Mike tyson inside my boiler!

Hi there,

I have searched through FAQ and cannot see an obvious cause for my dilemma from the very informative postings here, so here goes, i apologise in advance if I have missed something obvious.....
I desperately need some advice regarding my boiler, a halstead Best 50. Gas, with gravity fed cold tank and hot header tank...After operating the Heating (not hot water) this boiler runs like a dream for sometimes hours and then all of a sudden BANG, it sounds as if mike tyson is trapped inside it and is smashing his way out with a hammer.BOILER is situated in kitchen directly below hot water cylinder pump and motor valve in bathroom but i'm sure it's the boiler....This problem is not new, started about 6 yrs ago... no apparant cause...
This boiler has been serviced annually and in past 12 mths, and last week following boiler failure it had a new pcb, gas valve was fine but new honeywell motorised valve was installed due to perishing (rads were heating up even though heating off). system was obviously partly drained and then refilled and i believe relevant rads bled...
Having had this prob for some time and 5 different plumbers come out, the system has had more attention than most... it has been drained and flushed, descaled flushed and refilled and bled. There have been two alterations to the system. once when a power shower was installed in 2003, which takes a cold fill from mains in loft and hot from flange on top of hot tank, the other is the addition of two rads and a heated towel rail in an extension. All rads have thermostatic valves, towel rail is correct bcu rating, but probs were apparant before these installations...
When i asked latest plumber about the noise he said it was due to the gas pressure which was off the scale- which he said he has rectified, but as with previous call outs (must have spent over a grand so far) - problem still there... Has anyone got any suggestions? As i'm sure something may get damaged judging by the noise... imagine a length of steel pipe - then hit it with a hammer as hard as you can in an irregular pattern for 30 seconds.... It scares the c**p out of me everytime!

There definately seems to be air bubbling in the rads so i wonder if it's building up somewhere it shouldn't! After the drains and fills i thought that would be irrelevant unless it's like bleeding master and slave cylinders in cars- sometimes needs pressure bleeding if air lock won't shift???
Here's hoping one of you can shed some light on this aggressive boiler!
Thanks for reading,
Worried female (getting poorer by the minute )


removed email - rupert

Its very difficult to guess but I would wonder if the stat has failed and the boiler is seriously overheating?

I also would question the competence of some of the people who you have had changing parts.

Even on a Halstead I would question if it really had all those parts fail.

Tony

Thanks for that, strange you should mention the stat because i did point out the fact that the pump 'in airing cupb' sounded like it was boiling inside and it was like a boiling kettle to touch.. in other words untouchable as was most of the surrounding pipework, but that was put down to faulty valve leaking the hotwater which was too hot 'cos of the boiler stat being up full wack... seemed logical but then i haven't a clue have i....
Anyway the pump still fizzes but i think thats air bubbles whizzing through.. and it's not boiling hot anymore..