Heating of residential buildings is one of the main sectors contributing to the overall primary energy (PE) consumption worldwide. Therefore, many efforts have been done to make new buildings more energy efficient, such as increasing the insulation level and adopting more sustainable heating systems. In this field, heat pumps (HP) are seen as the most promising heating technology. However, they have some drawbacks that limit their spread at large scale, especially in existing buildings, for domestic hot water (DHW) production and in cold climates. The adoption of hybrid systems (HS) can mitigate this phenomenon through the application of a second generator, a condensing boiler, that helps the heat pump during its periods of inefficient operation. The purpose of the present research is the identification of the area of application and the best control strategies for HS, to allow them to be a valid substitute for fossil-based heating systems in existing and thus less insulated buildings.Different hybrid system configurations, as well as climates and types of building have been simulated. The results have been compared in terms of PE consumption. In addition, a cost evaluation has been conducted.The results show that HS can lead to important benefits, especially for buildings with high energy needs. The results of the research proved that HS could contribute to increase the PE savings of older and less insulated buildings, which represent the large majority of the building stock. In this perspective, hybrid systems are a viable solution to be applied for the energy transition towards more sustainable buildings.