A system of heating-plant and building was monitored in order to measure the heating-energy consumption necessary to maintain thermal comfort conditions during the occupation time of the building. Recorded data allowed the identification of a black-box model of the system, which relates the heating-energy consumption to the daily mean exterior temperature. Two different operating conditions of the heating-plant were examined, a continuous one and an intermittent one; the energy consumption in the heating period has been evaluated. The energy consumption in both conditions is comparable though heating time lapses are remarkably different. Continuous operation is best when the times of required thermal comfort are longer than in the monitored situation.