The TriSolHP project is developing a new heating system for multifamily buildings that combines a photovoltaic thermal (PVT) collector field with heat pumps (HP) using natural refrigerants. The system is designed to be more efficient and environmentally friendly than traditional air-source heat pumps (ASHP). The PVT used in the project has been optimized to be used as a heat source for HP. The first phase of the project focused on measuring the performance of the PVT HP collector in a climate chamber. The results showed that the collector can efficiently capture ambient heat, even when covered in ice. Then numerical models of a classical (sub-critical, with R290 refrigerant) and a transcritical (with R744 refrigerant) HP have been developed. The models were validated against experimental data and used to assess the performance of the TriSolHP system. Two hydraulic circuits for the TriSolHP system are then described together with the regulation principles and sizing rules. The first circuit uses a R290 HP for both space heating (SH) and domestic hot water (DHW) production. The second circuit uses a R744 HP for DHW production and a R290 HP for SH. The performances of the TriSolHP system for a variety of building typologies have been simulated. The results showed that the system can effectively meet the SH and DHW demand of multifamily buildings with SPF including PV autoconsumption by the HP ranging from 2.74 up to 4.97. The system also outperforms ASHP systems in terms of energy performance and emissions savings. The financial viability of the TriSolHP system were assessed. The results showed that the system has a levelized cost of heat (LCOH) of between 19 and 25 ctsCHF/kWh. This is slightly higher than the LCOH of ASHP systems, but the difference could be offset by rising electricity costs in the future. The diffusion potential and the decarbonation potential of the TriSolHP concept were estimated for the Geneva canton. The results showed that the system could installed in 53% of the building (corresponding to 30% of the Energy Reference Area). A large diffusion of the TriSolHP system could contribute to a CO2eq emission reduction of between 26 and 51% of the Geneva building stock. Overall, the TriSolHP project has shown that the system is a promising alternative to ASHP for multifamily buildings. The system is more efficient and environmentally friendly than ASHP systems, and it can be installed in a wide variety of buildings.