Hi, had diverter valve changed about 6 months ago and all was fine. Heating works but now the hot water is only Luke warm. When hot water tap is running or the shower the boiler keeps switching the burner off and on it might say be on for a minute then goes off say for about 30 seconds then comes back on. The pipe from the boiler is hot but the 1 on the heat exchange botom left is cold. Could the heat exchange be blocked boiler is 20 years plus old any help be much appreciated.