Boiler Kettling

[Longclough]

My boiler has very recently started Kettling.
The is Boiler a Suprima 40L
Connected to 10 Radiators (3 Bedroom Radiators with TRVs)
Combined F+E (no Vent Pipe)
Unvented Cylinder
Manual Bypass Gate Valve open 1 turn
Pump Set to 2
Boiler Stat at Max

The kettling started after draining down the system to change a bathroom radiator to Towel radiator as the plumbing needed adjustment. I did the pipe work one weekend and fitted the towel rail a weekend later. During the week inbetween I noticed the boiler was kettling slightly, so when I fitted the new towel rail I filled it with X100 and X200. I turned off all radiators, except the towel radiator and ran the boiler on low for 30 mins before opening all other radiators.

However 3 weeks later on I am still getting kettling from the boiler and think it is actually getting worse. Kettling occurs after the boiler has been running for about 20 mins. All radiators heat up consistently to a point where they a hot to touch (can be touched for a few seconds). I have set to pump to 3 for now to see if that helps.

Does anyone have any idea what could be causing the kettling problem? Could it be that the flow is somehow restricted (pump not fully working)
Thanks in advance for any advice provided

 

[petertheplumber12]

The boilers heat exchanger has become partially blocked with baked on sludge and could also be lime scale, the water can't pass through it fast enough so it starts to boil a bit inside the boiler especially when the return pipe temperature increases a bit as the system warms up. just like a kettle coming to the boil which is were it gets the name from. the system needs a good clean, which probably won't clean the heat exchanger much; but then you can add boiler noise silencer to the system, it make tiny little air bubbles in the water which act as a cushion effect to the noise. in the old days they used to put washing up liquid in the water but that is not recommended now.

 

[Longclough]

Petertheplumber thanks for your info. I currently have X200 circulating.

So you suggest adding something like x400 for a few weeks and then flushing the system. add X100 and X200 again and see how it goes.

A quick question about the suprima. I had the new PCB fitted over a year ago, which has been working fine. However I have a question about the pump overrun facility. Does it just run for a set time or does the boiler use the Stat to determine that the it has move enough heat away from the boiler. If the later is the case I would feel more confortable with the by-pass closed to a 1/2 turn to allow less flow through the bypass.

 

[45yearsagasman]

He mentioned "system silencer". Fernox and Sentinel have this type of product. The noise also may well disappear after a while without treatment.
What sort of zone valve do you have? 3 port or two 2 port?

 

[Longclough]

45yearsagasman thanks for your input.

However what is the difference between X200 and boiler silencers. The system has 2 x 2 port valves.

 

[petertheplumber12]

Boiler noise silencer chemical does NOT have a number. that is what you ask for that is what it is called, you add it to any other chemical you already have in the system. if you drain the system, put some more boiler noise silencer in