My gas combi's sealed system pressure drops from 1.0 to about 0.5 bar every
few weeks. It seems to stick at 0.5, but I've not left it long enough to be
sure. A few seconds on the loop valve and it's back to pressure - so at most a
pint or two of water.

I can't see anything obvious at and around the rads and pipework, but did come
across an encrusted main water inlet valve/unions on the boiler. This was an
issue soon after the boiler was installed, and I've only just noticed it now.
There's no sign of dripping water though. Could this be the cause of the
pressure loss?

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Cheers, Rob