Every day (for about 2 years) when the heating is on, I have to let water into my Worcester combi greenstar 28i boiler system. It drops to 0 again by the next morning. The pressure relief valve looks dry (i can't reach it to feel if it is wet) but there are no drips or water stains down the outside wall. No leaks have appeared in the downstairs flat (i have been waiting for a leak to appear). The radiators aren't leaking. The pressure rises fairly quickly once the heating goes on, but it never goes into the red. I therefore assume the PRV doesn't blow and that it is unlikely to be faulty, especially if I don't see water outside. Does the expansion vessel need replacing? If the diaphragm in the EV is punctured, then surely the pressure would build up very quickly and it would blow out of the relief valve. I would see the gauge go into the red and also notice water outside, so assume it isn't the EV either. Therefore I am unsure what is causing the pressure to drop and where the water is going. I have had 3 plumbers who don't really know what to suggest other than to wait for a leak to appear (hence 2 years of observing) or to try changing the EV or the PRV and see if that solves it. Would love advice on what the problem might be before having to incur anymore cost.