency curve was shown to work well for time steps larger than the collector residence time. However, for lower time steps, the model was found to be inaccurate due to the hypothesis of a single control volume for the fluid analysis. For the same reason, besides the assumption of a first order temperature profile in the fluid flow, the model was not capable to predict a physical behavior when submitted to strong variations of the fluid inlet temperature. (C) 2013 Elsevier Ltd. All rights reserved.