The gray knob on the bottom of the 553 Series AutoFill (or 573 Series AutoFill Combo) is just a shut-off valve which is used to isolate the feed water from the system. It does not affect the pressure setting. Leave it open if you want to maintain system pressure and automatically top off the system, otherwise close it to isolate once the system is filled and stabilized. Standard rotation applies; CW to close, CCW to open. Be sure to open it all the way during filling!