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Air to water electric heat pumps are one technological solution to achieve energy defossilisation goals for heating of residential building stock. Nevertheless, they may not necessarily be the only solution for all residential building stock. A case in point is where extensive fabric refurbishment is impracticable or where electric heat pumps are installed where low ambient temperatures prevail and/or high water delivery temperatures must be utilised. For such instances, hybrid (gas and electric) heat pumps offer an alternative option by facilitating fuel source switching between electricity and gas, when ambient temperatures are low or high water supply temperatures are required. In the current study, the effectiveness of an air-to-water electric heat pump and hybrid heat pump are examined for different building retrofit scenarios for a residential dwelling located in Ireland. This is achieved by means of a sensitivity study of a validated building simulation model, incorporating both heat pump systems, subject to different building retrofit scenarios. Relative to a conventional oil-fired boiler, for a deep building retrofit scenario, the hybrid and electric heat pumps achieve primary energy reduction of 128 kWh/ m 2 /year (72%) and of 123 kWh/ m 2 /year (70%), respectively. Considering the associated carbon footprints, the reductions were found to be 29.7 g C O 2 e / m 2 /year (74%) for the hybrid heat pump, and 27.6 g C O 2 e / m 2 /year (68%) for the electric heat pump. Finally, the deployment of either an electric heat pump or hybrid heat pump for deep building fabric retrofit achieves approximately half of the heating system capital cost return within 20 years. • Analysis of a hybrid gas-electric heat pump system in a residential building. • Benchmarked against, oil boiler, gas boiler and electric heat pump. • Integrated validated simulation model utilised for retrofit options analysis. • Electric and hybrid heat pumps with deep retrofit are the most economically attractive options. • Electric heat pump and hybrid heat pump systems with deep retrofit exhibit the lowest C O 2 emissions. [ABSTRACT FROM AUTHOR]