This paper discusses a simulation-based fault detection approach for detecting faults during the operation of low-temperature solar thermal systems. An effective fault detection approach for solar thermal systems can ensure a quick response and reparation of occurring malfunctions that cause a reduced solar energy yield. Several fault detection methods can be applied, e.g. manual fault finding, algorithms, spectral analysis or simulation-based fault detection. The simulation-based fault detection approach is based on an hourly, daily or monthly comparison of measured and simulated energy yields. The method was applied and evaluated for three field test systems with different hydraulics and a collector area between 15 and 1290 m(2). Several faults were successfully detected. Faults in the solar loop can be detected as long as the fault causes a large enough energy loss in comparison to the uncertainties of the simulated and measured energy yield. Fault diagnosis turns out to be more difficult and is expected to greatly improve by integrating the simulation-based fault detection approach in a larger framework that includes data management and algorithm-based fault detection. (C) 2013 Elsevier Ltd. All rights reserved.