worcester greenstar water keeps going cold

[shirlzababe]

We have a Worcester Greenstar condensing combi boiler and after 6 trouble free years, water keeps going cold. First noticed in shower and hot for 2 mins then cold. Have to turn tap off and on again half way and keep doing that to get hot water for about 2 mins at a time. Similar problem when filling the bath. Often a loud knocking noise from the boiler at the time.
Had diverter replaced but problem back after a week or so.
No problem with centralheating - please help!

 

[Solagas]

It is most likely a blocked DHW Heat Exchanger. Sometimes it is the shower thermostat clogged with lime scale. Having said that, you mentioned it is happening with the tap as well. This points me back to a blocked Heat Exchanger.