Details about modelling the heating system of an ecological building are given in this paper. Solar air heaters provide thermal energy for various needs and for driving a vapor compression heat pump for space heating. The heating system has two different operating modes. In the first mode the heat pump operates in combination with the solar thermal collectors array. In the second mode the building is heated by the heat pump alone. Switching between the two operating modes mainly depends on the level of incident solar global irradiance but other control parameters are also taken into account. The heat pump COP is higher in the first operating mode. Also, the electric power required by the heat pump's compressor is smaller (with up to 8%) in the first operating mode. (C) 2002 Elsevier Science Ltd. All rights reserved.