commonly the domestic heat exchanger will scale up/block does the boiler fire up then off ,cycling on and off as this can indicate the boiler reaches temp internally then cuts as it senses too much heat,but the water temperature is moderate,
secondly check too see if the central heating pipes heat up at the bottom of the boiler as this could be the diverter valve letting by,in this case the boiler would remain on and not cycle on/off
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