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Thanks - but any idea what could be wrong and what I need to do? Is it the diaphragm as suggested or something else? As I say, the hot water always works when the central heating is on. Simon

Hi - the hot water on my Vaillant Sine 18 only comes on now when the central heating is on (apart from very rare occasions). I don't know if this is relevant, but the hot water flow also seems reduced. An engineer looked at it - though he didn't go inside - and says I may need a new diaphragm...