Heat pump integration in residential building retrofits: Energy, cost, and carbon dioxide emission analysis in South Korea

The transition to carbon neutrality in the residential sector heavily relies on electrifying heating systems, prominently through heat pump integration. However, the actual environmental and economic efficacy of heat pump retrofits is highly contingent on regional grid characteristics and operational conditions. This study evaluates the energy, economic, and environmental impacts of replacing a conventional electric boiler with an air-source cascade heat pump system in a South Korean residential building. Using field measurements from a retrofitted single-family house operating on off-peak electricity, the heat pump system was found to yield substantial energy savings of 31.5%–57.7% and heating cost reductions of 16.5%–46.6%. The economic feasibility was further confirmed by an estimated payback period of 7.95 years and a return on investment up to 317.4%. Conversely, dynamic simulations based on an empirical biquadratic performance model revealed a seasonal carbon emission reversal, indicating a lack of carbon reduction benefits despite the heat pump retrofit. Part-load performance degradation during intermediate seasons, combined with South Korea's high grid carbon intensity (0.4747 kgCO2/kWh), caused the heat pump's annual carbon emissions to marginally exceed those of a gas boiler by 0.1%. To secure an absolute environmental advantage, this study proposes three critical mitigation strategies: improving the system's part-load average coefficient of performance to at least 2.22, decarbonizing the national power grid below 350 gCO2/kWh, and integrating on-site photovoltaic systems to offset intermediate-season emission deficits. Crucially, as architectural space constraints physically limit PV capacity, preventing solar energy alone from achieving emission parity, the study underscores that true net-zero residential heating requires a multifaceted approach integrating technological innovation, grid decarbonization, and on-site renewables. The novelty of this study lies in coupling a field-measured, dynamic part-load COP model of the heat-pump system with the national grid emission factor to quantitatively analyze the system's intermediate-season and winter carbon emissions, and in further deriving the break-even thresholds in COP, grid carbon intensity, and photovoltaic capacity as a more practical pathway toward an absolute carbon benefit. These findings offer practical insights for optimizing heat pump retrofits and advancing regional building decarbonization policies.