The UK Government has committed to the annual installation of 600,000 heat pumps (HPs) in existing houses by 2028 as part of its Net Zero strategy. However, there is a gap between predicted and in-situ HP performance. The aim of this study was to offer a deeper understanding of the socio-technical parameters influencing HP performance in the field and address the emerging enablers of high HP performance in the domestic sector. Systems thinking was utilised as the integrating framework for the interpretation of the qualitative and quantitative data collected through field monitoring and in-depth site investigations on 21 case studies across the UK. The collected material was inductively analysed and the results were fed into a series of interlinked causal-loop diagrams, specifically focusing on the parameters and feedback processes that are likely to influence HP performance. The analysis identified seven interconnected focal areas influencing HP efficiency and energy consumption. The structure of the final cumulative systems thinking qualitative model, revealed important feedback structures and served the identification of four high-leverage interventions. The work mapped for the first time the complex network of the parameters influencing HP performance based on an extensive range of interacting boundaries and achieved new insights into the requirements for well-performing HPs.