Ignition Systems: Standing Pilot, Intermittent Pilot, Hot Surface and Direct Spark
What the exam tests
Pilot and direct ignition types, flame sensing, ignition timings, and the electrical differences between standing pilot, IPI, HSI, and DSI.
Core rules
- Standing pilot: a thermocouple or pilot generator holds the safety valve open. Thermocouples produce about 25–30 mV; the circuit must have clean connections and correct pilot-flame coverage.
- Intermittent pilot (IPI): spark ignites a pilot, then the main valve opens after the pilot proves. It uses flame rectification or a pilot generator.
- Hot-surface ignition (HSI): a silicon-carbide or nitride igniter heats to 1,800–2,500°F, then the valve opens. It draws several amps at 120 V or is 24 V on some boards; it is fragile and should not be touched with bare hands.
- Direct spark (DSI): high voltage sparks directly at the main burner; flame is proven by rectification.
- Flame rectification needs a stable ground, correct polarity, and a clean sensor. Typical minimum is about 0.5–1.0 µA DC; below that, the board locks out.
Worked example
An HSI furnace ignites, burns for 5 seconds, then drops out and retries. The igniter glows and the valve opens, so ignition is present. Measure flame current: 0.3 µA. Clean the sensor and ground; if current rises to 2 µA, the board holds. The fault was a dirty flame sensor, not the gas valve.
Traps
Do not test an HSI with an ohmmeter alone; cold resistance can look normal while the element fails hot. Do not assume a spark means flame proof. Reversed polarity or a poor ground will cause rectification lockout even with a good flame.