A raw-water source heat pump for a thermal storage tank in a vertical water treatment building was dynamically simulated by TRNSYS. Two different configurations of a heat pump system with either one or two heat pumps were investigated by considering different heating zones in the building. The average COPs of the single heat pump system were 4.79 and 3.76 for cooling and heating, respectively. When the size of the thermal storage tank was changed from 5 m(3) to 15 m(3), the highest COP was found at 10 m(3). The electric power consumption of the heat pump was significantly affected by weather conditions. The average COPs of the two-heat-pump system were 4.92 and 3.68 for cooling and heating, respectively. Except April and October the monthly average COP of the two-heat-pump system was higher than that of the single-heat-pump system. (C) 2013 Elsevier B.V. All rights reserved.