hi i bet u really understood mr k answer i had trouble & i've been in the business 27 years, anyway i guess your old system had roof tanks that are no longer there, the plumber would have cut off the mains cold feed to these this piece of pipe will have air in it as the air compresses it can force water back in2 the boiler causing it to fire, so either get rid of the air in this pipe, get rid of the pipe altogether, fit a non-return valve or there is availible a valve that goes in2 the pipework to prevent water hammer this may work as well, which ever u choose it would be down to your installer to do this free of charge if he gives a warrentee on his work.
hope this helps.
Keith Gillyon Plumbing, Heating & Building Services (southampton)
May 2009
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