Flow interaction between thermal plumes and vertical air distribution and the resulting airflow structures were investigated under increasing heat load conditions. The main objective was to investigate the large-scale flow patterns, airflow fluctuation and frequency of the flow field. The flow interaction between thermal plumes and ventilation provides random flow motion and vortical structures that further effect the airflow characteristics such as velocity and temperature fields, turbulence intensity and frequency of the fluctuations. Fourier analysis was conducted in order to observe the energy levels of air speed records. It represents the frequency distribution from the set of discrete values of the given variable over the time-interval by providing the sinusoidal components of original function with certain frequency. The novelty of this study comes from the Fourier analysis of this flow interaction. The flow interaction was investigated in a test chamber of 5.5 m (l) x 3.8 m (w) x 3.2 m (h). Thermal plumes were produced by using 12 symmetrically installed cylindrical thermal loads of 0.4 m x 1.1 m that gave a thermal load range of 40-80 W/m2-floor. Omnidirectional anemometers were installed into a measuring mast at the heights of 0.1 m, 0.6 m, 1.1 m, 1.7 m, 2.3 m and 2.9 m. The highest mean air speed was observed near the floor at the height of 0.1 m and the lowest near the top of thermal loads at the height of 1.1 m. The smallest turbulence intensity in turn was near the floor at the height of 0.1 m, whereas the highest intensity was found at the height of 1.1 m. The draught rate DR was below 21 %, and it increased with heat load. The highest draught rate was observed near the floor and the smallest at the height of 1.1 m, similarly than in the mean air speed and conversely than in the turbulence intensity. Overall, the results indicate a clear correlation between the thermal load level and the mean air speed as well as the airflow fluctuation. The observations showed that the mean air speed, the airflow fluctuation and the power spectral density increased with thermal load.