On Tuesday, 3 March 2020 18:34:48 UTC, Chris B wrote:
I am going to have to fit a PRV in the cold supply for the benefit of a
new thermostatic shower mixer (max allowed pressure 5 Bar) current cold
pressure 7.5 bar and hot pressure regulated to 3 bar by a pressurised
hot water system.


If I do this, do I also have to fit a mini expansion vessel (as in 0.5
or 1lt - like the one linked below) into the cold supply?

https://www.advancedwater.co.uk/prod-775-147-0006.html

As I see it the water that has just entered the house at 3 or 5 C will
go through the PRV and be reduced to 3 bar. It may sit in the cold feed
pipe for some hours and warm up to say 20C. How is this expansion
accommodated?


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You should take your cold water supply for the shower from downstream of the same PRV for the hot water system thus ensuring the pressures are exactly equal.