This paper presents a frequency domain transfer function methodology for thermal characterization and design for energy flexibility of zones with hydronic radiant heating/cooling systems and significant thermal mass. The modeling methodology is validated with experiments in an environmental chamber. Using the developed frequency domain model of the zone, key transfer functions are calculated for a case study. By means of transfer functions, the effect of different levels of thermal mass on the zone thermal response and quantification of the energy flexibility in response to grid signals is studied. The model is used to evaluate different design and operation options on a relative basis to enhance energy flexibility for different scenarios. It is shown how transfer function analysis provides insight into the building thermal dynamics without the need for simulations. The transfer function that relates the radiant floor heat source at the bottom of the slab to the zone air temperature is derived and the delay between the heat input of the radiant slab and zone air temperature is calculated. Experimental measurements in an environmental chamber are used to validate the key design parameters obtained from the frequency domain transfer function regarding the operational strategies for energy flexibility of the thermal zone. Finally, it is shown how to utilize the quantified energy flexibility that is possible with the floor thermal mass through application of appropriate control strategies while maintaining satisfactory comfort conditions. (C) 2020 Elsevier Ltd. All rights reserved.