This paper presents a TRNSYS simulation to evaluate the performances of a solar water heating system (SWHS) used for greenhouses according to Tunisian weather. The SWHS is mainly composed of two solar collectors, with a 4 m(2) total surface, related to a 200 L storage tank and a capillary polypropylene heat exchanger integrated to the greenhouse. During the simulation all combinations possible of the two solar collectors (series and parallels) are investigated. The parameters which have influence on the storage system, like the inlet flow rate, tank volume and collector area are also investigated. The results of simulation show that, by increasing the tank volume, the temperature at the collector outlet decreases. In addition, high flow rate minimizes the phenomenon of stratification and increases the efficiency of the system. We notice that decreasing the exchanger inlet flow rate is good solution to reduce heating loses. The best storage system we found will be used as our greenhouse heating system. In fact, the choosen heating system according to this parametric analysis, mainly composed of a flat plate collector and a 200 L tank, can increase the inside air temperature of the greenhouse by 5 degrees C and make it suitable for tomatoes agriculture. (C) 2013 Elsevier Ltd. All rights reserved.