Vaillant ecoTEC plus 637 Unvented System - Boiler Pressure

[Squiral1]

Hey I was hoping someone could help re boiler pressure with my Vaillant ecoTech plus 637.
The pressure keeps dropping down to 0.4 bar, I top it up a little to get it back above 1 bar, than when I turn on the hot water, rads and underfloor heating, the pressure increases to above 3 bar. See photos attached.

I have released air from the expansion vessel and no water is coming out, the pressure in the vessel is a constant 3 bar, I increased it up to 4 bar.
The system as a 6 bar PRV, it seems to be working ok, I can twist to release water, otherwise it look dry, so cannot see the system is releasing any water via the PRV.

Not sure what else it could be. Any help would be appreciated.
Thanks

 

[jj4091]

Isolate the flow and return heating pipes and see if it still drops after a couple of hours

 

[Squiral1]

The pressure drops after a few days, could be longer, but depends on usage.
Just having the hot water on can cause the pressure to drop after a week or so.

Could the expansion vessel be faulty?

 

[CountryFan]

Could the expansion vessel be faulty?

Or flat, especially since you've depressurised it

 

[Squiral1]

Attached is the photo of tune vessel label, what should the pressure be and how to put pressure back in?

 

[CountryFan]

Attached is the photo of tune vessel label, what should the pressure be and how to put pressure back in?

That looks like an unvented cylinder vessel and you shouldn't be touching that as you need to be G3 registered/competent.