An air handling unit (1) for cooling down an indoor airflow (A1) including at least one fan (3) circulating the indoor airflow inside the air handling unit (1) and a first and a second cooling subsystems (5, 15) including a refrigeration apparatus (50, 150) comprising an evaporator (500, 1500) and a condenser (504, 1504), a first water circuit (52, 152) connected to the condenser and comprising at least one outside heat exchanger (520, 1520) exposed to outside air (A5, A15), a second water circuit (56, 156) connected to the evaporator and comprising at least one indoor heat exchanger (560, 1560) exposed to the indoor airflow, water connection means (62, 64, 162, 164) for selectively connecting, depending on a temperature of the outside air.