vaillant VCW-sine 18 T3 W

[Kensal]

The pressure in the boiler is usually at 1.2 bar, but every couple of weeks I see the pressure slowly decrease to about 0.8 bar.By the morning the pressure has dropped to 0 and of course the heating will not come on. When I re-prime the system the pressure gauge rapidly goes up to 1.0 bar and then slwly increases to my shut off point of 1.2 bar.
Has anyone got any suggestion as to the cause of the rapid drop on the pressure gauge and why the rapid increase back up to 1.0 bar?.

 

[Bunnyman]

Sounds like ur expansion vessel is flat but I would just thank god it's still going Must be at least 27 years old now!

 

[Kensal]

Thanks Bunnyman for your suggestion.
The boiler is actually28 years old. I have replaced the expansion vessel a number of years ago when I had a leak from it but since then it has worked beautifully.
I pumped up the bladder to 0.9 bar a while ago, and replaced the schreider valve at the same time. The water pressure seemed to hold for a couple of weeks before I could detect the needle on the pressure guage starting to go down from 1.2 to 1 bar, then down to 0.8 bar.
I haven't checked the air pressure since the replacing of the schreider so I will check it again.
Thanks for your suggestion and if you think of anything else please let me know.