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  1. Central heating is ok, but DHW is not.

    The fault can only be at:
    • The diverter valve,
    • microswitches,
    • flow switch,
    • dhw thermostats/thermistors,
    • dhw heat exchanger,
    • water governor,
    • pcb.
  2. No dhw, diverter valve diaphragm pass type.
    Split diaphragm.
  3. No dhw, diverter valve Venturi type.
    • Split diaphragm.
    • obstruction in Venturi ie scale, rust etc.
  4. No dhw or dhw not at temperature and CH circuit warms. Diverter valve.
    • Wax capsule lost calibration.
    • capsule scaled.
  5. No dhw or dhw not at temperature and CH circuit warms. Spindle does not move.
    • Pump failing/failed.
    • venturi pipe work blocked.
    • filter/strainer blocked.
    • diaphragm perished/split.
  6. No dhw or dhw not at temperature and CH circuit warms. Slow movement of spindle.
    • Pump failing.
    • spindle shaft scaled due to leak.
    • venturi partially blocked due to rust or sediment.
    • water flow slow due to sealed domestic heat exchanger.
    • mains pressure low.
    • pressure loss at spindle sealing unit.
    • air may be trapped in valve after repair.
  7. No dhw or dhw not at temperature and CH circuit warms. Spindle moving but no response.
    • Not enough spindle force due to faulty diaphragm causing pressure loss.
    • not enough spindle force due to sealing unit leaking.
    • microswitch faulty.
    • PCB not sequencing, fan not working, ignition sequence (sparking) or faulty gas valve etc.
  8. No HW, CH ok.
    • Thermistor connections/terminals are dirty/loose.
    • flow switch stuck in off position or faulty.
    • thermistor open circuit/infinity caused by water ingress.
    • thermistor lost calibration.
    • thermostat contact points worn/burnt out/pitted.
    • thermocouple phial out of position.
  9. DHW not at temperature.
    • Sludge/scale build up in main heat exchanger often causing noise - kettling.
    • sludge/scale or detritous in dhw heat exchanger or thermostat.
    • incorrectly sized gas pipe.
    • water temperature potentiometer set too high.
  10. Hot water alternating hot and cold.
    Sludge/scale build up in domestic hot water heat exchanger.
  11. Reduced dhw flow rate.
    • Filter/strainer partially blocked by detritus.
    • cartridge partially blocked by detritus.
    • temporay low mains supply pressure.
Card Set:
2016-01-04 09:14:26
fault finding
Combi boiler fault finding
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