just fixed a problem after years of ongoing speculation , including "trained" homeserve engineers replacing the prv , twice! As you look at filling loop tap horizontal is closed and vertical is open whilst turning it clockwise , HOWEVER!!!! as I have just discoverd if you turn it a fraction to far so its at 12:01 rather that 12:00 position it slightly opens it back up and therefore ................releases water into the bolier , problem solved at no cost at all , thanks.