Abstract: A fitting for joining a radiator to the main flow pipe of a single duct heating system comprises a housing which defines an inflow duct for connection to the upstream section of the main flow pipe, an outflow duct for connection to the downstream section of the main flow pipe, an inlet duct for connection to the inlet conduit of the radiator and a return duct for connection to the return conduit from the radiator. The return duct consists of a piece of bent pipe, arranged at the interior of the housing, said return duct entering in the space of the outflow duct at right angles with respect to the axis of the said overflow duct, and then making a bend so as to become parallel to the mentioned axis, in order to discharge the return water in the same direction of flow of the water in the outflow duct.