The widescale adoption of most solar water-heating systems in colder climates is hampered by the fact that they require freeze protection, which inevitably leads to an increase in the unit cost. Integrated collector/storage solar water heaters (ICSSWH), due to their large thermal masses and simple compact structures, give good resistances to freezing in most climates. The experimentally determined performance of two full-size ICSSWHs for the winter period in Northern Ireland is reported. The ICSSWHs tested incorporated a novel heat-retaining storage vessel. Details of the performance of the two designs tested for 3 months over the winter period are presented in this paper. (C) 2001 Elsevier Science Ltd. All rights reserved.