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 .