A new TRNSYS model of a solar domestic water heater is presented. It describes a thermosiphon system with a horizontal store and a mantle heat exchanger. The fluid inlet is placed at the top of the annular gap. This new TRNSYS 'Type' is developed from the standard Types 45 and 38, with sonic new features. Results of the new type arc compared with two sets of experimental data. The new model is in accordance with experimental data, comparing daily energy delivered and draw-off temperature profiles. (C) 2002 Elsevier Science Ltd. All rights reserved.