Ignition Systems
Standing pilot, intermittent pilot, hot surface and direct spark ignition, and how each proves flame.
Safety
What it is
An ignition system lights the main burner and proves that flame is present. If flame is not proven within the trial period, the gas valve must close. The proving method is what distinguishes the systems.
| System | How it lights | How flame is proven |
|---|---|---|
| Standing pilot | A pilot flame burns continuously | Thermocouple or thermopile generating millivolts |
| Intermittent pilot | A spark lights a pilot on each call | Flame rectification through the pilot |
| Hot surface ignition | A silicon carbide or silicon nitride igniter glows and lights the burner | Flame rectification through a flame sensor rod |
| Direct spark ignition | A spark lights the main burner directly | Flame rectification, sometimes through the spark electrode |
How it works
A thermocouple produces a small direct current voltage when heated, typically around 25 to 30 millivolts open circuit, which holds a magnetic valve open. Lose the pilot and the voltage falls, the magnet releases and gas shuts off.
Flame rectification uses the fact that a flame conducts current in one direction. The board sends alternating current through the sensor rod and the flame rectifies it to a small direct current, typically measured in microamps. No flame means no rectified current and the board shuts off gas.
Ground matters
What can go wrong
- Cracked or aged hot surface igniter that no longer reaches temperature.
- Oxide coating on a flame sensor reducing microamp signal.
- Weak thermocouple or a dirty pilot orifice on older equipment.
- Cracked spark electrode ceramic allowing the spark to jump to ground.
- Poor chassis ground or a loose burner assembly.
Safety
Gas and high voltage
Tools involved
- Multimeter with microamp and millivolt ranges
- Manometer for gas pressure
- Non abrasive pad for sensor cleaning
- Combustion analyzer
Local Code Notice
Symptoms
Practice & Mastery
- Ignition Systems Troubleshooting Scenarios2
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