Bleeding Car Heater?

IU have a clio and the car heater does not work. The bottom hose to the car heater gets hot, but the top hose is cold. There is a "screw" in the top hose which I thought was for bleeding the air. It just turns and clicks and doesnt appear to want to be removed. How do I bleed the heater circuit and what is the "screw" for? Many Thanks, John