Combination Boiler Fault Finding

Symptom: No heating

Procedure:

Check demand - Room stat

Check taps - dripping

Check thermistors

Check safety / low pressure

Check diverter valve

Check electrics

Check pump

Check rads

Symptom: No hot water (Heating ok)

Procedure:

Turn tap on (water ok) try other taps too

Flow sensor (check)

Check preheat (if it has one)

PCB faulty Pre-lim checks

Gas rate/Burner pressure

Check diverter valve (check microswitch)

Pump should be ok

Fan should be ok

Burner should be ok

Check heat sensing thermistor (resistance)

High flame comes on (modureg) - visual

Symptom: Neither heating nor hot water

Procedure:

Any power to boiler (Powercut/fuse)

Any gas to boiler (supply to meter)

Does boiler fire up?

Potentiometer fault (no call for demand)

Flow switch faulty

Diverter valve faulty (microswitch not working)

Pump faulty (not running)

Overheat stat tripped (reset)

Pump, heat exchanger blocked/ faulty thermistor

Fan (not running - air pressure switch)

Faulty PCB

Flame rectification / spark electrodes

Faulty gas valve (solenoid not opening to allow gas through)

Symptom: User unhappy with system poor performance

Procedure:

Check performance (Flow cup/Heat Rise)

Check inlet gas pressure

Burner pressure/Gas rate

Check thermostats, potentiometers, thermistors

Fan speed (modulating boilers only)

Pump failure

Scaled up heat exchanger & plate heat exchanger

Faulty gas valve

Faulty modulating solenoid on valve

Possible controls/PCB fault