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Gas absorption heat pumps represent an alternative to condensing boilers for space heating and domestic hot water production in existing buildings. In particular, they enable fuel saving and the exploitation of renewable energy even in heating systems based on radiators, which require high supply temperature. However, in order to provide useful indications to policymakers, manufacturers, and system designers, a fair comparison of two technologies has to be based, besides the energy consumption and the direct CO 2 eq emissions, also the environmental impact over the entire life cycle. Thus, in this paper, the environmental profiles of a condensing boiler and a gas driven absorption heat pump are compared as competing technologies to provide space heating and domestic hot water in old (constructed before 1980) and not refurbished buildings. The assessment was carried out for three buildings located in three representative European climates, using 1 kWh of thermal energy produced by the two systems as the functional unit. The Ecoinvent 3.6 was used as background database and the EF 3.0 normalization as weighting set method. Uncertainty and sensitivity analysis were also included. The results show that the use phase contributes for more than 97% of the total impact for both the energy systems in the three climate zones. Despite the higher electricity consumption, the gas driven absorption heat pump offered a lower environmental profile compared with the condensing boiler, mainly because of the lower amount of natural gas needed in the use phase. In particular, an average reduction of 27% was found for CO 2 eq, 25% for fossil resource consumption, and 22% for weighting results., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier B.V. All rights reserved.)