Influences of left-right tube insert regularly spaced with rod and spacer on heat transfer and friction factor characteristics in a thermosiphon solar water heater have been analyzed comprehensively. Results showed that the modified designs of twists reduce the pressure drop significantly between 47.2% and 8.9% lower than the drop offered by full length twist and improve the overall instantaneous thermal efficiency between 533% and 38.7% when compared with plain tube collector. Thus an attempt has been made to improve the internal convective heat transfer with lesser pressure drop between the tube wall and fluid in the collector and to test the possibility of using customized designs of left-right twist as tube insert. (C) 2013 Elsevier Ltd. All rights reserved.