heating works fine but water is hot and goes cold after a minute or so you can see the piston on the heating hitting the micro switch and diverting the water to the heating.
whats the make and model of the boiler would help ? the diverter valve diverts it to the radiators or to the dhw water heat exchanger. most likely the heat exchanger has become clogged and will need cleaning or replacing. if the diverters faulty then when running the hotwater u will notice the house radiators warming up u can test this by turning the heating off i the night then in the morning running a bath or shower and see if any rads are warming up in the house
