Heatrae Sadia multipoint 50 issue?

[macka09]

Hi folks
I’ve never installed or maintained the water heater above but today I’ve isolated the cold into the heater but lived up the mains to it so that it could feed the other appliances.
I noticed that water was passing through the tundish. As id isolated the feed into the heater I ruled out the temp/prv on top of the unit being the issue. I checked the vessel which was flat and full of water, so I replaced it. Still water passing through tundish. There was also a prv fitted on the feed to the unit so I assumed this was the issue. Changed it and still passing.
What I did notice was there was no pressure reducing valve fitted. I didn’t have my gauges on me to test the incoming but am I right in thinking that if the heater is isolated it clearly can’t be the prv on top and had to be the prv on the incoming feed due to high pressure??

I will be checking the incoming pressure but the fact theirs no pressure reducing valve I’m assuming the incoming must be opening the 6bar prv!

Anything I may be missing here?
Thanks

 

[John.g]

The MIs show a PRV set to 3.5bar on the cold feed to the cylinder.

 

[macka09]

Something like this from the mains?

 

[John.g]

That looks OK, don't think a double check valve, DCV, is required though, if thats what it is, then the expansion vessel and the expansion relief valve further on.