Ian Allen
Hi it sounds like the Diaphragm in the Diverter Valve has become worn or split ( more likely worn ). If the heat exchanger was blocked you would get symptoms of over heat as the heat exchanger would not be able to dissipate the heat. They are relativity cheap to buy & as long as the boiler is in good condition it should take around 1 1/2 hours to do. Kind regards, Ian Allen Aquian Plumbing