The paper describes the modelling and analysis of a 10 MW chilled water distribution system that is used for the cooling of parallel Data halls. During an unplanned site power failure, the Chillers may go offline for some minutes, affecting the temperature and delivery of chilled water to the Data hall cooling units. This may cause the air temperature within the Data halls to rise. The ultimate test of robustness is to ensure that the IT equipment is adequately cooled during this time. This work outlines the key findings from the thermal response of the cooling system to a site power failure. The entire chilled water network that comprised the primary and secondary plant was modelled, including components such as pumps, valves and Chillers. A transient analysis was conducted to predict the rise in water temperature at different locations in the system. Computational fluid dynamics was then adopted to predict the air temperature reaching the IT equipment, therefore confirming the capability of the design to steer clear of effects such as thermal runaway. The findings demonstrate that confidence in the omission of expensive buffer vessels and UPS backed plant could not have been achieved without the advanced transient analysis conducted here. (C) 2010 Elsevier B.V. All rights reserved.