Pipes bang bad when turning water ON

[Albopalbo]

Some help diagnosing how to fix this would be appreciated. (new build house)

About a month ago we had a plumber install a shock arrestor to our washing machine supply as it was banging when the water shut of abruptly. Issue fixed, but ever since then another issue has popped up:

When we turn ON the cold tap in the kitchen, or bathroom the pipes bang badly. The initial flow of water for 1 second is super pressured then flows normal. If I watch the pressure indicator and turn off the tap, the pressure slowly rises from 3.5 to 5 over about 30 seconds. Turn on tap, BANG, pressure drops to 3.
I initially thought it may be some air trapped in the pipes from the plumber draining the system, which was slowly getting compressed by the mains pressure, then causing a water hammer when the flow was released so I drained the system again and filled it by opening the upstairs taps. Still bangs. I repeated, this time opening the upstairs taps in a different order. No joy.

Am I right in my assumption? How do I go about sorting it? Or is this caused by something else. It never did it before the plumber drained the system..? Confused.

Many thanks

 

[rbranco]

you might need a PRV?

 

[Albopalbo]

I've had an investigate and there is a PRV installed. Tried turning it down and had to turn many many times before any difference was made. Now I've set it super low to about 1 bar, the kitchen taps a dribble, but when I turn the tap off, the pressure slowly creeps up to 6-7bar over 15mins. Faulty PRV?