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Unvented cylinder, Honeywell S-plan with an Ideal Icos boiler. Very gradual pressure loss (0.1 bar / two days on average). Seems to work fine, boiler serviced in October with no issues mentioned by the plumber.
I'm pretty sure it's not the heat exchanger in the boiler - fortunately - since I drained the condensate sump and left it switched off for some time. The sump was bone dry when I checked several hours later.
I briefly whacked the system pressure up to 2 bar and checked every radiator. One or two showed a tiny bit of weeping from the bleed caps, which I've tightened, but it was a truly tiny amount.
Auto-vents are dry. No sign of leaks on any visible copper pipes (rads are plastic piped). Over-pressure / over-temp drain is dry (I wondered if it was a trickle past the valve, but no).
Running out of ideas now...
Tie tissue paper below every joint.
Turn heating on.
Check every bit.
I was going mad with this , turned out i had a pin hole on a soldered copper bend and it was misting over time but drying almost instantly on the hot pipe.
Uv dye and uv light found mine
Fixed it up with a compression joint as twas all i had in the shed and drained the system - flushed out the uv dye and refilled with fernox antifreeze/inhibitor.
Also 0.1 bar over 2 days is a tiny amount.
It doesnt take much fluid loss to lose that much pressure.
Put a bag on the end of the prv pipe - and check it tomorrow - although from what you said i doubt its that - more likely to loose most of the pressure every other day .