Glow-worm Combi making loud banging noise!!

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My mother-in-laws Glow-worm combi has starting making an awful banging noise. Apparently it started about 5 years ago and successive yearly services by a couple of heating engineers has never come up with a fix. One suggested a pipework problem somewhere. In the last year it has become more violent and frequent.

My wife was there this week and heard it for the first time and was horrified saying the actual casing was shaking!! She was using hot water but the mother-in-law said it also happens on heating.

My wife was using the hot water tap but the noise started as soon as she opened the tap and stopped when she turned it off. The water appeared to be hot as soon as she opened the tap.

The boiler is about 15 years old. The system has never been drained down over the years.

Any ideas what might be the problem?
 
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what is the pressure guage reading when the CH has been on for a while
 
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Sorry, I've not been up to look at it but have to assume that the loop water pressure is high enough to allow the CH to work.

No, there are no leaking taps.
 
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Sorry, I've not been up to look at it but have to assume that the loop water pressure is high enough to allow the CH to work.

No, there are no leaking taps.
We are more interested if it is too high, not too low
 
Mother-in-law had the Heating Engineer visit to service it today and he thinks it’s the main heat exchanger which is scaled up and causing the banging. He said a replacement would be around £500 but he doesn’t think the old 38CXI fitted circa 2005 is worth spending the money on.

A replacement is going to be towards £2000.

What would you do?
 
He didn’t offer that solution. Is it possible? Would a de-rocking solution do the trick.
 
fernox and sentinell sell the proper solutions, best way is to close the isolation valves ,partially drain the heat exchanger then add ab out half a bottle of cleaner, leave the iso valves closed and run a hot tap for about half an hour, then drain and refill the heat exchanger, open the isos and it should be good to go, any experienced engineer will be able to do this in about an hour
 
Are you talking about the main heat exchanger or the plate exchanger for the hot water.
 
I know yours is a different boiler, but this video shows how it is done , you have a different set up but the principal is the same

 
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