The concept of an improved integrated storage collector is evaluated for Central European temperature and radiation conditions where integrated storage collector systems have so far not been used. The system studied works without a heat exchanger loop because care is being taken to prevent freezing. This is accomplished by a sufficiently large volume of water to provide inertia plus the use of high quality transparent insulation materials for the front cover. This extremely simple system is modelled using actual weather data for a year with exceedingly low temperatures. The conditions under which freezing is excluded are determined and the optimized system is evaluated for the entire year regarding solar fraction for a four person household and conversion efficiency. Also evaluated are different types of auxiliary heaters: Solar fraction ranges from 55.6 to 41.0% and efficiency from 35 to 26% for a 4 m 2 integrated storage collector. This is compared with other collector systems. It is found that the ISC is superior to active fiat plate systems and thermosyphon systems. It is not as efficient as a very good system based on vacuum collectors.