A Phase-Change Energy Storage (PCES) system was used to heat a greenhouse of 180 m(2). For the seasonal heat storage unit, paraffin was used as the phase change material (PCM). The system consists mainly of four units: solar air heaters, the seasonal heat storage unit, the greenhouse and a computerized data-acquisition/control unit. The absorber surface of the installed air heaters is 0.15 m(2) per square meter of greenhouse ground surface. The storage tank volume is 0.055 m(3) per square meter of greenhouse ground surface, and the storage volume is about 0.37 m(3) per square meter of the solar air heaters. The storage tank contains 6 000 kg of paraffin PCM, of which the thermal properties, as measured with a Differential Scanning Calorimeter (DSC), were: melting temperature range of 48 - 60 degrees C and latent heat of melting 190 kJ/kg. All the temperatures at the inlet and outlet of the storage tank, the solar air heaters, and in the tank and the greenhouse were measured by thermistors each second, and the 15-minute averages were recorded and fed into the data-logger and computer control system. In this paper, the preliminary results obtained during the charging period of the storage tank, August and September 1994 in the first year of the experiment are presented. During the charging periods of August and September of 1994, an average values of the air temperature at the outlet of the collectors were 59.6 and 56.7 degrees C, respectively. The average temperature of the paraffin reached 46.8 degrees C and 49.3 degrees C during August and September 1994, respectively.