Explain this valve to a noob

wingnut

I am attaching a photo of this valve. I know very little other than the plumber told me to try and make sure it was about 1.5 when the heating is on a while back. Pressure is down and mixer shower is not warm. What is the best way to adjust this: ie what does the grub screw at the bottom do vs the plastic adjuster at the top

cruiser202006

Black knob is for turning on and off Threaded piece at bottom is for adjusting pressure. Clockwise for more pressure. Should really get plumber back to suss out why it's loosing pressure. Check what air is in vessel etc

jca

Fairly shoddy looking installation tbh. Turn the black knob anticlockwise to allow water into the system. If the black knob is already fully open and nothing is happening on the gauge slowly turn the brass screw clockwise to raise the pressure to 1.0 bar with the system stone cold preferably having been off overnight. As these automatic filling valves are notoriously unreliable when the black needle rises to 1.0 bar, turn the black knob clockwise to stop water entering the system and over pressurising the system. The red expansion vessel beside the filling valve will have a shrader (car type) valve under a black cap on the bottom. Take off this black cap and press the centre of the valve briefly, a small hiss of air should come out. If water comes out you need a new pressure vessel if air comes out use an accurate tyre pressure gauge and bicycle pump to make sure the vessel is pressurised to 1.5 bar. Check the pressure vessel before letting any water into the system as this would be the most accurate method without draining down the whole system.