My Vaillant Combi boiler is not heating water, but central heating is fine


I am confused as to why my Turbo max boiler is not heating the water at all. It is coming through the pipes freezing cold. I have tried the cental heating and this works fine, when the heating is on this also heats the water temperature to luke warm, but no where near hot.
Can you advise what may be causing this before i have to spend money on calling a plumber, also how much should the repairs cost. The boiler is around 4 years old and regularly serviced.
Thanks.
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Sounds like a defective pressure switch sensing the call for hot water.

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Almost certainly the diverter valve diaphragm gone.
ken
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