"I have recently had a new Combi Boiler and mixer shower fitted ............. ever since the new shower has been fitted.........................awful whining noise."
First port of call should be the guy that fitted it then.
"I have been in touch with the manufacturer and he seems to think a pressure reducing valve is the answer."
Why would he say that unless he knows thats the answer 'cos they know they have a problem with this particular shower valve in certain circumstances? Cynical - moi?
"However the pressure showing on the Combi boiler is only between 2-2.5bar, and the shower can supposedly take up to 4 bar."
But the pressure showing on the Combi pressure guage is the pressure in the central heating part of the system. Thats nothing to do with the pressure on the DHW side which is fed directly from the mains. What is your mains pressure? Did the "manufacturer" ask you that? It may not be a pressure issue at all. What is the flowrate on the DHW side from the combi? What is the flowrate spec for the shower?
"So is a pressure reducing valve the answer? If so how easy / expensive is one to fit?" Is it the answer. Don't know from a distance. Yes they're easy and no not expensive and when you get the guy back in that fitted the shower he should be able to tell you if thats the solution to the problem or if its something else. Do get him back. If its been doing this since the day fitted it then he has an oblication to you. Did he supply it or did you?
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