I've been looking into using a venturi shower and was kindly sent a
large amount of data by Trevi. One thing that does puzzle me is that
their diagrams show the DHW head to be to the *bottom* of the cold
water tank and not the internal water level as I would expect.

Surely the head is the effective weight of the total cold water column
and therefore should be measured to the water level in the cold water
tank. I appreciate that most tanks aren't that deep to make a
significant difference but am I wrong or are they wrong ?

Rob