Banging Boiler! - maybe?

[jblink]

In the last 6-8 months when the central heating boiler fires up, in the first few seconds there can be a series of very loud banging sounds from the system, lasting 5 seconds or so. It sounds like someone is hitting the pipes with a hammer. It only occurs when the system is fairly warm (i.e. never on first start up in the morning) and it only occurs about once or twice a week. I flushed and descaled the system then added Fernox at Christmas. The system remained quiet for about three months but has since started the banging again. The boiler is around 15 years old. Now I know that means it may be due for replacement but is it reasonable to think that this noise may be a symptom of impending boiler demise?

Thanks in advance for any help anyone can give. I would be very interested in the technical explanation. So if it is the boiler, what is the cause?

Cheers

 

[Slugbabydotcom]

You can get a boiler silencer additive but before I say too much read this
//www.diynot.com/forums/viewtopic.php?t=6026

 

[jblink]

Thanks - I guessed I had missed something vital like what type of boiler. It is a Myson Apollo Fanfare S, - wall mounted fan assisted gas unit with pilot ignition. It is running on its max output setting of around 50000bthu. Around 7-8 years ago it started kettling and I descaled the system then and added Fernox inhibitor. Kettling has never returned but as stated, the banging started about 6 months ago. It only bangs for a few seconds when the system first ingnites but only during the PM or evening start up. first thing in the morning, after cooling down completely overnight so far it has been quiet.

The pump is running okay although I wonder if it somehow was slow to spin up on the odd occasion then the already slightly warm water would rapidly boil in the boiler. The pump is original but has not shown any signs of not wanting to run normally.

One question - which may or may not be related. There is a bypass valve on the pump outlet, before the 3 way, to recirculate the water back to the boiler inlet. It is a simple gate valve. How critical is the setting of this? I appreciate that it must not be too wide open for obvious reasons. At present it is set as - closed, then open half a turn. If there was a kettling problem I could see maybe this might influence it but I cannot really see how it could effect the start up......but what do I know

 

[Slugbabydotcom]

That gatevalve is a notorious problem with these boilers. I cant go in to too much detail now cos i am on someone elses computer but open it up a bit so that the temperature difference between the flow and return pipes is always 11C. I know its a little difficult if you dont have the right gear .
Like I say Im in a rush now but Im sure someone else in the forum will take you further if this doesn't sort it as I just skimmed through what u posted

 

[jblink]

Slugbaby - thanks! I actually do have a thermocouple adaptor for a DMM but the bl*ody thing has failed so I cannot measure 11C. However have opened the GV a bit to see what happens. I have no objections to paying one of you Corgi pros a sensible amount to change my boiler. I am very used to working with pressure systems at 350 bar plus, but they are hydraulic - (try tightening a compression fitting the same way! - will explain if you want me to...) but I draw the line at low pressure water!

I have been quoted £2500 to change the boiler by British Gas. Now I know how they pay for the flash cars, flash suits and laptops with wireless printers to give an instant quote.

 

[Slugbabydotcom]

If you open the valve a half turn a day till the banging stops this should do the trick. The 11c thing is just what we are told by the manufacturers. It's academic really and the system is actually going to be at its best if you do it the half a turn a day method and find the balancing point yourself.
Because the heat exchanger is copper and conducts the heat from the flame to the water so efficiently that sudden temperature change causes the water to actually boil where the water comes into contact with the copper, resulting in kettling which manifests itself with the banging noises.
A faster pump speed moves the water along more quickly resulting in the water not being left in the same place long enough to 'kettle'. Turning your pump speed up, making sure that it is working efficiently and that the system is sludge free are the next things to look at if altering the valve doesnt work

 

[jblink]

Cheers Slugbaby - great info. will let you know