Unrenovated multifamily houses in Germany are mostly heated by fossil heat generators; therefore, measures are required for CO₂ emission reduction. The use of air–water heat pumps is restricted by high required flow temperatures and heat output but can be mitigated by hybrid heat pumps. To limit additional operation costs by the heat pump, a new operation strategy is introduced in this study, which allows to maintain a target CO₂ emission. The operation strategy is applied in a field trial in a small unrenovated multifamily house built in 1964. A thermohydraulic simulation model is verified and is used in full-year simulations to apply improvement measures and compare the new control strategy with existing optimizing strategies. The results show that the control onto target emissions is possible and limits additional costs but can also result in higher CO₂ mitigation costs, making it less favorable. The hybrid heat pump reduces the direct fossil CO₂ emissions by 61% (in total by 22%); thus, it is a relevant solution for multifamily houses, especially within a further decarbonized electrical grid.