© 2008 by Taylor & Francis Group, LLC. Heat transfer by natural convection in interconnected systems finds multiple applications in devices such as domestic oil heaters, electrical and electronic equipment to name only a few. Interconnected systems consist of two elements: a region where heat is generated (main system) and a region where heat is dissipated (secondary system). Both regions are connected by ducts. In the present study, the main system contains a heat source at the bottom which increases the temperature of the fluid near the wall of this source (Fig. 15.1). This fluid tends to rise due to buoyancy forces as its density decreases. The fluid exits from the top of the main system to the secondary system which acts as a radiator by dissipating heat to the atmosphere. The fluid descends through the radiator as it cools down and enters the main system through the bottom duct completing the cycle where flow is induced by a thermo-siphon effect. Heat is transferred between the main and secondary systems by natural convection.