If you own a gas-fired furnace, the ignition system is critical for safe, efficient, and reliable operation. Each time your furnace turns on, it must ignite the natural gas that powers the burners and allows the unit to heat your home. Older units kept a tiny flame (known as a standing pilot light) continuously burning, but this straightforward approach created obvious efficiency issues.
Modern furnaces typically use hot surface igniter systems, but older furnaces that remain in service may use other methods. This guide will discuss the three most common systems found in relatively modern furnaces and a few repair issues you may experience as your furnace ages.
1. Hot Surface Igniters
You probably have a hot surface igniter if you've recently replaced your furnace. This incredibly simple ignition system works by heating a small piece of metal until it is glowing hot. As gas and oxygen move across the hot surface, the heat triggers ignition and your furnace lights. This system typically only contains the igniter and the wiring necessary to connect to the furnace control board.
Despite their simplicity, hot surface igniters can fail. Common issues include:
- Overheating and failure due to excessive gas pressure
- Contamination from soot due to inefficient furnace operation
- Electrical or control board issues
- Physical damage during replacement or other furnace repairs
Testing and repairing a hot surface igniter will require some electrical knowledge, so it's a job best left to a professional HVAC contractor.
2. Intermittent Pilot Light Igniters
Intermittent or electronic pilot lights are a slightly more modern version of the older standing pilot light. An intermittent pilot light is more efficient since it doesn't need to keep a pilot flame burning continuously. Instead, these units use an electronic spark to ignite the pilot flame when your thermostat demands heat. While uncommon in modern furnaces, older units may still use this design.
Unlike hot surface igniters, intermittent pilot lights have numerous components, leading to various possible failure points. These include:
- Faulty ignition control modules
- Dirty or broken flame sensors
- Clogged or damaged pilot gas tubes
- Dirty, misaligned, or damaged pilot nozzles
An incorrect repair of an intermittent pilot system can lead to gas leaks, so always rely on a qualified and licensed technician to diagnose and fix any problems.
3. Direct Spark Ignition
Intermittent pilot lights work by using a spark to ignite a pilot light, but why not use that spark to light your burners directly? This simplified design is precisely how direct spark ignition works. These systems are less common than hot surface igniters, but you can still find them on certain modern furnaces. Direct surface igniters are a more straightforward design, but they still have several failure points:
- Faulty spark control module, usually due to age
- Dirty or damaged spark head
- Wiring issues preventing the spark module from receiving adequate voltage
- Underlying control board failures
Most direct spark ignition problems can be complex to diagnose, and attempting to fix this problem alone may lead to unnecessarily replacing the expensive spark control module. Instead, always rely on a trained technician to perform a more thorough diagnosis to ensure you don't replace parts that still work.
For more information on furnace repair, contact an HVAC technician.
