Boiler kettling?

After trying to balance my system as best as possible, I'm pretty sure that its balanced properly. I still have the problem though that I'm getting banging from the boiler towards the end, and after the burn period.
I don't ever remember this happening prior to replacing the boiler thermostat (where it was turned off for 3 days or so). I'm pretty sure that after that, the banging noise started. The heating worked for a few days until I then replaced all the rad valves with TRVs and flushed all the radiators of sludge. I didn't use any flushing agent before doing it though (so the pipes would still have sludge in) and I realised that when refilling, the sludge at the bottom of the F&E tank was disturbed by the water filling the tank and therefore would have been introduced into the system. I put in inhibitor of course when refilling!
I'm not 100% sure the noise came about after replacing the thermostat or after the TRVs (though I'd say 80% or so).
From my new IR thermometer, the boiler flow temp is just under 80 degrees when thermomstat on max, and the return is about 10 - 15 degree drop depending on whether its heating or not heating the water at that point in time. All radiators are nice and hot with very similar return temps.
The boiler is banging loudly towards the end of its firing period and for 30 seconds (guess) after the burner is off but room stat is still calling for heat.
Until the boiler stat failed, I don't remember hearing any noises from the boiler. When the boiler stat failed, it boiled water intermittantly for a while (up to a max of an hour or so but don't remember hearing the noise of it more than twice.). Could this have produced any significant increas in scale on the heat exchanger? The boiler is old (old Baxi WM 531 RS boiler) and I've got a sneaky feeling that the old drain cocks leaked (they were literally covered in blue/white crusty bits and wouldn't turn on or off and felt wet - possibly like this for years). Based on this leaking, the apparent lack of boiler servicing, I suspect that the inhibitor may have been diluted and fresh water being present, so scale build up may have been possible over time. Would actually boiling the water caused more rapid scale build up or even blockage of parts of the heat exchanger? Why is it suddenly doing it now, when it didn't do it before?
The pump seems to work okay (quite noisy on higher speeds but I don't think its a flow problem) and increasing pump speed does not appear to alter the banging noises, nor does changing the boiler stat setting.
Based on all this, does this sound like scale/blockage of the heat exchanger? Could the sludge I introduced when filling the F&E tank blocked the heat exchanger?
I'm thinking that I should try some boiler descaler (the type you can keep in there afterwards) and then, if the noise hasn't gone after a number of weeks, maybe flush with some de-sludging stuff and then replace inhibitor again.
Sound sensible?
Thanks
D
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On Sun, 28 Dec 2003 18:10:36 +0000, David Hearn wrote:

AIUI kettling happens due to small particles in the heat exchanger which acts as catalysts [1] for local boiling the resulting vapour pocket has an extremely short live expectancy in water less than 100C. As it implodes the resulting shock wave is noisy.
I may be wrong but, it is my belief backed up with some experience that there is nothing that can absolutely remove the problem except boiler replacement. Nevertheless cleaning out, desludging, and descaling may reduce the noise much. INHO the only product which I would _expect_ to work is the Fernox Boiler Siliencer Gel which comes in a large yellow (sealant gun) tube and will probably cost in excess of £25. If nothing else my experience is that the noise will be softened to 'thudding' which is less obtrusive. Converting to a sealed system would also definately help where this is a reasonable course of action.
HTH
[1] Nucleation sites for pedants.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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_expect_ to

yellow

nothing
IANAE (by a long way) but I've had good results with this product: http://www.screwfix.com/app/sfd/cat/pro.jsp?idC140&ts 920
It could be that your CH alterations have reduced the flow rate of the water through the boiler, making kettling more likely (by the mechanism described by Ed) - if the flow rate is reduced, the water temp inside the boilder can build up more. Whatever the pump is doing, if the TRVs are cutting the amount of heat used then they are doing so by reducing the rate of water flow through the system.
Or maybe your new thermostat has too high a cut-off temperature. Why not try turning it down a bit?
W.
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wrote:

16 or thereabouts at Plumb Center if you smile sweetly ;-)
Ed, on this subject - if you had to drain down (open vented system), refill, cleanse, noise silence and inhibit - all using Fernox products - what order would you do the Noise Silence/Inhibit in?
You have been fingered as a bit of an authority on these things ;-)
Cheers Dan.
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On Tue, 30 Dec 2003 20:05:37 +0000, Dan delaMare-Lyon wrote:

The only product that would be from Fernox would be the Silencer Gel itself. The first half of the inhbitor would go in the header tank as soon as I was sure the drain point was not going to leak, and the 2nd half as the filling was nearing its end. The gel would go in a radiator that cut off from both ends then partially drained the injected into the bleed hole with the blank plug removed.
-- Ed Sirett - Property maintainer and registered gas fitter. The FAQ for uk.diy is at www.diyfaq.org.uk Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
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wrote:

Gotcha - thanks.
Gradually sorting out the little problems with ours - and given it's a hard water area and the boilers old (but well maintained) I think I'll give this a run if it clears it's next service OK. A good clean out will no doubt do it good - and since I'm going to change a couple of lockshields and thermostat valves as well makes sense to do it all in one :)
Cheers Dan.
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