Hello all,

I've a Ikon-t gas combi boiler that's been installed for about four
years. Unfortunately it's developed a fault. When hot water is called
for, either by a hot tap or the heating is switched on, the boiler
hums then the gas lights up. All as normal.

However, after a few seconds the needle on the pressure gauge jerks
around as high as it can go and the boiler starts to make a knocking
sound. Then the safety cut-out light comes on. After that the needle
on the pressure dial slowly drops back to normal.

I've checked that the tap on the filling loop is closed.

What might it be?