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View the thread, titled "Glow Worm Fuelsaver 40B Mk2 Heat Exchanger Kettling" which is posted in UK Plumbers Forums on UK Plumbers Forums.

I have an old Glow Worm Fuelsaver 40B Mk 2 installed on the kitchen wall.
In November, the heat exchanger began making a kettling noise.



It’s done this before. In the past, simply draining and refilling the system to flush out loose deposits was enough to stop the noise.
In November 2021, I tried using Sentinel X400 cleaner which I left in for a couple of weeks. I flushed the system and added Fernox MB-1 and 500ml of Sentinel X200 noise reducer.
This seemed to help for a while but the kettling noise is now back.

I have a bottle of Fernox F3 but when I tried that in the past it made little difference.

I considered trying an acid based cleaner such as DS-40 but am a bit worried about it causing leaks in the radiator valves.

Directly above the boiler is a drain cock used to empty water in the pipe that runs between the boiler and the airing cupboard upstairs. The heat exchanger also has a drain plug at the lowest point. I wondered whether it would be feasible to drain the heat exchanger via the plug and use the drain cock above to fill it with acid cleaner (using a short hose and a funnel). I could leave the acid in place for a day or two (the heat exchanger is made of copper) and then drain and flush it via the plug.
That way the acid cleaner would not enter the radiators or the rest of the heating system.
One concern would be whether the drain plug would actually unscrew after not being touched since the boiler was first installed years go. If attempting to remove it damages the heat exchanger then it would be game over. Does anyone have experience removing the heat exchanger drain plug? Does it need a new (copper?) washer when re-fitting? If I have to get a new boiler, I would rather wait for warmer weather.

An alternative might be to try just draining the system again and adding more X200 as a stop gap measure.

Any suggestions?
 
The pump is working / not 15 years old / failing ?
I don't think there is a problem with the pump.
It's set to the lowest speed (1). I have tried running it at faster speeds (2 & 3) to see if it would help clear loose debris from the heat exchanger.

I have a pipe thermometer on the boiler outlet. The boiler stat is set about half way (2.5).
That corresponds to a water temp of just over 60degC measured at the outlet pipe.
The kettling noise only happens as the water temp reaches above 50degC.
 
Update:

I put up with the kettling noise for a while but last week decided try the Fernox F3.
I wasn't convinced it would help if the problem was limescale in the heat exchanger.

I left in it for a week and ran the heating morning and evening as usual.
Yesterday I drained and flushed the system and added MB-1 inhibitor + Sentinel X200 noise reducer.
The water that came out looked pretty clean - just a slight brown colouration from the F3.
For the last 2 days, the boiler seems to be back to normal and has been running quietly. Fingers crossed.
 

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