The result of kettling = air?

[mahogany]

My old boiler keeps on kettling even though I have used all the methods mentioned here. The resulting air finishes up in the highest rad so I bleed it off.

I am puzzled how the same water circulating in the CH system can keep producing air after localised boiling (= kettling ?) has occurred. I thought that once the air dissolved in the water had been boiled off, there was no more to come out.

Presumably the presence of that air in the highest rad increases the water level in the F/E tank. So long as that increased level does not reach the overflow, no fresh water will be fed in via the ball valve when the air is bled off so no fresh water and no more air.

There is a reasonable margin between the F/E tank water level and the overflow so I don't believe any water exits via the overflow. Also, when I look at the overflow outlet pipe under the eaves, I see no drips.

Can you experts point out the flaw in my reasoning and explain why I need to keep bleeding the air.

And BTW, I'm pretty sure it is air and not corrosion gases as it smells OK and I have always had plenty of inhibitors in.

 

[bengasman]

Despite your inhibitor, I still suspect blockage in your system. Either that, or it is installed wrong.

 

[Weedrip]

Air
Because it is unlikely to be air in the sense you mean, it will hydrogen. It is a by-product of the corrosion processes that occur with the heating system.

Kettling
often caused by the process of localised boiling that then collapses as the local steam bubbles come into contact with cooler water, this results in the noise often described as such because it sounds like a kettle boiling.

The kettling is also usually a symptom of the corrosion process due to the deposits of sludge that can result from the corrosion processes that produce the hydrogen accumulating within the heat exchanger.

hope that helps.