Help with potterton combi boiler bangs when cooling down

[jonkaw]

My boiler constantly bangs after its been shutdown for an hour or so..sometimes it's 5 or 6 getting shallower other times it's a few more 10 to 12.Its always after I've Ean the hot water on my potterton combi boiler. I live in uk and would love some help with this annoying issue.
Thanks.. Jon

 

[oz-plumber]

Disable pre-heat function on boiler by altering the parameter settings

 

[John.g]

Install a small expansion vessel on the mains supply to the boiler and close to it, some boiler manufacturers insist on this if they have preheat.

 

[jonkaw]

Disable pre-heat function on boiler by altering the parameter settings

This is a 6 yr old boiler that has just recently started with this problem

 

[John.g]

What is the make/model of the boiler?.

 

[jonkaw]

What is the make/model of the boiler?.

Potterton heatmax combi 28HE

 

[jonkaw]

Potterton heatmax combi 28HE

Any ideas what my issue might be?

 

[John.g]

You said its always after DHW usage only?, you might narrow down the reason by switching off the boiler immediately after turning off the HW tap, then reopen the hot tap and run (cold) water through it for a few minutes then turn it off and see does the banging reoccur.
You may have a blocked plate heat exchanger.

 

[jonkaw]

You said its always after DHW usage only?, you might narrow down the reason by switching off the boiler immediately after turning off the HW tap, then reopen the hot tap and run (cold) water through it for a few minutes then turn it off and see does the banging reoccur.
You may have a blocked plate heat exchanger.

Sorry been away.Ran hot for a while then boiler off with hot tap running until cold plus a bit more. No banging. The system is still banging like I say after 90mins of running hot..very strange any more suggestions

 

[John.g]

Sorry been away.Ran hot for a while then boiler off with hot tap running until cold plus a bit more. No banging. The system is still banging like I say after 90mins of running hot..very strange any more suggestions

When you say no banging after running the water cold, did it still bang ~ 90 mins later or was DHW used again before this time had elapsed and then did its banging?
If it didn't bang after running the cold water through it then maybe worth installing a small E.vessel on the cold feed just before the boiler.