Potterton promax 40erp HW issue

[mrpled]

We have this boiler installed and it's started to give issues.
The service was done and passed ok. But only as there was no noise for the guy to hear.
Radiators are fine.
When hot water is being used it generally seems OK, but sometimes there's a large rumble noise 10-15 secs then it stops.
I haven't been close at this time but have also noticed when showering the boiler setpoint is exceeded. Flame cuts out. Drops temp,and refires. Temp increases, exceeds and flame cuts out.
The expansion vessel is at 1bar so doesn't seem it's that, but other than calling baxi. What can I check next?
Video to show the temp fluctuations available.


Thankw

 

[John.g]

Could be the PHEX (plate heat exchanger) is fouled, it may run OK at low HW demand but trip out on high demand.
"have also noticed when showering the boiler setpoint is exceeded" Is this the DHW setpoint thats exceeded by the actual DHW temperature?. If so could be different problem. Can you run two HW taps at full (HW) flow and see if the same problem occurs.

 

[Deleted member 120897]

No video attached, also how old is the boiler? Could be plate hex as above, but can be other things too.

 

[Knappers]

HEX or sensor - shouldn't be a difficult find/fix for a gas man