• An alternative indicator of thermal response of radiant systems is studied (HSE). • HSE allows comparing systems with complex thermal behaviour by a single value. • HSE ranged from 3% for fast systems to 56% for slow systems. • Effective thermal output (ETO) that considers the thermal response is introduced. • ETO was up to 57% lower than nominal thermal output for thermally active systems. An alternative indicator of thermal response of radiant heating and cooling systems called heat storage efficiency (HSE) has been tested. The heat storage efficiency was compared with established indicators represented by the time constant τ 63 , response time τ 95 , and thermal energy stored (TES). The comparison was performed for three wall cooling systems with various combinations of pipe location, configuration of material layers, and materials of the thermal core. Considering the whole response curve (HSE) instead of focusing on one specific point on the curve (τ 63 , τ 95) allowed comparing the thermal response of complex systems by a single value. It also permitted predicting thermal response consistently regardless of the system and core material. Thermal energy stored predicted the thermal response of certain systems, but it may not be suitable for comparing the thermal response of radiant systems with different thermal admittance. Besides, a novel indicator of thermal output called effective thermal output (ETO) has been proposed. The effective thermal output considers both the steady-state thermal performance represented by the nominal thermal output and dynamic thermal performance represented by the heat storage efficiency. For thermally active building systems (TABS), effective thermal output was up to 57% lower than the nominal thermal output. [ABSTRACT FROM AUTHOR]