In schools the temperature of the indoor air is one of the most important factors in terms of the indoorclimate. The resulting indoor air temperature is depending on many factors; the temperature of thesupply air being one of the more significant one. One fan-assisted natural ventilation system includesa supply air system where the outdoor air should be passively heated respectively cooled when passing components in the ground before it is supplied to the classrooms. The objective is to examine the ability of the supply system to transfer heat to/from the outdoor air. This has been studied by measurements of the air temperature at several measurement points in a real school building. The results show that the supply air system has an ability to heat respectively cool the outdoor air. The heat transfer results in a supply air temperature within a rather narrow interval, regardless of outdoor air temperature. The consequence of cooling hot humid air leads to high relative humidity which means that a risk for mould growth may occur which should be avoided. This must be observed and handled if designing this form of system.