CH Losing Pressure

Sound like the expansion vessel needs repressurizing, or maybe replacing as it has a punctured diaphram.

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.

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

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.

Done - I'll check when it's next on, thanks.

Yes, it usually goes up 0.5 bar or so.

Thanks, I'll take a look.

I don't know whether it's the whatsit bottle or the valve, but encrustation does indicate a leak over time.

NT

Don't think it's either. The expansion vessel is at 0.6 bar, with a specified range of 0.5 to 0.75. Nothing in the plastic bag on the overflow (after a couple of showers and a few hours of heating).

While in, I had a closer look at the fitting and I'm pretty sure it's historic, and no sign of any drips or leaks. The only other thing was the condensate trap water looked very murky. That'll need cleaning at some point, but it isn't the cause of the pressure drop.

So, under the floorboards for me. Joy.

Well, at least it's not time to dig up the concrete floor.

NT