Definitely got some pressure loss happening. 🙁 Only a couple of seconds of the filling loop tap open to bring back up to pressure every few days / week. Only noticed it since having the heating back on this winter.

There doesn’t seem to be any discharge from the external vent so I think the PRV and expansion vessel (internal to Gloworm condensing boiler) are intact.

Can’t see any damp patches in ceilings or rad joints, so I’m thinking the likely culprit is an downstairs underfloor weeping joint. Some is plastic under suspended floor, the rest copper is under a screed. Has anyone had any joy with the sealing fluids?
Otherwise, even finding the leek is gong to be a right pita.

Mine was a pinhole leak in the expansion vessel. Try a bike pump on the expansion tank valve, blow up to pressure rather than fill up with water.

chuck in a liter of a decent leak sealer. do nothing else. put a kids ballon over the prv pipe outside then you ll know for cert if thats the issue. i ve never pumped up an expansion vessel and couldnt reccomend it.

Minor leak in the boiler itself can cause these symptoms. The lost water is evaporated by the heat of the boiler.

Pressurising the boiler cold then leaving it off for a week might help identify the leak, but won’t be popular for 6 months plus.

I’ve had most success with fernox leak sealer

Yep, fernox ls1 often works, cheap fix too.

Cheers folks. I’ll try some fernox then, and double check any discharge from the vent.

Get the stuff in a can then you can squirt it straight into your filling loop. F4 Express you need to ask for.