single check valve pressure drop

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?
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Matthew Barnard

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Thanks David,
The shower is a concealed fitting - and is now fully fitted into a tiled wall. Detaching the back will be rather difficult.
Your recollection of the mechanism is entirely correct. You may well be right that the cold flow is reduced by the venturi valve. Although I have found via Google a description of another venturi shower which achieves 8 to10 L/min. I believe this unit should be broadly similar.
I have contacted the manufacturers - and will pursue them - but so far their technical advice has constituted : have you flushed the pipework?...
As you suggested partially closing the hot supply results in an acceptable temperature and a reasonable shower. However the shower is new and is supposed to provide water from cold to hot - which it currently does not do. The water pressure with this arrangement is reasonable - but not much different to a low pressure system. This is not what I wanted - and I would like to exclude my setup before pursuing the manufacturers.
Thanks again
--
Matthew Barnard

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