Bosch worcester greenstar 28i boiler won't maintain 1.5bar pressure when off

[PHIL269]

My Worcester 28i boiler won't maintain pressure of 1.5 bar pressure.
I try repressuring it the normal way with key underneath but after hot water or central heating runs it drops towards zero bar when no heating on.
Is there anywhere else to top up the central heating via boiler as it is a combi with no tanks in loft.
Can't see any leaks but would it help to flush and put some treatment in not sure what how do you top up or flush a combi 28i worcester boiler?
Thanks for any help please
Phil

 

[lightning]

Sounds like expansion vessel needs recharging or replacing

Paul

 

[Stadan20]

If you look outside some where you will see the prv outlet , usually 15mm copper pipe is water coming out of there ? Does the key actually pressurise the system ?

 

[AGAS]

if no water coming out the pipe mentioned above ( if so replace PRV and charge ex/vessel at the same time ) if not you may have a leaking heat exchanger or leaking on system somewhere .
fill the boiler back up then isolate the flow and return pipes under the boiler and leave overnight , if you lose the pressure still your heat/ex is fooked