Read an advertising article by no less than Grundfoss today explaining to us all how head should be measured from the bottom of the storage tank to the showerhead. However as every schoolboy knows from his elementary physics head is measured from the water level to the point of outlet. If any diy'ers have been mislead by this teutonic wisdom check out this test and determine the head at A,B andC
And I'm sure Grundfos know that. However, as the water level in the tank does not stay constant, Grundfos use the bottom of the tank as a "worst case" level, which ensures the head is always greater than that.