Ignition Systems & Flame Safety Controls
Thermocouples, thermopiles, flame rectification, hot surface igniters, intermittent pilot, direct spark ignition, gas valves, and flame sensor diagnostics.
- Explain thermocouple and thermopile operation and output values
- Describe intermittent pilot, direct spark, and hot surface ignition systems
- Explain flame rectification and its use in modern gas appliances
- Identify gas valve types including combination controls and modulating valves
- Perform flame sensor diagnostics including microamp measurement
Lesson 1
Standing Pilots, Thermocouples & Thermopiles
Standing Pilot Systems
A standing pilot burns continuously to ignite the main burner when heat is called for. While being phased out in favor of electronic ignition, standing pilots are still found on older furnaces, water heaters, and gas fireplaces.
Thermocouples
A thermocouple is a safety device that senses the pilot flame and holds the gas valve open. It generates a small DC voltage when heated by the pilot flame:
If the pilot goes out, the thermocouple cools and voltage drops below the holding level, causing the gas valve to close within 30 seconds. This prevents unburned gas from accumulating.
Thermopiles
A thermopile is a series of thermocouples wired together to produce a higher voltage - typically 250-750 millivolts. Thermopiles provide enough power to operate the gas valve and even a thermostat without external electrical power. They are commonly found in gas fireplaces and millivolt-rated heating systems.
Exam Tip - Thermocouple vs Thermopile
A thermocouple produces 20-30 millivolts (just enough to hold a safety valve). A thermopile produces 250-750 millivolts (enough to power the gas valve and thermostat). The exam tests these voltage ranges frequently.
Thermocouples produce 20-30 mV and hold the pilot safety valve open. Thermopiles produce 250-750 mV and can power gas valves and thermostats. If the pilot goes out, the gas valve closes within 30 seconds.