The transition to a sustainable energy system with high penetration of renewable energy sources increases the need for demand-side energy flexibility. One way to achieve such flexibility is to utilize the thermal inertia of buildings; flexibility can be provided by storing energy in the thermal mass and using it for load shifting. The storage potential in the mass of buildings depends, among other things, on the type of heat emission system. In this paper, the flexibility potential of a thermally activated building system was compared to an embedded surface system and a radiator system, using an existing well-insulated multi-family house as case building. Based on one representative apartment of the case building, a simulation model was established using the simulation software TRNSYS. The model was validated against operational data from the management system of the case building, using the indoor air temperature, heat supply temperature, and heat return temperature for comparison. The validated model was used to simulate three flexibility scenarios: (1) the heat emission system was turned off until the lower thermal comfort limit was reached, then turned on with maximum power, (2) the heating power was suddenly increased until the upper thermal comfort limit was reached, and (3) the heat emission system was run intermittently with heating only during the night. It was found that the thermally activated building system performed better than the other systems; it had the longest response time, shortest recovery time, greatest storage capacity, and smallest temperature fluctuations. © 2025 The Authors