Component	Function
Burner	Mixes gas and air to generate a flame that heats the system
Fan	Draws combustion air in and pushes flue gases safely out
Air Pressure Switch	Proves fan operation before ignition, ensuring safe combustion
Ignition Electrodes	Spark or hot surface ignites the gas/air mixture
Primary Heat Exchanger	Transfers burner heat to the primary circuit water (for heating or DHW indirectly)
DHW Plate Heat Exchanger	Transfers heat from primary water to incoming mains cold water for instant hot water
Diverter Valve	Switches flow between central heating and DHW circuit
Pump	Circulates water around the central heating system
Expansion Vessel	Absorbs pressure increases from thermal expansion in the sealed system
Pressure Relief Valve (PRV)	Releases pressure if it exceeds safe levels
Temperature Sensors (NTC)	Monitor water temps for burner modulation and safety
Flow Sensor / Hall Sensor	Detects when a hot tap is opened, triggering DHW mode
Printed Circuit Board (PCB)	The brain of the boiler – controls ignition, safety, temperature, pump, and diverter
Condensate Trap / Pipe	Collects and drains acidic condensate from flue gases (in condensing boilers)
Flue Assembly	Safely expels combustion gases and draws in fresh air (balanced flue)

🔁 Sequence of Operation – Heating Mode

Here’s what happens when the room thermostat or programmer requests heating:

Call for Heat

• Room thermostat closes the circuit or sends a signal to the PCB.
• PCB initiates pre-ignition checks.

Fan Activation

• Fan starts to draw air in and purge the combustion chamber.
• Air pressure switch confirms fan operation.

Ignition Sequence

• PCB sends a signal to open the gas valve.
• Ignition electrodes create a spark.
• Burner ignites under control of the flame rectification circuit (flame sensor confirms flame).

Primary Heat Exchanger Heats Water

• Burner flame heats the water in the primary heat exchanger.
• Circulator pump begins moving heated water through the central heating circuit (radiators).

Temperature Control and Modulation

• NTC thermistors monitor the flow and return temperatures.
• The PCB modulates the gas valve and fan to maintain setpoint temperature efficiently.

Room Temperature Reached

• Thermostat or programmer breaks the circuit.
• Boiler enters post-purge cycle: fan runs briefly, pump overruns to dissipate heat.
• Burner shuts down.

---

🚿 Sequence of Operation – Domestic Hot Water Mode

When a hot tap or shower is opened:

Tap Opened / Flow Detected

• Flow sensor detects movement of cold mains water.
• Signal sent to PCB to switch to DHW mode.

Diverter Valve Activated

• Diverter valve shifts from CH mode to DHW mode.
• Flow is diverted through the plate heat exchanger.

Ignition and Burner Operation

• Fan starts, air pressure switch confirms operation.
• Burner ignites via electrodes.
• Flame heats the primary circuit water.

Heat Transfer in Plate Heat Exchanger

• Cold mains water flows through the secondary side of the plate heat exchanger.
• Heat is transferred from primary water to domestic water instantly.
• Hot water flows to the tap at the target outlet temperature.

Modulation and Flow Regulation

• Temperature sensors monitor DHW outlet temp.
• Gas valve modulates to maintain stable hot water (often ~55–60°C).
• If tap closes: flow stops → burner shuts down → pump/fan overrun.

---

🌡️ Modulation and Efficiency

Modern combi boilers feature modulating gas valves (often 1:6 or 1:10 ratio) meaning they adjust their flame output based on demand — not simply ON or OFF. This ensures:

• Higher efficiency
• Reduced gas consumption
• Lower wear on components

Boilers with weather compensation or load compensation can further fine-tune operation to match conditions outside or inside the home, further saving energy.

---

⚠️ Key Safety Systems Inside a Combi Boiler