Theoretical analysis and computational modelling of closed wet cooling towers (CWCTs) are presented. Experimental measurements for performance of a prototype tower are used to define the tower transfer coefficients. Tower flow rates and number of tubes and rows are optimised for the required cooling load and to achieve a high coefficient of performance (COP). The performance of a cooling system used to cool office buildings is simulated using the transient system (TRNSYS) simulation environment. The cooling system consisted of a CWCT, chilled ceilings, pumps and a fan. A control strategy. including night cooling, is introduced in the simulation. The results indicate efficient performance and high COP values for the system.