Why does the expansion vessel fail, well the cylinder gets nice and hot, the water is nice and toasty, but the heat causes extra pressure
which has to go somewhere safe. (Expansion vessel) it takes up the extra volume. But we didn't service it for years. so the air inside the
expansion vessel has gone! the cylinder is hot, plenty of showers but the cylinder cant expand its extra pressure, so where does it go?
Safety valves will open and allow the pressure to flow to the waste or drains, but sometimes before that happen it leaks through the immersion heaters and other valves, never good to see, but the cylinders are superbly made, and we rarely see damaged to the cylinders.
So other damage! the expansion vessel has lost it's air charge, water has taken it's place
and can cause the expansion vessel to corrode from the inside out, I get called in to service I see a lovely painted expansion vessel, but I do
not know what state it is in, inside? (My advice is to replace the vessel)
Here is what is happening in Manchester Now! letting agents, and management companies, are sending
out letters to service the unvented cylinders because if the vessel fails mains pressure water will escape from it, at three bar, and could
flood the apartment! If this happens to you, you need to know where to turn off the water (stop cock or mains stop valve) open a hot tap
gently, then you (MUST) turn off the energy to the cylinder. Normally I would turn off the energy first, but if the water is spraying out of
the expansion vessel just turn off the water supply and open the taps
this will calm it down, then isolate the electricity to the cylinder.