This is a very common problem with this boiler. The nice people at Worcester deny there is a fault with this manifold but I personally have change countless numbers of them.
The only thing that could possibly help, is to fit a water expansion vessel on the cold main as it enters the boiler. If you read the installation instructions, positioning is detailed in there.
Unfortunately however, it seems that this is just a badly designed manifold and you may have to resign yourself to changing the part every few years.
Carefree
April 2011
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