I have a Termet stove, dual-purpose, for clarification. When I have the refill valve open, the pressure in the system is the same as in the mains, when it is closed it drops after a while, even quite quickly. Every now and then I unscrew and equalize to 1.5 bar. At home, I would close the valve and vent some water or wait. I don't know if there is a similar relationship here.
I won't say which valve in the picture is, I can't see a bit too much, I put it in my blind and twirl my fingers, the knob is brass, incised (a bit like paste). If it is the case here, then the knob does not require the use of any key.
