Sequence of Boiler Operation

What is the sequence of boiler operation for domestic boilers? This guide will take you through the boiler operations that happen when the boiler is being used.

Table of Contents

Combi boilers are highly efficient and convenient heating systems that provide both hot water and heating in a single unit

Understanding the sequence of operation for a combi boiler is essential for maintenance, troubleshooting, and repair purposes

Start-Up Procedure

  1. Ensure that all the electrical and water connections are properly made
  2. Check that the pressure gauge reading is within the recommended range (usually between 1 and 1.5 bar)
  3. Turn on the water and heating system isolating valves
  4. Turn on the electrical supply to the boiler
  5. Press the reset button (if any) and turn on the boiler using the control panel or thermostat
  6. Wait for the boiler to reach the desired temperature and pressure level

General Sequence of boiler operation

  1. Boiler in standby mode
  2. Demand called for via room thermostat/timer
  3. Circulator Pump runs – activating primary flow switch then micro switch from NO to NC
  4. The diverter valve remains in at “rest” position (central heating port open only)
  5. Printed circuit board sends voltage to the fan
  6. Boiler Fan runs ok
  7. Air pressure switch activates from C and NC position over to C and NO position (Sending signal back to the Printed circuit board)
  8. Overheat cut out thermostat is checked for a closed circuit (continuous circuit) Primary thermistor is also checked for correct thermal resistance
  9. Spark generator and ignition electrode(s) ok
  10. Gas valve energised and ignites ok (low rate)
  11. Flame detection electrode ok (sparking finishes – ionisation proved)
  12. Gas valve ramps up to range rated set pressure (Controlled via the thermistor and potentiometer at p.c.b)
  13. Boiler control thermostat or room thermostat is satisfied or the timer shuts down the boiler
  14. Boiler Fan and pump over run continue to dissipate residual heat (over run time will vary)
  15. Boiler returns to standby mode

Hot Water Mode – boiler operation

  1. Boiler in standby mode
  2. Demand called for activating hot water flow switch then micro switch above from NO to NC (paddle, turbine, wheel or pressure differential type)
  3. Pump runs-activating primary flow switch then micro switch above from No to NC
  4. Diverter valves internal hydraulics moves over from “rest” position (central heating port open) to domestic hot water position (port to secondary plate exchanger open only)

    Note: not all models incorporate a diverter valve and plate to plate heat exchanger, but instead utilise a twin pass main heat exchanger

  5. Printed circuit board sends voltage to the fan
  6. Boiler Fan runs ok
  7. Air pressure switch activates from C and NC position over to C and NO position (sending a signal back to the Printed circuit board)
  8. C/H and H/W overheat cut out thermostats are checked for a closed circuit (continuous circuit), primary and secondary thermistors are also checked for correct thermal resistance
  9. Spark generator and ignition electrode(s) ok
  10. Gas valve energised and ignites ok (ignition rate)
  11. Flame detection electrode ok (sparking finishes – ionisation proved)
  12. Gas valve modulates according to the water flow rate/ desired temperature setting (controlled via the thermistor and potentiometer at p.c.b)
  13. Boiler shuts down when hot water demand is satisfied
  14. Boiler Fan and pump overrun continue to dissipate residual heat (over run time will vary)
  15. Boiler returns to standby mode

Please note: The above sequence of operation for both hot water and central heating are not identical for every boiler out there, however, the sequence of events will not progress any further forward, if there are no feedback signals or if they are incorrect. See also our boiler servicing procedure page.

Boiler Main Components

  • Boiler - Check boiler manuals for details
  • Programmer
  • Ignition - Electronic Pulse, Direct Spark, etc.
  • Heat Exchanger - Single Pass or Crossflow.
  • Pump - Speed (single, two speed, three-speed), Modulating, Twin Pump, etc.
  • Diverter Valve - Diaphragm Pass, Wax Capsule, Electric, Venture.
    Sometimes called a 'mid-position valve' due to the mechanism of diverting hot water from the boiler to either the radiators or the hot water cylinder or both
  • Gas Valve - Zero Governor, Automatic, Intermittent or Permanent Pilot.
  • Calorifier - Cylinder and Coil or Plate.
  • Primary Flow Switch - Pressure differential, Diaphragm, Reed.
  • Secondary Flow Switch - Paddle Diaphragm, Diaphragm, Reed, Thermistor.

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