could be one of a few things....
If water is leaking through the flow pipe even when boilers is off and system is cold, firstly ensure system pressure is between 1-1.5bar, if up around 3bar, bleed off excess water pressure through a radiator bleed valve and ensure your filling loop is closed. If pressure is correct,and still leaks when system is cold you may need to clean or replaced your 3bar safety valve in the boiler.
If pressure is correct, but leak only occurs when system is hot (when hot the pressure is raising to 2.5bar or above) you may need to recharge you expansion vessel or replace it.
T, May 2008
