Sick of all this crap , why don’t you go post this stuff on another forum , it’s getting boring really quick.
Have you checked if the discharge pipe is wet, or you could put a ballon over it to see if it is discharging
If you have such good pressure why not go for a unvented cylinder .
I got brand new one of eBay couple of months ago
Get the installer back and explain what’s happening
It would help to tell us what boiler you have
What I was saying that the ultimate 3 is exclusive to city plumbing yes 5 and 10 year warranties.
City plumbing only do the ultimate with 10 years warranty
Have fitted loads and no one has ever complained of the flow rate
Without seeing the house it’s hard to judge , Vaillant system 37 kw can be range rated , would have thought you would need 28 mm for gas...
Flow turbine faulty
Open the two back handles till 1.5 bar then close them
Why don’t you just fix the cm927 it takes about 10 mins to do
Do all radiators have trv’s on , if so take one off when the noise happens to see if it stops
Check the expansion vessel , also where is the expansion vessel sited
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