Does a single check valve in a cold mains pipe result in a pressure
drop?

I have installed a venturi shower (Trevi Boost) exactly according to the
manufacturers instructions. Unusually the water temperature is
excessively hot. After pretty exhaustive investigation I believe the
problem may be due to inadequate cold mains pressure. However the mains
pressure to the house is certainly adequate. As recommended by the
manufacturers installation guide the cold water mains supply to the
shower has an isolating stopcock and a single check valve. The
instructions state that the check valve is ideally 22mm but can be 15mm
on the cold side. I have installed a 22mm on the hot side and a 15mm
inline check valve on the cold side.

The cold mains gives over 12 litres/minute at the kitchen tap, but less
than 6 l/min from the shower head. Could the check valve be causing a
reduction in flow/pressure?
--
Matthew Barnard