By shutting off the boiler servicing valves that isolate the boiler from the radiator loop you can find if the leak is somewhere in the boiler or in the pipework. If the pressure drops with these taps shut it must be a leak in the boiler. (Dont try to use the boiler with the valves shut).
When you do use the central heating, does the pressure go up a lot when the radiators heat up? (if they do your expansion vessel may flat or faulty, the expansion vessel compensates for small changes in water volume+-, either due to thermal expansion or very minor leaks).
Leaks caused by nails driven through C.H. pipes running below ground floor floor boards etc can be very difficult to locate.
cd, March 2007
