Impacts of solar shortwave radiation are not taken into account in the standardized design methods in the current radiant system design guidelines. Therefore, the current methods are not applicable for cases where incident solar is significant. The goals of this study are to: (1) use dynamic simulation tools to investigate the impacts of solar radiation on floor cooling capacity, and (2) develop a new simplified method to calculate radiant floor cooling capacity when direct solar radiation is present. We used Energy Plus to assess the impacts of solar for different design conditions. The simulation results showed that the actual cooling capacities are in average 1.44 times higher than the values calculated with the ISO 11855 method, and 1.2 times higher than the ASHRAE method. A simplified regression model is developed to improve the predictability of ISO methods. The new model calculates the increased capacity as a function of the zone transmitted solar and the characteristic temperature difference between the hydronic loop and room operative temperature. (C) 2016 Elsevier B.V. All rights reserved.