> "perhaps the water is a bit pressurized as well?"
I suppose in such a large storage vessel, most of the water is naturally pressurised by its own weight. So most of it won't boil, even at 100°C, except right at the surface?
If the water is not being heated from the bottom then I'd expect all the hottest water at the top/and or at the point of entry to the system. Heat loss into the holding substrate will cause rather rapid convection and surface boiling.
I suppose in such a large storage vessel, most of the water is naturally pressurised by its own weight. So most of it won't boil, even at 100°C, except right at the surface?