- An air handling unit for cooling down an indoor airflow (A1) to be blown into a room of a building, comprising at least one fan circulating the indoor airflow (A1) inside the air handling unit , wherein the air handling unit comprises a first and a second cooling subsystems comprising each: - a refrigeration apparatus comprising an evaporator and a condenser , - a first water circuit connected to the condenser and comprising at least one outside heat exchanger exposed to outside air (A5, A15), in which the water releases heat to the outside air (A5, A15) under action of a fan , - a second water circuit connected to the evaporator and comprising at least one indoor heat exchanger exposed to the indoor airflow (A1), in which the water draws heat from the indoor air flow (A1), - water connections linking the first water circuit and the second water circuit , - water connection means for selectively connecting : - the first water circuit to the condenser and the second water circuit to the evaporator and disabling the water connection , in a mechanical cooling mode where the refrigeration apparatus is operating or - the first water circuit to the second water circuit and disabling water circulation in the evaporator and the condenser , in a free cooling mode where the refrigeration apparatus is stopped; wherein the at least one indoor air heat exchanger of the first cooling subsystem and the at least one indoor air heat exchanger of the second cooling subsystem are placed in series with respect to the indoor air flow (A1), and wherein the air handling unit comprises control means configured for operating the mechanical cooling mode or the free cooling mode of the first cooling subsystem and the mechanical cooling mode or the free cooling mode of the second cooling subsystem depending on a temperature of the outside air. - a refrigeration apparatus comprising an evaporator and a condenser , - a first water circuit connected to the condenser and comprising at least one outside heat exchanger exposed to outside air (A5, A15), in which the water releases heat to the outside air (A5, A15) under action of a fan , - a second water circuit connected to the evaporator and comprising at least one indoor heat exchanger exposed to the indoor airflow (A1), in which the water draws heat from the indoor air flow (A1), - water connections linking the first water circuit and the second water circuit , - water connection means for selectively connecting : - the first water circuit to the condenser and the second water circuit to the evaporator and disabling the water connection , in a mechanical cooling mode where the refrigeration apparatus is operating or - the first water circuit to the second water circuit and disabling water circulation in the evaporator and the condenser , in a free cooling mode where the refrigeration apparatus is stopped; - the first water circuit to the condenser and the second water circuit to the evaporator and disabling the water connection , in a mechanical cooling mode where the refrigeration apparatus is operating or - the first water circuit to the second water circuit and disabling water circulation in the evaporator and the condenser , in a free cooling mode where the refrigeration apparatus is stopped; <EOS>