Mystery combi fault

[OldScrawn]

My fairly new Wickes / Halstead is losing pressure, but only when the CH is on. On DHW only I don't think I have had to top it up since the spring. With CH on it drops from 2 bar to 0.5 bar in a day. I can't spot any leaks (but it is a funny old house and there are inaccessible places where it might conceivably be leaking). I'm 95% certain it isn't overpressurising and venting through the overflow (I had to replace a leaking relief valve, and the leak then was fairly obvious). This sounds like a leak from the CH but only when hot, and which I have not yet located. Or am I missing something?

[Ed Sirett]

Certain comression joints can start leaking when hot but hold when cold. (Are there other types that do this I've only seen this on badly made compression joints).

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