You must have a leak somewhere in the system, it could be in one of 2 locations.
1. Heating loop - that is the plumbing to your radiators and back into the boiler - check all joints in the pipework, if pipework runs under floor - check the ceilings beneath for signs of staining, check radiator unions and valves also check radiators for pinholeing, they can rust from inside if air is not bled from radiators annually, also check all the radiator bleed valves.
2. The boiler.
i. Pressure relief valve - trace where the drain pipe from the valve exits the property and tie a plastic bag around its end - if water accumulates here in conjuction with a pressure drop then the valve needs replacing.
ii. Elswhere in the boiler - unless there are obvious traces of water or water stains this is a little more diffiuclt to trace - but his should help - Turn off the electrity supply to the boiler, leave the system to cool completely to ambient (room) temerature, bring the system pressure up to 2 bar by using the filling loop, imediately after doing this close the service valves on the feed and return of the central heating unions (located where the heating pipes join the boiler) and make a note of the EXACT system pressure, what you have done now is created two isolated pressurised locations ie the boiler and the house plumbing. Leave it in this state for at least a day checking the system pressure every now and again. Finally check the system pressure - if the pressure has fallen then the leak is somewhere in the boiler - this will be confirmed when you open the service valves - if the leak is in the boiler preassure should have remained in the plumbing and when the valves are opened the pressure gauge should rise. However if the pressure has remained constant while the service valves were closed and upon opening the service valves the pressure in the gauge drops then the leak is in the plumbing. Third scenario is that prior to opening the service valves there is a drop in pressure and upon opening the service valves the pressure remains the same or even drops - this will show that both boiler and plumbing ar at fault.
Sorry to be so long winded but I hope this helps trace the fault.
Iori, November 2009
