boiler kettling on hot water

26 Aug 2005
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United Kingdom
My brothers 10 yr old combi boiler is kettling on hot water demand, the central heating is fine, no kettling at all. The heat exchanger and the system has been descaled/flushed.
We both have the same combi boiler, got a deal at the time. Mine was doing the same thing after 10yr but a descale cured it.

Any ideas please
Thank you

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Could be his plate heat exchanger that is scaled. The dhw side of this would not have been cleaned/descaled at the same time as a system flush ;)
The heat exchanger for dhw has been out and descaled.
Its a coil type.
bunser said:
The heat exchanger for dhw has been out and descaled.
Its a coil type.

well from the sounds it it it has not be cleaned very well

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Its totally clean, the chap removed it and pumped a chemical through the dhw heat exchanger for ages and got rid of all the scale etc then flushed it with clean water under pressure.

this was done to all inlets and outlets on the dhw heat exchanger

would a pump cause this problem?

Kettling is caused by scale ( normally lime scale ) on the primary heat exchanger.

If its kettling then the primarily heat exchanger has not been cleaned properly. But you only mentioned the secondary heat exchanger !

You can play about with all sorts of pumping but if you dont do it property with the correct chemicals then it will not clear the problem.

What makes you think he "got rid of all the scale" ?

Only the dhw heat exchanger was removed and chemically cleaned.

The correct chemicals were pumped through the exchanger properly which left a load of bits in the bottom of the cleaning tank.

The kettling can be heard coming from the dhw heat exchanger.

Thank you
The kettling can be heard coming from the dhw heat exchanger.
Well it isn't happening there!

It's either scale in tthe primary heat exchanger, or boiling due to poor circulation , due to a knackered pump or blockage elsewhere within the boiler.
Or a temp sensor is letting it get too hot.
I see what you mean chris but surely if it was a partiall block primary heat exchanger or the pump the beast would rattle and hum in heating mode

I have never succeeded in clean out a plated heat exchanger yet, believe me I have tried, so i reckin your better replacing the thing
The dhw heat exchanger is a coil type.
Perhaps we will get the flow rate of the pump checked because like you say theres no kettling when the heating is on. any ideas what the norm flow rate should be?

Thank you
bunser said:
The heat exchanger and the system has been descaled/flushed.

I assumed from this quote that you had had the radiators and primary heat exchanger flushed, Now that we learn you haven't it will be the primary heat exchanger as you have already been told. Kettling is the water boiling at each little bit of scale that is deposited on the inside of the heat exchanger.
Primary heat exchanger is probably scaled up. Its quiet on central heating since the boiler output may heave been set up for the size of system. This means the burner operates at considerably lower power output than for hot water.
Sorry for the confusion there.

Thanks for all your help
Kettling by ( professional ) definition only occurs on the PRIMARY heat exchanger !!!

However we are led to believe that this is a first that its occuring on the SECONDARY heat exchanger in this particular case!

My sceptical mind wonders if its not kettling at all but perhaps air rushing noises?


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