In message .com,
Graham writes
Vaillant VCW GB 242E Over presure valve leaking - when the heating is
off and the pressure is set to 1 bar -

The over pressure 'saftey valve' is leaking slightly .. ie the system
pressure is slowly dropping and the there is a slow 'drip' from the
vent pipe out side ..

I see the valve appears to be part of the boiler - can be un-screwed -
leaving the housing behind ..

Question is ... can it be serviced ? or is it a sealed unit ? if you
unscrew the 'insert' do you also need to remove the housing to
change it ?

1/ Are you sure that it's the pressure valve leaking ?

2/ if so, you could try opening it - give it a good blast, you could
have some crap on the seating

3/ You really should have enough grey matter to answer regarding
replacement or repairability, it's the sort of thing that should be
obvious to anyone with a clue

--
geoff