Its your heat exchanger 'kettling', more than likely due to a partially blocked heat exchanger, and more than likely a scaled or sludged up ch system.
An technican may change the heat exchanger and the problem will go away for a year or more if lucky, but if the system is scaled or sludged up it will not last long.
Total answer would be to have a 'good' 'recommended' plumber come and chemicly treat and maybe power flush the entire system, change the heat exchanger is nessesary (the treatment may improve the heat excahnger blockages too) and finally fit a magnetic cleaner (something like a magnaclean) to the system to keep it clean.
This may seem expensive, but in the long run it is the one and only way to finally put the system right!
T
April 2008
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