A combined heating and cooling system for a building is described. The system includes a heating circuit and a cooling circuit. The circuits are coupled to heat energy storage reservoirs. Each reservoir defines an elongate flow path along which water can flow in the form of a column. Using heat from the heating circuit, one or more of the reservoirs can be charged with hot water which is stored in the reservoir(s). The stored hot water is subsequently used to boost the temperature in the heating circuit. Similarly, water cooled by the cooling circuit can be stored in other reservoir(s) for subsequent use in boosting the cooling action of the cooling circuit. A heating system or a cooling system may also be provided.
