firstly, is the filling loop passing, ie open when it is shut off? If you have an integral filling loop remove the key! Does it leak water?
Is the expansion vessel full of water?
If not the above your water to water heat exch is cross leaking! If you lose the pressure out of the system and do not turn boiler on does the pressure rise on its own? if it does turn the cold isolation feed off..does it stop rising...then your heat exch is cross leaking! replace-easy job!