Heat pump unit comprising at least one main circuit adapted to perform a main heat pump cycle (HPCM) with a respective operating fluid, said at least one main circuit comprising: - a main condenser adapted to perform the condensation of the operating fluid of said main heat pump cycle (HPCM) and intended to be connected to an external circuit of a first thermal user plant in a heating operating mode of said heat pump unit; - a first heat exchanger, connected downstream of said main condenser and upstream of the expansion means of said at least one main circuit, adapted to perform an undercooling of the operating fluid of said main heat pump cycle (HPCM) after the condensation of the same in said main condenser, and - a main evaporator adapted to perform the evaporation of the operating fluid of said main heat pump cycle (HPCM) and intended to be connected to an external circuit of a heat sink in a heating operating mode of said heat pump unit, wherein the heat pump unit further comprises a secondary circuit adapted to perform secondary heat pump cycle (HPCS) with a respective operating fluid, said secondary circuit comprising: - a secondary evaporator adapted to perform at least the evaporation of the operating fluid of said secondary heat pump cycle (HPCS) and in heat exchange relationship with said first heat exchanger to transfer heat power released by the operating fluid of said main heat pump cycle (HPCM) during said undercooling to the operating fluid of said secondary heat pump cycle (HPCS), and - a secondary condenser adapted to perform the condensation of the operating fluid of said secondary heat pump cycle (HPCS) and intended to be connected to the external circuit of said first thermal user plant, characterized in that both said main condenser and said secondary condenser are intended to be connected to the external circuit of said first thermal user plant and are connected to each other in series, with the secondary condenser upstream, in a line arranged in said heat pump unit for the connection to said external circuit of said first thermal user plant. - a main condenser adapted to perform the condensation of the operating fluid of said main heat pump cycle (HPCM) and intended to be connected to an external circuit of a first thermal user plant in a heating operating mode of said heat pump unit; - a first heat exchanger, connected downstream of said main condenser and upstream of the expansion means of said at least one main circuit, adapted to perform an undercooling of the operating fluid of said main heat pump cycle (HPCM) after the condensation of the same in said main condenser, and - a main evaporator adapted to perform the evaporation of the operating fluid of said main heat pump cycle (HPCM) and intended to be connected to an external circuit of a heat sink in a heating operating mode of said heat pump unit, wherein the heat pump unit further comprises a secondary circuit adapted to perform secondary heat pump cycle (HPCS) with a respective operating fluid, said secondary circuit comprising: - a secondary evaporator adapted to perform at least the evaporation of the operating fluid of said secondary heat pump cycle (HPCS) and in heat exchange relationship with said first heat exchanger to transfer heat power released by the operating fluid of said main heat pump cycle (HPCM) during said undercooling to the operating fluid of said secondary heat pump cycle (HPCS), and - a secondary condenser adapted to perform the condensation of the operating fluid of said secondary heat pump cycle (HPCS) and intended to be connected to the external circuit of said first thermal user plant, characterized in that both said main condenser and said secondary condenser are intended to be connected to the external circuit of said first thermal user plant and are connected to each other in series, with the secondary condenser upstream, in a line arranged in said heat pump unit for the connection to said external circuit of said first thermal user plant.