At Stuttgart University, a solar heating system for an office building with laboratories and lecture rooms was installed in 1985. The heating system consists of 211 m2 of unglazed solar collectors, a 1050 m3 water-flooded pebble bed heat store, and a heat pump. This installation is a pilot facility and is included in the activities of the International Energy Agency (IEA). The experimental heat storage contains all heat exchanger systems that are used for ground stores: direct water exchange and a 4853 m long tube for heat exchange. Heat can be supplied to the store from the solar collectors or from a power station (as waste heat). The store and its behaviour ate described. The whole system has worked successfully for 5 years under varied strategies.In the first two heating periods, the heating strategy was aimed to collect as much solar energy as possible. Thus, about 60% of the heat demand could be covered by solar energy; but the yearly heat pump coefficient-of-performance (C.O.P.) was only around 2.76. With an improved heat pump, which was installed during the last 2 months of the second heating cycle, a monthly C.O.P of 3.6 was obtained. Heat losses from the storage amounted to about 20%.