You can't have *water* coming out of a shower at 120C, it becomes
steam at 100C at atmospheric pressure.

Yes you can. When it is in the pipe, it is pressurised. It takes a finite
time for it to evaporate when it emerges into atmospheric pressure.

But the issue here is one of using a hot water TANK on UNPRESSURISED hot
water supply from a CISTERN type header tank.

Well that's what you're thinking of. I'm thinking of a malfunctioning system
that has boiled and gone overpressure with a malfunctioning vent. After all,
why else would the water be at 120C?

Christian.