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Hi there,

I have a 10-15 year old Worcester Combi Junior 24i that kettles only on Central Heating cold start. This winter is the first time it's started doing it. When it kettles, if I turn off the boiler via the thermostat for a few minutes then turn it back on again it runs no problem until the system cools down.

When running CH all radiators get hot. Pump appears to be working fine. I bled the pump and checker the impeller was running freely with a screw driver.

I've been the owner of the flat in which the boiler lives for 5 years.

Last week I added Fernox F3 to the system and left it in for 7 days on regular heat cycle. The severity of the kettling noticeably improved but after a week it still remained.
I then flushed the whole system 3 times. Quite a bit of dirty water came out (to be expected as not sure when it was last cleaned).
I then added Fernox F1 Inhibitor and Fernox F2 Boiler silencer to the CH system as a final prayer.

It seems like the system is running much more efficiently and is generally much quieter sounding.

But, the kettling remains, albeit a bit quieter. Now either the kettling stops after a few minutes of it's own accord - seems as if the boiler can get rid of the heat eventually.
Or again if I turn the boiler off for a minute then turn it on again it runs perfectly for the rest of the time that the CH heating is on.

It seems like as long as the water in the CH system is above a certain temp then there are no problems.

Does anyone have any ideas as to what could be causing this problem?

New boiler is last resort as it appears our kitchen was built around the current boiler, which means we would need a new kitchen as well....

Thank you in advance!
 
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Drained the system then poured it into the radiator nearest the boiler. Then refilled the system.
 
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