Why do you need a ball valve? Wouldn't a simple U bend do
the trick?

I don't see how it would....Unless they were quite tall and very well
insulated.

The hot water in the tank rises to the top of the U, and stops.
it's not going to to down the far side, because the water
downstream is colder and denser than the water in the top
of the U. you might get some convection between the top
of the U and the heater tank, but that ought to be it.