Boiler kettling
If a boiler is kettling its 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.
Sludge/scale in the boiler ... Remedy = fernox system restorer or similar as you suggest. 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 crud and this was in a soft water area!
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 condusive to an even flow resulting in slow moving water in areas not directly between the flow and return pipe. So 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
Inefficient pump. This only applies to boilers which are in a fully pumped system. Remedy = Turn the pump speed up or renew the pump if its on its last legs.
Poor gravity circuit ... If a gravity circuit is undersized or not constantly rising to its peak or has excessive horizontal runs [often unavoidable] then the water will not flow quickly enough. An airlock in the pipework would reduce the effective bore of the pipe and therefore the flow would also be reduced. 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.
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.
Pump over-run stat / wiring problem. [Not applicable in this case but I'll use this post as a draft for the FAQ section seeing as I have got this far!] 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
If a boiler is kettling its 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.
Sludge/scale in the boiler ... Remedy = fernox system restorer or similar as you suggest. 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 crud and this was in a soft water area!
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 condusive to an even flow resulting in slow moving water in areas not directly between the flow and return pipe. So 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
Inefficient pump. This only applies to boilers which are in a fully pumped system. Remedy = Turn the pump speed up or renew the pump if its on its last legs.
Poor gravity circuit ... If a gravity circuit is undersized or not constantly rising to its peak or has excessive horizontal runs [often unavoidable] then the water will not flow quickly enough. An airlock in the pipework would reduce the effective bore of the pipe and therefore the flow would also be reduced. 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.
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.
Pump over-run stat / wiring problem. [Not applicable in this case but I'll use this post as a draft for the FAQ section seeing as I have got this far!] 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