Combi boiler pressure part 2!

Yes. Check its charge pressure. Sounds as if all pressure has gone.

Is this a new phenomenon, or has it always done it?

If it's recent, and was OK in the past then it's likely that the pressure vessel has lost pressure and needs to have some air added via (e.g.) a footpump, or it is possible that the diaphragm inside has sprung a leak, in which case that means a new pressure vessel.

If it has happened from new, then it could be that the expansion capacity in the system is inadequately sized and a larger or an additional vessel is needed. Often in these cases, the pressure relief valve lifts and water is ejected. Then on cooling the pressure drops below what it was before - a vicious circle because you keep topping up to no avail.

.andy

To email, substitute .nospam with .gl

Possibly, see FAQ below.