The burner is the heart of gas-fired boilers. When it fails, production stops. Here is a technical breakdown of common ignition failures.
The Ignition Sequence Explained
Understanding the sequence helps locate the fault:
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Fan On: Pre-purge to clear gases.
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Ignition Spark: Transformer creates a spark.
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Pilot Valve Open: Gas released to pilot.
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Flame Detected: Sensor sees pilot flame.
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Main Valve Open: Full firing.
Common Failure Points
1. Pre-purge Failure (Fan won't start)
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Check: Motor contactor and fuses.
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Check: Air Pressure Switch. This safety device confirms airflow. If the fan is dirty or flue is blocked, it won't close, and the burner will lockout.
2. Spark but No Flame
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Check: Gas supply ball valve. Is it open?
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Check: Ignition electrode gap. Should be 3-5mm. If too wide, spark is weak.
3. Flame Starts then Trips
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Check: Flame Sensor (UV or Ionization). If the sensor is dirty (soot), it cannot 'see' the flame. Clean it with a soft cloth.
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Check: Grounding. Most sensors rely on the frame ground to complete the circuit.
Expert Tip: Count the seconds before lockout. If it's immediate, it's an electrical interlock. If it's after 3-5 seconds, it's likely a flame detection issue.