Have you recently had to top up the system pressure? If you have, have you closed the filling loop fully?
Could also be pressure gone in expansion vessel or expansion tube blocked? Does the pressure rise to 3bar before the water starts to leak out? If it does, either of the above faults may be present.
A competent (CORGI eng) will be able to check the expansion vessel for you. They're usually charged up with air to a pressure of 1 bar. If on testing he gets water out of the vessel, it needs replacing. If he gets nothing at all out of it(water or air) it needs repressurizing. If he gets air from it, its probably a blocked expansion tube.
Baz (CORGI Eng), May 2007
