On 29/08/2020 19:06, Harry Bloomfield wrote:
After serious thinking RJH wrote :
My gas combi's sealed system pressure drops from 1.0 to about 0.5 bar
every
few weeks. It seems to stick at 0.5, but I've not left it long enough
to be
sure. A few seconds on the loop valve and it's back to pressure - so
at most a
pint or two of water.

Check the PRV (Pressure Release Valve), usually the exit pipe is outside
- put a plastic bag tight over the end, with an elastic band. If that
collect some water, that's where it is being lost.

Agreed

Does the pressure increase, when the boiler is burning? Could be that
the expansion vessel has no air in it, so as the pressure rises, it is
forced out the PRV.

Agreed. If it repressurises quickly that may be an indicator of a failed
expansion vessel. Often a PITA to replace, easier to add an external one
if you have basic plumbing skills. It doesn't have to be right next to
the boiler.

If the pressure relief valve has been weeping there may be erosion
damage on the seat, so that it always leaks after the primary problem is
fixed. They are usually a generic product, available from BES rather
cheaper than the manufacturer's version. You just have to make sure you
have the right size and gender. Not usually too difficult to replace.