On Sun, 22 Nov 2020 15:49:25 +0000, AJH
wrote:

It's probably a failed expansion vessel

When the British Gas engineer visited recently, he said something
about "capillary".

Basically the pressure dial drops well into the red zone, and when you
tap it, it goes up into the green zone.

Would that be a problem with the dial itself, "capillary" or whatever,
or would it be a faulty expansion vessel?


Sounds like a sticky needle in the dial gauge. If it were the faulty
expansion vessel the pressure would drop below the point at which a
pressure switch would prevent the boiler from firing.

How would it be fixed? Can you replace the dial gauge, would that
solve it?