... I am afraid to return ...

Unfortunately, the hand shower with the built-in my water heater is not compatible.
The water is either too hot or is suddenly cold at some point.
I see the problem but rather in my Through Lauer heater and the water pressure (too old energy guzzlers).

A clue I have.
The straight design of the hand shower a right bracket should be in place.
For me, I can respect the bracket. Angle of inclination not put down far enough.
Therefore, the water jet does not come from above but from "angle back" ...