High penetration levels from renewable energy sources in large-scale power systems demand a high degree of flexibility in the transmission and distribution system. This paper presents a method for utilization of flexible demand in the low-voltage distribution system using the thermal mass of a building to defer power consumption from electric space heating. The power consumption for heating is controlled by an operational virtual power plant, which is sending a set point for requested power consumption to the building management system. An optimization problem is formulated such that the discrete dispatch of power from ten electric space heaters is following the power set point given constraints on the indoor comfort that is defined by the users of the building. The controlling method has been implemented in an intelligent office building and used for demonstration of flexible demand in the low voltage network.