CSLB C-20 HVAC ContractorHeating SystemsEasy
A new 95% AFUE furnace uses an electronic ignition system with no standing pilot. A technician notices a small ceramic rod glowing orange near the burners at the start of the heating cycle before gas flow begins. What ignition component is this?
- ASpark electrode
- BHot surface igniter
- CThermocouple
- DFlame rectification sensor
Show answer & explanationAnswer & explanation
Correct answer: B. Hot surface igniter
A hot surface igniter (HSI) is a silicon carbide or silicon nitride element that glows red/orange hot to ignite gas directly, eliminating the need for a standing pilot. Thermocouples only sense pilot flame heat, spark electrodes create an arc rather than glow, and flame sensors detect flame after ignition rather than igniting it.
Why the other options are wrong
- A. A spark electrode produces an arc, not a steady glow.
- C. A thermocouple senses heat to hold a gas valve open, it does not glow to ignite gas.
- D. A flame sensor detects an existing flame via microamp current, it does not ignite anything.
Hot Surface Igniter (HSI)
An electronic ignition device that heats a ceramic element to a high temperature to ignite the gas-air mixture directly, replacing standing pilots.
- Made of silicon carbide or silicon nitride
- Very fragile — avoid touching with bare skin (oils cause failure)
- Common on modern 80%+ and 90%+ AFUE furnaces
Memory trick: Hot rod glows, gas flows.