Unvented system losing pressure again...

[Fluffster]

Tape a plastic bag on your prv warning valve and see if any water leaks out. Yes they can let by at any pressure of the spring is weak.

Pete

 

[dcawkwell]

Latest..!
Pressure drop is only when we run the CH.
HW pressure goes from 1 to 1.5 bar then back to 1bar.
CH goes from 1 to 2 bar then as cools pressure drops to less than 0.5bar.
The system is protected by a 3 bar blow off valve rwc cartridge type. Is it possible that this is letting slightly at about 2 bar? How reliable are they, or should they be replaced once they blow off?
Or.. Have we got a leak... But if so where's the water gone? No sign anywhere or have I hot to get all the floor boards up?
Is the sentinel leak sealer worth a shot or a complete no no?

If you can't see any leaks from pressure relief pipes you have a leak and it's under the floor.
I suggest isolating different parts of the system upstairs downstairs etc so you can narrow it down.
Alternative disconnect the boiler drain down and then use an air compressor to pressure the system up to something like about 3 or 4 bar. You should be able to hear the air escaping.