Biological fouling (biofouling) on wetted cooling coil surfaces decreases heat transfer efficiency, increases air-side flow resistance and may eventually lead to more energy consumption by fans and chiller plants. Applying ultraviolet germicidal irradiation (UVGI) systems in air handling units (AHUs) has the potential to clean coils, improve coil performance and save energy. In this study, the effectiveness of a coil irradiation system in improving coil performance and saving energy was investigated through a field test in a hot and humid climate. A commercially available coil irradiation system was installed downstream of a cooling coil in a variable air volume (VAV) AHU. The duration of the field test was 14 months, with four months before UVGI intervention and 10 months after UVGI intervention. The effectiveness of UVGI was evaluated via a "before UV" and "after UV" comparison of coil performance. The coil overall thermal conductance increased by 10% and the pressure drop decreased by 13%, with the improvement being most rapid over the first month after UVGI intervention. Fan energy use fell by 9% over the ten months of UVGI operation. Savings in fan energy were 39% greater than the energy used by the UV lamps. (C) 2016 Elsevier B.V. All rights reserved.