hot water cutting out and boiler making rattle noise

Hi my alpha 240e boiler when turned on hot water mode is making a loud rattle noise and the pressure gauge goes to maxim,um then the pilot light goes out and the gauge goes back to minimum setting and water is then cold. the central heating works fine.

I believe by looking at other messages on this site that it might be the DHW diaphragm that needs to be replaced and i know this is only about £ 11. is it easy to fix yourself as i have the installation manual

A worn diaphragm would NOT cause those symptoms.Your boiler is overheating and is more likely to be a blockage in hw components,eg secondary heat exchanger etc.Parts need to be removed and checked.A worn diaphragm usually just stops the boiler from working on HW.

I agree.

But if its got to such a bad state then the hot water delivery must have been very poor for quite a long while!

Tony