Recent studies have shown that the heat load characteristics under radiant cooling are different compared with those under convection systems. Therefore, heat load calculation methods that simplify radiant heat transfer may not be suitable to radiant systems. In this study, an indoor environment was observed under a 24-h radiant cooling condition in order to examine the heat transfer of each surface. Each surface temperature was compared under only radiant cooling, radiant cooling and active chilled beam combination, and only active chilled beam conditions. The results showed that the use of the overall heat transfer coefficient was not appropriate for calculation under radiant cooling conditions during daytime. By using the results obtained from the experiment, calculations were carried out by the heat balance method and the obtained results were compared to those calculated by the equivalent temperature difference method. Thereby, it was determined that the heat balance method was more accurate.