Description
General Sequence of Operation for Boiler Fault Finding
Page Contents
When fault finding on a gas boiler, sometimes it’s useful to know a general sequence of operation for either hot water or heating.
Central Heating Sequence of Operation
- Standby: Boiler is idle.
- Demand: Room thermostat or timer calls for heat.
- Pump Start: Circulator pump runs. Activates primary flow switch; micro switch changes from NO to NC.
- Diverter Valve: Stays in “rest” (central heating port open).
- Fan Activation: PCB sends voltage to fan. Fan starts.
- Air Pressure Switch: Switches from C/NC to C/NO, signalling PCB.
- Safety Checks:
• Overheat cut-out thermostat checked for continuity
• Primary thermistor checked for correct resistance - Ignition: Spark generator and ignition electrodes activate.
- Gas Valve: Energised. Burner ignites at low rate.
- Flame Detection: Flame detection electrode proves ionisation (ends sparking).
- Modulation: Gas valve ramps up to set pressure (controlled by thermistor and PCB potentiometer).
- Satisfaction: Boiler or room thermostat, or timer, satisfies demand and shuts down.
- Overrun: Fan and pump run to dissipate residual heat (duration varies).
- Standby: Boiler returns to idle.
Notes: Important notes about this general boiler sequence of operation
- Sequence may vary between models.
- Operation will not proceed if feedback signals are missing or incorrect.
DHW Sequence of Operation
- Standby: Boiler is idle.
- Demand: Hot water flow switch and micro switch (NO to NC) triggered (may be paddle, turbine, wheel, or pressure type).
- Pump Start: Activates primary flow switch; micro switch changes from NO to NC.
- Diverter Valve: The hydraulic valve moves from “rest” (CH open) to DHW (secondary plate heat exchanger open).
Note: Some models use a twin-pass main heat exchanger instead. - Fan Activation: PCB sends voltage to fan. Fan starts.
- Air Pressure Switch: Switches from C/NC to C/NO, signalling PCB.
- Safety Checks:
• CH and HW overheat thermostats checked for continuity
• Primary/secondary thermistors checked for correct resistance - Ignition: Spark generator and ignition electrodes activate.
- Gas Valve: Energised. Burner ignites at ignition rate.
- Flame Detection: Flame detection electrode proves ionisation (ends sparking).
- Modulation: Gas valve modulates based on water flow rate and target temperature (controlled by thermistor and PCB potentiometer).
- Satisfaction: Boiler shuts down when hot water demand ends.
- Overrun: Fan and pump run to dissipate residual heat (duration varies).
- Standby: Boiler returns to idle.
Notes: Important notes about this general boiler sequence of operation
- Sequence may vary between models.
- Operation will not proceed if feedback signals are missing or incorrect.
If you’re in Lincolnshire and you’d like to book one of our engineers for your boiler repair, you can use any of the methods below:
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