The carbon dioxide (CO2) heat pump water heater is recognized as a potential technology for the production of domestic hot water (DHW) and space heating (SH). In this paper, the performance of a transcritical CO2 heat pump water heater with a tri-partite gas cooler is discussed using a numerical model. The heat pump operates in three modes: (1) DHW mode, (2) SH mode, and (3) DHW+SH mode, which provides space heating at 35 °C and hot water up to 70 °C. The simulation model is validated with the experimental data. The effects of different parameters on system performance are investigated, and the coefficient of performance (COP) of the system under different operating conditions is evaluated. The results show that higher heat sink outlet temperatures lower the COP and increase SH/DHW-Ratio for the investigated cases. The maximum COP is investigated for various heat loads by continuous high-pressure (HP) modulation, reaching highest values at 50 % to 60 % of maximum heat load. The SH/DHW-Ratio is investigated for the presented simulation cases in DHW+SH mode, reaching 0.68 to 1.06 for different heat loads.