Reversible air conditioning unit comprising a refrigerant circuit wherein an external coil is designed to be placed in an outside ambient and is designed to establish a heat exchange with the air of the outside ambient; the refrigerant circuit also comprise an internal coil placed in a housing with a first air inlet designed to communicate with an inner ambient and configured to suck air from the inner ambient and a first air outlet designed to communicate with the inner ambient and configured to provide respectively cool or warm air to the inner ambient if the reversible air conditioning unit is operation in summer mode or winter mode, the air conditioning unit providing, during winter mode, a defrosting operation wherein the refrigerant circuit is reversed to warm the external coil and remove the ice that is formed on the external coil, wherein the housing is provided with a second air inlet configured to communicate with the outside ambient and second air outlet configured to communicate with the outside ambient; shutter means being provided to the first inlet and to first outlet and to the second inlet and to the second outlet and being controlled by an electronic unit that is configured, during forced defrosting operations in which said refrigerant circuit is inversed, to close said first inlet and said first outlet and to open said second inlet and said second outlet to provide a flux of air toward the external ambient thus preventing that the flux of air that comes from the housing is provided to the inner ambient; characterized in that the electronic unit comprises first calculating means designed to calculate the time τ_fd needed to perform a free defrost operation without reversing the refrigerant circuit from winter mode to summer mode and with stopped compressor; the electronic unit comprises second calculating means designed to calculate a time limit τ_lim for defrost operation, based on a number of parameters; and second comparing means checking if τ_fd is smaller than τ_lim to perform a free defrosting operation; said air conditioning unit is configured to execute free defrost operation in case the result of said second comparing means is positive. unit comprising a refrigerant circuit wherein an external coil is designed to be placed in an outside ambient and is designed to establish a heat exchange with the air of the outside ambient; the refrigerant circuit also comprise an internal coil placed in a housing with a first air inlet designed to communicate with an inner ambient and configured to suck air from the inner ambient and a first air outlet designed to communicate with the inner ambient and configured to provide respectively cool or warm air to the inner ambient if the reversible air conditioning unit is operation in summer mode or winter mode, the air conditioning unit providing, during winter mode, a defrosting operation wherein the refrigerant circuit is reversed to warm the external coil and remove the ice that is formed on the external coil, wherein the housing is provided with a second air inlet configured to communicate with the outside ambient and second air outlet configured to communicate with the outside ambient; shutter means being provided to the first inlet and to first outlet and to the second inlet and to the second outlet and being controlled by an electronic unit that is configured, during forced defrosting operations in which said refrigerant circuit is inversed, to close said first inlet and said first outlet and to open said second inlet and said second outlet to provide a flux of air toward the external ambient thus preventing that the flux of air that comes from the housing is provided to the inner ambient; characterized in that the electronic unit comprises first calculating means designed to calculate the time τ_fd needed to perform a free defrost operation without reversing the refrigerant circuit from winter mode to summer mode and with stopped compressor; the electronic unit comprises second calculating means designed to calculate a time limit τ_lim for defrost operation, based on a number of parameters; and second comparing means checking if τ_fd is smaller than τ_lim to perform a free defrosting operation; said air conditioning unit is configured to execute free defrost operation in case the result of said second comparing means is positive.