Why is my oil boiler kettling?

[regsmyth]

Some history...

//www.diynot.com/forums/plumbing/oops-oil-boiler.428202/

Well the chuffing noise is kettling, I guess. And it's still going on, but the boiler and system seem to be working fine.

So I'm trying to understand kettling a bit more. Am I right in thinking the chuff is caused when a bubble of steam collapses?

Boiler thermostat is set to 50-60, pump is only medium speed.

There is a Spirotech air separator about a metre from the boiler on the flow, and an MB3 dirt separator on the return. I get no air from the air remover, so the steam bubble must disappear before it gets there.

I'm beginning to think that maybe the previous blast tube incident has left the casing damaged or thinned, and that the water in that area gets too hot? Or maybe there's sludge or dirt in it restricting the flow of water. Any thoughts?

And if I decided to bite the bullet and replace it, what would be the easiest/best wall mounted system boiler to replace it with?

 

[AlanE]

What happens if you set pump on max?

 

[Burnerman]

Wondering if you had the pump pressure set, and a gas analysis done at some time.....just in case the burner is over firing?
John

 

[regsmyth]

Haven't tried setting the pump to max speed yet. Might reveal something I guess.
No I didn't get the boiler checked so perhaps it is over-firing. But what does this mean? I thought it simply meant the thermostat was set too high and that the excessively high temps inside would start to damage the casing/target brick/blast tube - but they all seem ok, and the exhaust gases don't look or smell unusual.
There is another issue - some air leaks around the door seals - causing some localised sooting. Not sure if it's worth the effort to replace them if the boiler is on it's last legs.
Can you explain "over firing"?
Thanks.

 

[slapper]

Chuck the bloody thing out and fit an External Grant. Seemples

 

[Burnerman]

With over firing, basically the burner flame is too big.....there's too much oil pressure at the nozzle, and by increasing the air supply means that the emissions are generally ok too.
However, if the flame is heavily impinging on the heat exchanger in the boiler, the water can locally boil in that area.
I had this on my own ancient boiler....no one knew what the pressure settings were until I found some info to reset it - scribed on papyrus, it was
John

 

[Terrywookfit]

Live with it. It's well overdue for a leaking heat exchanger!!

 

[regsmyth]

Ok, time to drag this one out from the archives again....

So in April/May I had the boiler checked out. Oil pressure was a little high so that was set correctly. Boiler stat checked and confirmed accurate. Flue gases checked - all ok.
I didn't use it at all over summer but when I began to use it again I realised it was still kettling.
I drained the system and put a hose through the boiler and got a good mugful of bits out.

But still kettling.

My guess now is that there is a blockage or restriction somewhere in the pipework so the water's not moving through the boiler quickly enough. So I'm going to drain the system again and start cutting out bits of pipe to see if there's any blockages. If anyone has any better ideas I'd be happy to try....

 

[JohnD]

You might as well give it a chemical clean. It is a cheap and easy DIY job. It sounds like an old system so is most likely pretty dirty. Sediment and limescale reduce efficiency and cause kettling, even though you may have other problems. I know nothing about oil boilers.

 

[JonG1]

It would be worth checking the output of the boiler and its range rating (via the size of the nozzle) to the demand you have. You also need to ensure that the pump is sized and set up to move the heat around the system.

Oil boilers don't modulate so it may be that at this time of the year with lower demands the output of the boiler is far adrift from actual requirements that the heat is not being absorbed and ends up overheating the jacket.

Do you have TRV's on the rads?

Does it also cycle on/off regularly which would indicate oversizing and if it does, it may be that it is sooting up too, because a short burn time will not allow it to burn soot off and achieve clean stable combustion.

More simply though the boiler stat may be out of calibration.

 

[regsmyth]

Thanks both.
All was working fine after fitting the Spirotech air remover and dirt separator. But then there was the incident with the burner and I had to replace the blast tube. That's when the kettling started. I've just bought some Sentinel X200 so I'll get that into the system in the next couple of days and give that some time to work. Fingers crossed....

 

[JohnD]

X200 is the long-term limescale reducer.

Try X400 first for a few weeks, then rinse well, it will remove black sludge and sediment.

 

[regsmyth]

I already checked that the system is generally clean last year by removing a few rads and finding them all pretty clean. The system works well and heats the house well. I hope it'll turn out to be a build up of scale inside the boiler so I think the X200 is probably the best thing to try first. We shall see...

 

[regsmyth]

Well I'm flabbergasted. I drained the system down to below the level of the pump, poured in the X-200, topped up and bled the system, turned on the boiler, and immediately - NO KETTLING! I've turned the boiler stat up to 70 and had it running for a few days now - all sorted! Amazing!