A hybrid plant for ambient-air conditioning of an indoor environment for civil or industrial use, which plant comprises: - at least one first thermal device comprising a burner which is adapted to heat a first working fluid contained in a closed circuit; - a chimney for ejecting the combustion products of said burner; - at least one second thermal device; - at least one heat pump having a second secondary circuit containing a second secondary fluid, and operably connected to said second thermal device; wherein said chimney comprises a condenser for at least partial recovery of sensible and latent heat of evaporation of the combustion products; wherein said condenser comprises a first secondary circuit containing a first secondary fluid and operably connected to said second thermal device for at least partially returning the recovered heat to the environment by means of the first secondary fluid; wherein said closed circuit and said second thermal device are placed in the same environment to be air-conditioned, said heat pump being adapted to selectively operate in heating or cooling mode, fluid connection means being provided, for selectively connecting said second thermal device with said first secondary circuit and/or with said heat pump, and the condenser for heating the environment in combination with said first thermal device, or disconnecting it for allowing the heating of the environment by means of the first thermal device only, said fluid connection means being further configured to selectively connect said second thermal device with said heat pump via the circuit and disconnecting it from the condenser either for cooling the environment or heating it by means of the heating pump only, said closed circuit being fluidically independent from said first and secondary circuits. - at least one first thermal device comprising a burner which is adapted to heat a first working fluid contained in a closed circuit; - a chimney for ejecting the combustion products of said burner; - at least one second thermal device; - at least one heat pump having a second secondary circuit containing a second secondary fluid, and operably connected to said second thermal device; wherein said chimney comprises a condenser for at least partial recovery of sensible and latent heat of evaporation of the combustion products; wherein said condenser comprises a first secondary circuit containing a first secondary fluid and operably connected to said second thermal device for at least partially returning the recovered heat to the environment by means of the first secondary fluid; wherein said closed circuit and said second thermal device are placed in the same environment to be air-conditioned, said heat pump being adapted to selectively operate in heating or cooling mode, fluid connection means being provided, for selectively connecting said second thermal device with said first secondary circuit and/or with said heat pump, and the condenser for heating the environment in combination with said first thermal device, or disconnecting it for allowing the heating of the environment by means of the first thermal device only, said fluid connection means being further configured to selectively connect said second thermal device with said heat pump via the circuit and disconnecting it from the condenser either for cooling the environment or heating it by means of the heating pump only, said closed circuit being fluidically independent from said first and secondary circuits. wherein said chimney comprises a condenser for at least partial recovery of sensible and latent heat of evaporation of the combustion products; wherein said condenser comprises a first secondary circuit containing a first secondary fluid and operably connected to said second thermal device for at least partially returning the recovered heat to the environment by means of the first secondary fluid; wherein said closed circuit and said second thermal device are placed in the same environment to be air-conditioned, said heat pump being adapted to selectively operate in heating or cooling mode, fluid connection means being provided, for selectively connecting said second thermal device with said first secondary circuit and/or with said heat pump, and the condenser for heating the environment in combination with said first thermal device, or disconnecting it for allowing the heating of the environment by means of the first thermal device only, said fluid connection means being further configured to selectively connect said second thermal device with said heat pump via the circuit and disconnecting it from the condenser either for cooling the environment or heating it by means of the heating pump only, said closed circuit being fluidically independent from said first and secondary circuits.