The performance of a new solar hot water system with an integrated heat-pipe is investigated theoretically and experimentally. The now system uses a wickless gravity assisted loop heat-pipe for the heat transfer from the collector-evaporator to the tank through a heat exchanger-condenser. A detailed heat transfer experimental study is performed, focused on the thermal behaviour of the different parts of the system. The results show that the system can reach satisfactory efficiencies which, in combination with manufacturing simplicity, absence of moving parts and good behaviour under freeze conditions, make it an interesting technological solution. Furthermore. a theoretical model for the collector is proposed and comparison with experimental results is performed. This model can be used for the optimisation of the system design. (C) 2002 Published by Elsevier Science Ltd.