The UK Government views heat pumps (HPs) as an increasingly important technology that can contribute significantly towards heat decarbonisation in the building sector and has committed to the annual installation of 600,000 HPs in existing houses by 2028. However, the efficiency of HPs in real life is often very different to that predicted by technical design experts. The aim of this study was to offer deeper insights into HP performance influencers and address the emerging enablers to high-performing domestic HPs in the UK. The research design was based on a multiple case study approach, utilising the quantitative data of the larger Renewable Heat Premium Payment field trial population as the basis for the selection of 21 case studies. Systems thinking, in the form of causal loop diagrams, was utilised as the integrating framework for the interpretation of the qualitative and quantitative data collected through field monitoring and in-depth site investigations. The findings showed that HP performance relies on an extensive network of complex socio-technical system interactions, ranging from the strictly technical system aspects to a variety of boundary conditions acting within the building envelope and including interactions with the household. The study achieved new insights into the requirements for well-performing HPs, which have important implications for policy makers, manufacturers, installers and users of HPs.