A valve including a body that defines first and second ducts that open out into a third duct. The first duct is provided with a first shutter and the second duct is provided with a second shutter connected to drive means arranged to move the shutters in a normal mode of operation in which the second shutter is in the closed position and the first shutter is adjustable in position between an open position and a first closed position, a stop mode in which the second shutter is in its closed position and the first shutter is in its first closed position, and a secondary mode of operation in which the second shutter is in its open position and the first shutter is in a second closed position.