Leak detection

nofoollikeold

When the weather allows it, turn the heating off, raise the pressure to 1.5 bar, and close the isolation valves on the boiler flow and return pipes (under the boiler). Leave it like that for 24 or better still 48 hours. If the pressure at the end is still 1.5 bar, then the leak is not inside the boiler, but must be in the central heating system.

As suggested above, wrap a piece of rag, or tissue paper tied in a bow with the "ears" sticking up. Run the system. Any leaking valves will show up by wilted ears.

Only if both the above fail is is likely to be worth digging up flooring.