The CTSS Method was developed by ITW in Germany and is based on separate tests of the most important components like the collector (according to EN 12975-2), the store (according to ENV

12977-4) and the controller (according to ENV 12977-2). Based on the parameters evaluated for the components, the thermal performance of the complete system is predicted using a component — based system simulation program such as TRNSYS [7]. The application range of the CTSS method is very flexible because of its component-oriented approach. It is possible to apply the method on nearly every system configuration, including solar combisystems. The performance can be determined for different reference conditions (meteorological data, load profiles) by means of annual system simulation [5]. The performance indicator is the energy used by the auxiliary heater which implies that the solar combisystem is always tested in combination with the heater.

One of the major advantages is the flexibility of the method but it is quite difficult to examine the system control strategy in real, because links between the components are just simulated.