Invented pressure rising!

[Stratf01]

System Oil boiler has been loosing water through the pressure release valve so when system is cold the guage is showing pressure is too low and needs topped up.
(This shows to be the isolation valves for top up loop are fully shut off)

Expansion vessel was replaced with new one and still having same issue

Pressure gauge showing pressure going up to 3 bar so looks like pressure release valve ok.

There is no automatic bypass but there are 2 towel radiators without thermostat valves.

Any ideas as to what could be happening here please?

 

[Stratf01]

I forgot to add that this setup has 2 motorised valves one for heating the other to immersion tank

 

[Knappers]

I'd recommend getting a professional to take a look.
You have several issues here that tell me it needs assessing by a qualified engineer.
1: pressure shouldn't get near 3bar in normal use
2: pressure loss could be several things
3:the use of 'invented' rather than 'unvented'

 

[Stratf01]

Hahaha please don't penalise me for a typo

 

[John.g]

How many rads do you have and if double or single panel and can you read off the expansion vessel volume and we can do a few calcs which may help. What is the boiler output.