Could be airlock but sludge is also a possibility causing local overheating. Clanking is caused by steam bubbles forming and collapsing. (Boiler is still hot, water circulation has stopped) Causing the pump to run on after the boiler ceases firing is a "fix". (You can try this to prove the cause)
Boiler needs to be flushed out, ideally by removal and hose pipe applied outside in the garden.
Most likely in an old system with no water treatment.
I was getting localised boiling (kettling) in my GlowWorm boiler when the pump turned off. I've added ScrewFix Boiler Noise Reducer and that problem seems to have gone. I now get an alarming clanking instead which I'm sure wasn't there before.
Any ideas as to what is causing it and how to stop it?
Air lock? Try turning the pump to full speed for a few minutes.
You might think you have.
A con, "power flushing" has very little effect.
Since there is no change of water in a heating system, there can be no "furring up".
This is not steam bubbles, I've got rid of those.
There is pump overrun .
It was power flushed two years ago.
Old system, yes, water treatment also yes.
Are you in a hard water area? Could be accumulation of limescale caused by failure or lack of additives or just many years use. Can be fixed by Furnox descaler though it's a bit of a hassle. My boiler tends to sound like my 3KW electric kettle when it's furred up.
rusty
You are wrong there.
We've found the source of the noise. One of the zone valves was sticking and closed with a "bang" rather than gently easing shut. This one had a manual lever and I've opened/closed it a few times. Noise gone.
I used to get that with my old cast iron Potterton boiler. Cleaning didn't seem to help. I added a pump over-run circuit which kept it running for a minute or so after things told it to stop.
There's no need to state the "Bleedin' Obvious" back at Harryagain in this news group. Most everybody here _know_ he's wrong. :-)
Since he 'harries' this NG again and again, his nom de plume is well chosen.
Systems that leak do change water. This can easily go unnoticed for a long time if automatically refilled from a header tank.
I wonder if it has a pump run-on timer which has died?
No. Was a BE type.
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