oh... after a bit of fiddling I found that the white knob (red arrow in pic) is a water isolation valve, and is already fully open.
Not sure what to try next...
Hi
Vaillant Thermoblock gas combi boiler... it is heating water, but I noticed that the pressure gauge is always at 0 when cold. When radiators are being heated, the needle wobbles around 1 bar. I thought it should always be up there, or a bit higher...
Might explain why the heating...