Draft FAQ post contributions welcome

[Slugbabydotcom]

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

 

[JohnD]

the bit about the cause being water not flowing fast enough - you can get it even with a good flow, if there is scale on the exchanger, because steam form in the scale.

 

[Slugbabydotcom]

Thanks for input
I suppose the the water in the scale that forms the steam isn't moving very quickly, till it boils. Also scale reduces bore and therefore volume of water. So I think that what I have there should be enough to explain without going into too much detail.
Any more constructive criticism anyone?

 

[Gasguru]

Don't forget the need for a bypass on some boilers (and correct adjustment), some manufactures who put automatic bypasses on their boilers as standard also advocate fitting an additional automatic bypass in some cicumstances.

Fabricated heat exchangers (simple copper tube designs) can kettle to the extent that the users are afraid to use the boiler.

Often it is a minimal coating of scale on the pipe wall. The boiling is exacerbated by the fact that this scale is formed between the coils of the turbulator wires and the outer wall.

The majority of non condensing boilers do not measure return water temperature. Although flow temperature is measured if there is an insufficient temperature differential across the heat exchanger this can result in kettling.

Most temperature sensors used have a slow thermal response, the boiler may modulate down but it lags behind - a design flaw in my opinion.

For example the Glowworm Compact ( and equivalent Saunier Duvall) have a fixed heating output - they shouldn't be range rated down.

On small heat loadings (bedsits with a couple of rads) they start to kettle badly just before the burner modulates. I suspect it helps sell more heat exchangers - customers get worried by the noise.
Adding a return sensor (£1.00 cost to manufacturers) would resolve the problem.

 

[croydoncorgi]

1. I strongly suggest we (people that respond here) adopt the convention that 'scale' means limescale, as is found inside cylinders, heatstore HW coils, etc., and adopt a different word for iron oxide-based deposits (black or brown) which are found inside radiators, caked onto low-water-content primary heat exchangers, in the bottom of CI HXs, in pump inlets, etc.

There's a load of confusion about different types of 'scale' and what chemical or other treatment will shift it.

2. There are at least two different types of kettling:
- where the whole water contents of the HX approaches boiling point and 'singing' noises start, followed by 'bumping' or banging as boiling actually starts.
- where parts of the HX are covered by a layer of crud (usually NOT stuck on) which acts as insulation preventing heat from passing efficiently from metal to water. Local boiling happens in the layer of crud as small amounts of water penetrate it, getting hotter as it approaches the metal surface. Eventually, it boils but only very locally, creating a bubble of steam which disturbs the nearby crud. A much larger amount of cooler water can now reach the metal surface and the local temperature drops rapidly back below boiling point. The noise (I believe) is caused by the bubble collapsing (like in pump cavitation) and usually sounds like crackling, buzzing or rapid ticking.

One of the reasons why boilers kettle, then cut out on overheat instead of responding by modulating the burner down is because of bad positioning of different sensors. On a number of designs, the overheat sensor is right next to the outlet port of the HX but the main Flow sensor is much lower down, presumably to avoid the additional cost of wiring and another sensor inside the firewall. as a result, with poor flow, overheated water trips the OH sensor before the heat reaches the main sensor.