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DHW is ok but central heating is not. The fault can only be at:
- The diverter valve which is seized or sticking,
- flow switch,
- CH thermostat and thermistors,
CH not at temperature.
- Sludge/scale build up in main heat exchanger often causing kettling.
- pump speed slow due to sediment/sludge etc.
No CH/ DHW is ok.
- Room thermostat faulty so no signal to combi boiler.
- programmer set incorrectly.
- diverter valve faulty.
- dripping hot water draw off point preventing diverter valve moving.
- microswitches faulty.
- thermistor connections/terminals are dirty/loose.
- open circuit/infinity caused by water ingress.
- thermistor lost calibration.
- thermostat failed
- thermostat lost calibration.
- thermostat contact points worn/ burnt out/pitted.
- thermocouple phial out of position.