Could be the the automatic air vent device on the top right and rear of the boiler. This has a small screwed cap on the top which is screwed down for transit of the boiler and must be unsrewed 1-2 turns when the boiler is first commissioned. The device acts like a float valve similar to a car carburettor to allow air to escape but not water. Over time general crud stops the needle valve in the device from seating properly allowing leakage. You can check by reaching up and feeling the screw cap; if you find leakage here try switching off and electrically isolating the boiler and screwing the cap down firmly. If this stops the leak the automatic vent unit is at fault. Do not turn on the boiler again until the cap is unscrewed 1-2 turns.
You can clean and overhaul the automatic vent yourself if you are confident with such things. It is now an offence for a non qualified person to service boilers however. You need to drain the system, remove the right hand side panel, remove the automatic vent unit and unsrew the top plate, clean and descale the valve and replace. Take care not to damage the sealing gasket/O ring or be prepared to renew it. Replace and refill and bleed the sytem checking for correct operation of the expansion vessel. Do not attempt this if you are uncertain about the procedure.
tony, January 2008
