The goal of the work was to find out the best performance of a closed wet cooling tower system combined with chilled ceiling, serving a residential building. For this purpose a computational research was carried out where system and building simulation was combined with optimisation. The building simulation tool used was IDA-ICE. Together with the GenOpt optimisation package it formed an effective optimisation environment. One floor of a well-defined block of flats was used to represent the building. A closed wet cooling tower with an optional chiller and storage tank, connected to chilled ceiling units in the building was the system under consideration. This building-system combination was simulated using Helsinki 1979 reference year weather on an hourly basis. The highest yearly COP achieved was 8.28. This was achieved using the cooling tower only. The tower combined with the storage tank or the optional chiller gave a slightly lower COP value. The indoor air temperature with the tower-only option was exceeding a limit of 24.2 °C for 55 hours during the year, the maximum temperature being 25.0 °C. The other combinations resulted in slightly better thermal comfort.