Noisy banging boiler what is the cause?

All,
still looking for a little help with my boiler which has been making noises or the last 18 months (since I bought house).
A bit of background...
The system is a fully pumped CH and HW system with a motorised valve, cylinder stat, 1 room stat. there is a bypass from pipe from the pump back to the inlet of the boiler with a gate valve (what is this for?????)There are 9 rads 2 of which are fitted as bypass devices (shower room and bathroom). Bolier is wall moutned on first floor in an airing cupboard right next to HW tank.
The boiler is a Thorn EMI Apollo Fanfare 30/50i fitted in 1987 when house built In the last 12 months I have replaced the pump with a suitable Grundfos Alpha repalcement, tried to descale the system and replaced the auto air vent on top of the boiler. I have also tried to make sure that the system is properly vented but I am not really sure if I have achieved this. The noise has improved but not drastically - in fact the way the boiler behaves seems slightly changeable so it is possible that all my efforts are in vain.
The symptom...
On switching on (firing up) from cold everything works OK for few minutes. Then I get massive bangs that wake the dead/neighbours/neighbours dogs. Then system settles down after 1-4 iterations of the start-bang-stop cycle. Having gone into the loft to investigate I can see gas (I am pretty sure steam) bubbling/exploding out of the CH header tank during the banging - there is not a safety vent pipe above the tank as I have seen in other installs. The steam appears to be coming into the tank via the outlet that feeds water from the header tank to the main system.
Recently I have come of the opinion that the pump is switching off for perhaps 20 seconds (there is a green LED which shows pump is running) - the water in the boiler really does boil, steam evolves and the expansion forces it to the header tank - then the pump kicks back in again for no real reason (green light comes on) and unless the cycle is repeated everything settles down. Immediately after the bang there is a real whooshing sound in the pipes which I presume is the sound of tiny gas (steam) bubbles being circulated this soon settles and is not present in normal operation.
So I guess my line of questioning should be around the control logic which switches the pump on/off.
I know the boiler has a lo stat, high stat and an overrun stat - you can see these on the pipes when the covers are off. The overrun seems to work as the pump will keep operating (with light on) for some time after boiler has stopped heating. With the lo stat selected the bolier never really seems to get going jut starts, does a little banging and stops... In the High position this is the only thing that seems to kickstart the boiler and get it going - as I say once the banging is over in normal operation the boiler/pump work fine.
So - how does the logic work which switches the pump on/off?
Would faulty stat(s) cause these symptoms? I could understand if a stat failed that the pump might stay on for ever or never come on but why does it come on, go off for a bit (20 seconds) and then come back on as the boiler is boling and making a racket...
Can the pump switch itself off or is it, as I believe, a fairly dumb device which lights up and spins when it has power and switches off when there is no power.
What else could be causing this pump behaviour and then boiler noise(I am sure it is not loose connection etc. fault is too consistent) can the pump itself shutoff???
I am really desperate to try and fix this as I do not think there is necessarily anything wrong with the boiler and do not want to try and find 1500 for a new one. I am also reluctant to spend lots on fernox etc until I know bolier is not about to end up in landfill.
SOrry for long post but I tried to give detail. If there are any questions/suggestions please let me know and I will try them. I have a wife and baby who are getting increasingly unhappy with me for this :(
Thanks in advance Tim
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On 26 Oct 2004 05:32:40 -0700, snipped-for-privacy@hotmail.com (Tim) wrote:

(SNIP)
I am DIY. I hate expensive plumbers. TURN IT OFF NOW! Call a plumber!
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The banging is caused, as you seem to know, by the lack of flow through the boiler, causing the pint or so of water actually in the boiler to turn to steam. I would imagine the bang is when the steam suddenly condenses when it hits the cooler water.
As for the pump, they are in fact dumb, and should not turn on and off on their own. The boiler electronics may turn them on/off, or they will be on for the entire duration of the "CH on" cycle. Not that I have much experience, but I would have thought a broken pump would either work or not work (I have had one get weaker), so maybe the on/off action is caused by a loose wire. Maybe the system heating up moves the loose connection via thermal expansion, causing it to stop for a while.
I would check the wires are secure (waggling them while on would cause a cut out of the pump). Failing that, I would fit a new pump (about 30-40). My pump in the last house had a valve each side of it, making it easy to swap without draining the whole system.
You may need a tool to turn the pump head around (by removing 4 allen bolts). The pump must pump in a certain direction, but to make wiring easier, you can turn the pump head around so the junction box is accessible. You may be lucky and buy a pump that is the right way around though.
Bob
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Some (most) pumps are prone to seizing if not regularly spun up by the system. On some there is a large flat in an inspection cover on the end face - undo this ( it will drip) and turn with a screwdriver the spindle underneath - it should turn easily or you may need to free it with a twist to break the sticking - then it should turn fairly freely.
Nick
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On Tue, 26 Oct 2004 05:32:40 -0700, Tim wrote:
<snip>
The type of temperature sensors you have in the boiler are apt to drift way out of calibration over the years. So it is possible that either the high or low setting are not around 65C and 80C as they should be.
There is an over heat sensor (95C) which also ought to have responded to the boiling? However if there were no flow at all it is _just_ possible that the steam is being raised in the heat exchanger and never gets to the o/h stat.
The pump over-run stat is meant to keep the pump running at all times when the boiler is on and at any time the boiler is above a certain temp until the boiler cools down. It will only work if the boiler is correctly wired with both switched and permanent lives and the pump is connected to the boiler and not in parallel with the switched live of the boiler.
Now any or all of the above 4 'cans' can and do cause problmes with this model.
I don't know a geat deal about Grundfos Alphas. I do know that the idea of reducing the circulation effort as the system warms up has got to be bad news, unless the burner is turned down by the same control (as is the case in many modern boilers). In particular it sounds like the Alpha also tries periods of not pumping - that might work on a cast iron lump boiler but it's really bad news on a low water content boiler like yours.
I'd first replace the Alpha with a bog standard 30 pump. Then you will be able to diagnose the problems with the temperture sensors if any.
-- 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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Thanks for the reply Ed, Comments in line, anyone please feel free to chip in...

This is what I had thought. If one or more of the stats are out, I wonder if it could be responsible for the behviour of the pump though. I dont think any boiling occurs before the pump stops - as I say the green LED on the pump goes out so I am sure that the boiler has stopped feeding current to the pump by this stage. This does not explain why the pump comes on to start with...if there was a flowchart which showed how/when the pump got power it would help :) Is there some sort of cold running override which turns the pump on when the system starts up??? If you think about it there must be something which achieves this or otherwise the pump would not start until a threshold value had ben reached by one of the stats...

Perhaps it is this sensor which kicks in ad restarts the pump??? If this has creeped to about 100 then you ould imagine the water will boil and then the overheat kicks the pump off again Again this does not explain why the pump comes on to start with - see cold start theory above

This *seems* to work fine from an overrun point of view - sounds like boiler wis wired right...

:(
OK well I have been running the alpha in dumb mode on the old fashioned 1,2,3 positions - in this mode it does not try and do anything clever. So I think changing to another pump is something I will not try yet...I will put a multimeter across the pump tonight and see if, when it stops, it is being fed current (ie is the pump switching itself off or is the boiler doing it)

I think the position 1,2,3 trick will suffice here, but if I am wrong someone please correct me.
So does anyone know either how I can test the stats and where I can buy replacements and for how much??? Are the stats failry generic parts or are they bespoke per manufacturer and boiler???
Thanks Tim
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On Wed, 27 Oct 2004 04:50:33 -0700, Tim wrote:
<snip> I had not realized the Alpha could emulate dumb modes so that is worthwhile, certainly any mode which turns the pump off even though power is applied is going to cause problems.
The over-heat stat if it operates should have shut the boiler down requiring manual intervention to restart. On the old permanent pilots it simply killed the thermocouple circuit and the gas valve dropped out. However yours maybe an electronic ignition model?
The over-run stat works like this: The pump is connected to the common pole of the thermostat. When cold the over-run stat connects the switched lived to the pump when hot the permanent live is connected to the pump.
A failure mode of these devices is that a 'dead zone' can emerge between the cold and hot states leaving the pump completely unpowered at certain temperatures.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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Thanks again Ed,

agreed, I have tested and power is definitely not being delivered to the pump for a short period between hot and boiling...it then comes back in when the boiling has happened.

It is elec ignition, interestingly sometimes it fails differently - I am guessing here that if the pump does not kick back in the overheat stat shuts down the boiler and then we start again... starting to make sense now.

which gives the kind of cold start functionality I ws thinking of (all be it achieved in a far more straight forward way...)

That sounds exactly like the problem...so all I need to do is try and find which is buggered, where to get a replacement and replace it/them.
Thanks Tim
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Well after 18 months tis all fixed. You were spot on, I think there must have been a dead zone. I looked at the wiring diagram and the only thing controlling the pump in this way was the overrun.
Got locally from stock a Hi, Lo and overrun stat for about 22
Took the casing off and changed em all in about an hour. Fired her up and not a bang since.
Very pleased
Tim
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