The performance of a novel heat exchanger unit ('Solasyphon') developed for a solar hot water system was experimentally investigated under indoor and outdoor operating conditions. The 'Solasyphon' can be easily integrated to an existing single-coil hot water cylinder avoiding the need for costly twin-coil solar hot water storage. A series of tests were conducted under controlled indoor and real outdoor conditions to test and compare the performance of the 'Solasyphon' system with a traditional twin-coil ('coil') system. The analysis was based upon experimental data collected under various operating conditions including different primary supply temperatures (solar simulated); heating from ambient, heating with a partially stratified storage from ambient and finally under no draw-off and standard draw-off patterns. The outdoor testing was carried out on both systems separately over Summer/Autumn conditions in Northern Ireland. The results showed that the 'Solasyphon' system is more effective compared to a traditional twin-coil system for a domestic application where intermittent hot water demand is predominant and under a transient solar input particularly on intermediate or poor solar days. The 'Solasyphon' delivered solar heated water directly to the top of the storage producing a stratified supply at a useable temperature. The twin-coil system was found to be more efficient than the 'Solasyphon' system under a prolonged heating period. (C) 2010 Elsevier Ltd. All rights reserved.