Boiler Fault Finder

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: Diverter valve Micro switch Flow switch Central heating thermistor High limit cut out thermostat Printed circuit board External controls (room thermostat and timer) No domestic hot water Split diaphragm on flow switch (pin not moving) Blocked pressure differential pipes (scale deposits) Faulty micro switch (burnt out contacts) Faulty motorised valve head (burnt out windings) Diverter valve (diverter may be stuck in central heating mode, spindle seized, dhw micro switch stuck in NO position) Programmer (faulty switching contacts) Loose connection / faulty cylinder thermostat Faulty zone valve (seized, burnt motor, end switch) Faulty mid-position valve (seized at port B) Severely blocked plate exchanger (scale and sludge) Faulty printed circuit board (hot water side) Faulty thermistor (open circuit / infinity) Faulty high limit cut out thermostat (open circuit / infinity) Faulty pump (not at full capacity) Domestic hot water too hot Cylinder thermostat set too high or out of calibration Cylinder thermostat stuck in closed (calling) position Cylinder thermostat is loose Faulty thermistor and high limit cut out Burner pressure too high Faulty gas valve 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: Diverter valve micro switch flow switch domestic hot water thermistor high limit cut-out thermostat domestic hot water heat exchanger printed circuit board 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…

This content is for Annual Subscription and Annual Subscription Special members only.

Please login to view this content. (or Register here.)