Radiant floor heating systems (FHSs) are considered reliable heating systems since they maintain the indoor air temperature at the desired value and reduce its fluctuations more efficiently than conventional heating systems do. However, FHSs require an optimal control strategy especially when they are exposed to perturbations such as direct solar radiation or unscheduled crowding. The present study investigates the dynamic thermal response of a FHS exposed to a simulated direct solar radiation that is localized on its upper surface to quantify the potential effects of this exposure and identify the subsequent control issues of the FHS. Three experimental test scenarios were examined by varying the location of the sun patch and its duration. The experimental results show that the tested FHS, primarily regulated by controlling the indoor ambient air temperature, experiences a deficient regulation of the indoor temperature, which leads to an overheating period depending on the sun patch intensity and duration. An overheating of 4 degrees C compared with the recommended maximum desired value was recorded for duration of approximately 11.5 h after the sun patch was turned-off. The experimental results may be used in further numerical studies to predict the thermal behavior of FHSs exposed to sudden solar patches on their upper surfaces. The FHS design elements (material, thickness, etc.), as well as the control procedures, must be reconsidered to address the issue of overheating under such conditions.