17. A method of cooling servers in a data centre, wherein the data centre comprises:servers arranged in the data centre in rows separating alternating cold aisles and hot aisles,
a first personnel corridor in fluid communication with the cold aisles, and
an indirect air handling unit configured to receive warm air from the servers via the hot aisles and to provide cool air to the servers via the first personnel corridor and the cold aisles,
wherein the first personnel corridor is separate to the cold aisles and located on the same level of the data centre as the cold aisles, and wherein the servers comprise server fans;
wherein the method is performed by:
operating the indirect air handling unit to cool the warm air by exchanging heat with air outside the data centre thereby producing the cool air, and
operating the server fans to draw in cool air from the first personnel corridor via the cold aisles, transport the cool air through the servers thereby producing the warm air, and to exhaust the warm air into the hot aisles, wherein said operation of the server fans is sufficient to directly cause circulation of the cooling air along the first personnel corridor and along the cold aisles from the indirect air handling unit to the servers, cause circulation of the warm air along the hot aisles from the servers to the indirect air handling unit, and cause circulation of air through the indirect air handling unit.