Domestic Hot Water Faults

When the hot water is ok but the central heating is not, this eliminates all operating sequence possible faults such as the fan, air pressure switch, pump, ignition sequence, ignition detection, gas valve and domestic hot water thermistor.

Therefore, the fault can only be at the:

No domestic hot water

Domestic hot water too hot

Domestic hot water not hot enough

  • Blocked primary and/or secondary heat exchanger (scale and sludge)
  • Flow restrictor not fitted (as recommended by manufacturer)
  • Cold mains pressure/flow rate too high
  • Faulty diverter valve (diverter may be letting by through central heating port)
  • Faulty thermistor or scaled
  • Burner pressure too low
  • Undersized gas installation pipe work
  • Faulty gas valve
  • Cylinder thermostat out of calibration
  • Faulty pump (not at full capacity, dissipating the heat)
  • Hot water inlet strainer (if fitted) blocked
  • Blocked primary filter on plate exchanger (not applicable to all boilers)

Should the temperature differential between the flow and return thermistor be too great, the boiler will modulate down and even cut out.

A classic example is when the hot water temperature fluctuates!

Central heating fault finding:

When the central heating is ok but the hot water is not, this eliminates all operating sequence possible faults such as the fan, air pressure switch, pump, ignition sequence, ignition detection, gas valve and central heating thermistor.

Therefore, the fault can only be at the:

No Central Heating

  • Split diaphragm on flow switch (pin not moving)
  • Faulty micro switch (burnt out contacts)
  • Faulty motorised valve head (burnt out windings)
  • Diverter valve (faulty wax capsule or bobbin assembly)
  • Faulty programmer (faulty switching contacts)
  • Faulty room thermostat stuck in the open (satisfied) position
  • Faulty zone valve (seized, burnt motor, end switch)
  • Faulty mid-position valve (seized at port A)
  • Severely blocked heat exchanger (scale and sludge)
  • Faulty printed circuit board (central heating side)
  • Faulty thermistor (open circuit / infinity)
  • Faulty high limit cut out thermostat (open circuit / infinity)
  • Faulty pump (not at full capacity)

Central Heating Too Hot

  • Room thermostat set too high or out of calibration
  • Room thermostat stuck in closed (calling) position
  • Faulty thermistor and high limit cut out
  • Burner pressure too high
  • Faulty gas valve

Central Heating Not Hot Enough

  • Blocked heat exchanger (scale and sludge)
  • Blocked radiators / pipe work (scale and sludge)
  • Return manifold filter blocked (debris)
  • Faulty diverter valve
  • Faulty thermistor or dirty
  • Burner pressure too low
  • Faulty gas valve
  • Room thermostat out of calibration
  • Range rating potentiometer set too low for heat load
  • Dip switches on printed circuit board not at maximum temperature
  • Faulty pump

Other Issues to check

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