is my diverter valve faulty on the hot water. the heating flow pipe gets hot when the hot water is running and gradually over time the hot water supply is getting cooler.
The diverter valve require replacemnt as it is "letting by" spare spindle sets are available but I'd always advise replace whole unit, you need a gas safe registered engineer to do this for you, unqualified people could cost you more than just money!
The plumber
December 2009
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