If a boiler is kettling it is because the water is not flowing quickly enough through the heat exchanger. The temperature increase is too great in one slow pass of the water so it literally boils creating the noise.
To find a remedy you have to think of things that would slow down the flow or cause the excessive build up of heat within the heat exchanger.
Heres a few ideas.
1) Sludge/scale in the boiler and or system,
Remedy - Fernox system restorer or similar. If that doesnt work a chemical powerflush and if that doesnt work then your heat exchanger and or pipework is probably seriously crudded up. I did come across one case where a pumped 22mm Flow and Return to a cylinder was effectively reduced to 15mm by a coating of iron oxides.
2) Boiler not ‘cross flowed’
Boilers tend to kettle less when there is an even flow of water through them but the design of most boilers is a progression of the old solid fuel type. This is not equal to an even flow resulting in slow moving water in areas not directly between the flow and return pipe.
Even though the boiler may be correctly ‘cross flowed’ with the water flowing for example from bottom left to top right the remaining 2 corners dont have much movement of water.
Worth mentioning that some boilers kettle even if they are fitted in accordance with instructions. My own Thorn Olympic, circa 1983 is a prime example.
3) Inefficient pump. This only applies to a fully pumped system.
Remedy -Turn the pump speed up or renew the pump if its on its last legs.
4) Poor gravity circuit ... If a gravity circuit is undersized or has excessive horizontal runs then the water will not flow quickly enough. An airlock in the pipework would reduce the bore of the pipe and reduce the flow of the pipe.
Remedy - Check pipe sizes and that pipes constantly rise as much as possible from the boiler and there is nowhere for air to gather. Consider conversion to fully pumped system.
5) Faulty boiler stat. If a boiler stat is faulty or the probe is not fitted correctly in its pocket then it cannot sense the true temperature of the water in order to turn off the heat quickly enough resulting in boiling / kettling.
6) Pump over-run stat / wiring problem. A lot of boilers have a pump over-run device which circulates the water after the burner has shut down. This is to remove the residual heat within the heat exchanger that hasn’t yet got through to the water after the burner goes off causing kettling or an overheat situation.
Remedy - Replace faulty over heat stat/device OR When wiring to a Y or S plan diagram the boilers wiring instructions should also be checked as the pump should be fed its current through the over-run device in the boiler
7 ) Balancing problems. As a rule boilers are meant to have a temperature difference of 11ºC - 20ºC between the flow and return at the boiler.
This is achieved by careful adjustment of a by-pass valve which effectively short circuits some of the water to avoid a dramatic rise in temperature resulting in noisy operation. This is often critical for some older boilers such as the Apollo.
Please read this draft post for more detailed info.