Abstract: A heat transfer system is described for controlling the transfer of heat from a roof space into a living space of a building. The heat transfer unit (10) includes a flexible duct (14) and a ceiling vent (16) for transferring warm air from within the roof space (12) into a living space (18) below. An electric fan (20) located at the top end of the flexible duct (14) is arranged to draw warm air from the roof space (12) down through the duct (14) into the living space (18). A first temperature sensor (22) mounted adjacent the top end of the flexible duct (14) senses the temperature of the air within the roof space (12). A second temperature sensor (24) located in the room below senses the temperature of the air within the living space (18). An electronic controller (26) controls the operation of the electric fan (22) in response to temperature sensing signals from the first and second temperature sensors (22, 24) respectively.