Pressure loss when boiler off - Pressure OK when running?

[paulodipoggio]

Have a Ferroli combi - a few years old
Suffered pressure loss twice in the last 5 years
first was a leaky PRV,
second time plumber diagnosed a lack of pressure in the expansion vessel (against the diaphragm?) so got his bike pump out to top it up
This was probably 18 months/2 years ago

Problem now is that if i leave the heating on continuous the pressure stays at 1 to 1.5 bar for weeks on end, but if i set the boiler as timed the pressure will fall to below 1 bar within 12 hours.

can i pump up the Exp Vessel myself?
how much, etc or is it something more serious?

(I did have a weeping rad but this is now sealed)
Many thanks

 

[ChrisR]

How far below 1 bar? Half a bar or so drop is fairly normal, and the pump pressure can raise it a bit when it's on.

 

[paulodipoggio]

thanks
falls to 0.5 or so, but problem is that boiler is set to cut out at less than 1

 

[kevplumb]

whats the pressure cold ????

 

[paulodipoggio]

when it has fallen i top it up to just over 1 bar