Abstract: A heat transfer system for use in supplementing the operation of the heating/cooling system of a building and its hot water heating system includes a heat exchanger in the flue of the furnace as well as a heat exchanger in the fan chamber. A first liquid circulation loop couples the heat exchangers for transferring heat from the flue exchanger to the air moved through the fan chamber heat exchanger. A second liquid circulation loop includes the flue exchanger and the building hot water heater for supplementing the heating of water therein. In the summer months during the cooling mode of the systems operation, cold water employed, for example, for lawn sprinkling is passed through the fan chamber heat exchanger for cooling and dehumidifying air circulated in the building. A valve control system is employed to automatically control the flow path of fluid in the system as a function of detected temperatures.