Public awareness of energy in the early 1970s stimulated a number of projects on alternative ways of heating. The 'Institut fur Thermodynamik und Warmetechnik' (ITW) of the University of Stuttgart has been operating a solar heating system since 1985. Ever since, this system has been minutely monitored. In particular, the storage was painstakingly considered as it was intended to serve as a pilot facility for the much discussed problem of seasonal storage. This storage unit should be simple and cheap but heavily instrumented in order to obtain many and accurate data and it should be versatile in order to gain knowledge for operation. The solar heating is provided by collectors that are unglazed, so a heat pump is required for appropriate heating temperatures. However, the heat pump allowed for a combination of hearing and cooling in our system and this proved to be very advantageous, as cooling energy is more expensive and more in demand in our building than heating energy. The system was used in various seasonal cycles with changing conditions. It has now been operating for almost 15 years. During this period. neither storage nor collectors caused any trouble. Some difficulties were experienced with the heat pump. The first one had to be replaced; we made suggestions for improvement of the second one. This provided good COPs but there has been an occasional defect. The experience with our solar heating system was so satisfactory that, based on the knowledge gained from it, large housing projects in Friedrichshafen and Hamburg and an office building project in Chemnitz were conceived and built and are now being monitored. (C) 2000 Elsevier Science Ltd. All rights reserved.