The present engineering tendencies shows that, due to the development of HVAC system and control equipment, indirect evaporative cooling system variations becomes even more attractive for use not only in hot and arid regions, but also in European countries located in temperate climates. The aim of the present study is to investigate the impact of outdoor air moisture content variations on the chiller efficiency in real site conditions. The investigated system is located in restored historical building, The Art Museum Riga Bourse, which is located in the centre of Old Riga, Latvia. Electricity consumption, chiller operation stages, cooling fluid temperatures, and outdoor air parameters data has been acquired for the period of eight month, during the year 2012. Each minute recorded data was processed, and recalculated for hour’s average values. Data analysis for 7 different constant air temperatures showed that for the studied period of time chiller’s COP slightly depends on the outdoor air moisture and this dependence is inverse proportional.