Solar thermal energy offers efficient solutions for many thermal applications. However, as a prerequisite for the realization of its potential, the costs have to be reduced significantly. In a project funded by the Swiss Federal Office of Energy SFOE a thermal-hydraulic software tool has been developed. The tool offers the following features:

• Dimensioning of pipe network and pump
• Proof of de-aeration capability
• Dimensioning of membrane expansion vessels
• Proof of stagnation safety

The application of the software allows to cost-optimize solar circuits and at the same time prevents malfunctions and defects caused by design errors. An interface to the simulation tool Polysun provides a seamless integration into the established planning process.