It is designed to protect the unvented cylinder by controlling the main supply pressure that goes to the tank and is not designed to regulate the pressure of the entire property.Caleffi is a decent make, I understand, but these fellows - well, the safety-release-valves - are prone to allowing a drip through after they've been activated. li>

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My understanding is the pressure reducing valve ( 3 bar) is to maintain a level and balanced pressure for cold flow and thus level flow and equal pressure on hot water. Pressure reducing valves are devices which, when installed on a private plumbing system, reduce and stabilise the inlet pressure from the water mains. I'm afraid I don't have much knowledge of these items - I do have a PRV fitted in my own home (and a Caleffi as it happens) but I'm not aware of it having a safety release valve on it. Another possibility that I forgot to mention is that the expansion vessel could be undersized - if the EV cannot cope with the amount of hot water expansion in the system it will then have to go somewhere else (the PRV).