What makes this project special is that an existing high-temperature residential building was decarbonised without replacing its radiators, without rebuilding the hydronic heating system, and without installing a new low-temperature distribution system. The NIK Systems transcritical CO₂ heat pump was integrated in parallel with the existing gas boilers. Since commissioning, the gas boilers have not been required for normal operation. The system supplies approximately 70–80°C water for space heating and domestic hot water. This represents an extremely demanding operating condition for a heat pump and, in particular, for a transcritical CO₂ system, because very high supply and return water temperatures normally make it extremely difficult to maintain high efficiency. What is especially important is that the high COP was achieved specifically at these demanding high-temperature operating points. During recent optimised operation, the system demonstrated COP values reaching approximately 4.5 while operating with very high supply and return water temperatures. This result demonstrates the effectiveness of NIK Systems’ proprietary transcritical CO₂ system architecture and control strategy, and shows that R744 can be used not only for domestic hot water, but also for highly efficient high-temperature space heating in existing buildings. The project therefore challenges the common assumption that existing buildings with high-temperature heating require major renovation or complete conversion to low-temperature heating before fossil-fuel boilers can be replaced by heat pumps. Using natural refrigerant R744, the project demonstrates a practical and scalable pathway for decarbonising a large part of Europe’s existing high-temperature building stock