In large non-retrofitted MFBs, monovalent air-source heat pump (ASHP) systems are often constrained by space, electrical capacity, investment cost, or a planned envelope renovation. In such cases, hybrid systems combining an ASHP with the existing gas boiler can accelerate decarbonization through an immediate and progressive fuel switch. This study assesses how hydronic integration and ASHP sizing affect energy performance, CO2 emissions, and annualized costs. Dynamic simulations, validated against long-term in situ monitoring data, are applied to postwar Geneva MFBs (1946–1980). The analysis compares single and double HP configurations, sharing or dedicating capacity between space heating and domestic hot

water production, together with parallel and alternative boiler operation using modulating and non-modulating boilers. Results show that hybrid systems can achieve emission reductions of about 60% relative to a gas-only system and annualized costs approximately 16% below those of monovalent systems, with installed HP capacities corresponding to only 40–50% of the monovalent design. Although hybrid systems have slightly higher emissions than monovalent ones, they capture most of the emission reduction at lower cost, enabling earlier HP integration where monovalent operation is not yet feasible. The sizing and performance charts support configuration selection according to project-specific constraints.