Intermittent kettling on Volkera compact bioler - fix?

Our flat has a Volkera Compact 25 HE boiler which is kettling occasionally (I don't know quite how oocasionally yet).
It was installed five years or so ago and has been serviced (and passed its gas safety tests) but probably hasn't had a 'major' service.
The installer suggests that the system needs a 'power flush' at quite considerable cost. Is this really likely to be the cure and/or necessary or should we be looking at other things first?
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Chris Green
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On Saturday, 3 December 2016 15:33:04 UTC, Chris Green wrote:

Check the system pressure is as recommended. Low pressure can cause kettling.
See if the system will run satisfactorally with a lower pump speed. Pressure changes around changes in cross sectional area can cause kettling.
The other cause is local over heating which CAN be caused by build up of cr ap on heat exchanger surfaces. Either side, so check if the dry side of the boiler needs cleaning before g oing for the expensive option.
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On Saturday, 3 December 2016 17:00:35 UTC, harry wrote:

Water flow too low or overrun failure are most likely. Low flow can be caused by a bad pump, bad valve or crud buildup.

Only if your primary is running very hot

that will worsen it. Increasing pump speed may fix it.

NT
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On Sat, 03 Dec 2016 15:29:57 +0000, Chris Green wrote:

I assume that you have inhibitor in the system?
If in doubt you can empty, refill with a flushing compound and run for a week or so then drain and refill adding some inhibitor.
As far as I know a power flush is mainly used to clear the loads of clag that slowly gather in the bottoms of steel radiators over the years when there is not enough inhibitor in the system.
Usually detectable by cool/cold spots along the bottom of the radiator where there is crud instead of water.
Cheers
Dave R
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