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The research presented in this paper has two main objectives. First, it aims to generate an assessment tool for ranking and selecting the most sustainable domestic water-heating system (WHS) (with the lowest economic, environmental and social impact) that could be applied in any location and with any demand. Second, it aims to ascertain which WHS is the most sustainable in places with a climate and solar radiation like that of Barcelona, Spain, where a minimum solar contribution to domestic water heating is compulsory for new buildings and significant renovations. Multi-criteria decision analysis was employed to create the optimised flexible assessment tool. The Delphi method was followed to perform the surveys, and to provide the objectivity required in the identification of impacts, the definition of indicators and the assignment of weights. The most relevant criteria were determined: annual cost, material consumption, energy consumption, GHG emissions, space requirement, visual impact and occupational risks. The resulting tool was tested by analysing twelve domestic WHS, including two conventional systems, and ten combinations of five solar thermal technologies with two conventional systems as backup for a changing room in a sport centre located in Barcelona. The two conventional WHS studied were a natural gas-fired condensing boiler and an electric water heater. The five solar thermal technologies were: a flat plate with a harp design, a flat plate with a serpentine design, a heat-pipe evacuated tube, a direct-flow evacuated tube, and a direct-flow evacuated tube with CPC. The dynamic thermal simulation programme T*SOL was used to dimension the solar thermal systems. Two sensitivity analyses were carried out: one on weights and one on references. The tool proved very useful in the assessment of these systems, and could also help in decision-making processes to select the most sustainable WHS for other locations and domestic hot water demands., Peer Reviewed, Postprint (author's final draft)