I have seen reports of inadequate pressure and cold temperatures - but
my problem is that the water is too hot! I don't understand how this can
be given the design of the shower valve.

Recently had a Trevi Boost venturi shower fitted. The plumbing appears
to be correct - with feed from cold mains and top of hot cylinder. There
is at least 1.5m head on hot tank. From new the water temperature was
very hot - and turning the temperature control to "cold" still resulted
in pretty hot water. The pressure seemed low and it cut out after a
minute or so. I thought the high temperature thermostat was cutting it
off. The hot water tank was at about 70 degrees. So I turned down the
temperature of the hot water to under 50 degrees. The pressure is now
better - so perhaps I was correct. However the temperature control of
the shower still results in water which varies from hot to pretty hot -
with no cool or cold water ever coming out. Additionally the hot water
from taps/bath is now under desired temperature.

I don't understand why the temperature can't be adjusted to cold. I also
believe my first hypothesis may have been wrong as if the temperature
cut-off was the issue - any water which did come out should have been
cold - and it was very hot!

So any explanations as to why a venturi shower should produce water
which is too hot? And why I can't turn the temperature down to cold? And
why the shower pressure/flow increased when I decreased the temperature
in the hot water cylinder?
--
Matthew Barnard