Vokera Syntesi 25e VERY loud shuddering banging

[rayhollidge]

Vokera Syntesi 25e
CH heats up OK but after 40-60 minutes all hell lets loose. There is a terrible shuddering, banging, noise like very heavy drilling vibration. It is resonating to such an extent it seems to be all around and it is very difficult to be confident about where it stems from. It has been noticeable in rads & pipes near the boiler.
The system HAS to be turned off immediately. Once it has cooled down it can be turned on and is OK until it gets to a high temperature again.
At times lot of “vapour” is coming out of external duct
The pressure gauge is at 1-1.5 when the system is off and at 3-3.5 when it’s on.
Any suggestions would be much appreciated.

 

[dcawkwell]

Possible circulation problem or pump failing for some reason.

 

[picasso]

3 to 3.5 on the pressure gauge expansion vessel flat and the prv trying to open I would have thought.

 

[Gasguru]

Yep, it's the pressure relief valve open and reasonating on its seating. Get the vessel sorted out and a new PRV. Sounds like the filling loop is still connected and leting by since the pressure doesn't drop to zero.