Depending on the model it could be the diverter valve... When the heating's off and you turn a tap on does anything happen at all? Have a quick search on here under Worcester Diverter Valve or Giannoni for more details...
Handyman
February 2007
I have a similar problem - CH works a treat but the DHW is only just warm. I've tried adjusting the flow rate, but it only gets hot at really slow flow rate.
According to the manuals fault finding chart it could either be the DHW sensor or the DHW potentiometer.
I think the cause of the problem in this case was DHW was operated when there was no water in the CH system.
Simon
February 2007
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