Finding Duct Leaks

Understanding Intermittent Pilot Ignition Failures

Modern gas furnaces are surprisingly uniform. Most newer furnaces use direct spark ignition or hot surface ignition controlled directly from a single, integrated control board. However, ignition systems on older furnaces were often more varied. While very old furnaces often used standing pilot lights, slightly newer models relied on intermittent pilot ignition. Many of these furnaces are still in service.

If you have an older furnace with an intermittent pilot ignition system, it's a good idea to understand how it works, how it can fail, and what's necessary to get your heat working again.

Intermittent vs. Standing Pilot Explained

A standing pilot light is an incredibly simple way to power a gas-fired appliance. A natural gas flame is self-sustaining if it has sufficient fuel and air to continue burning. However, the flame in your furnace still requires a heat source to start the combustion process. Standing pilot lights provided this source by keeping a small flame burning at all times.

Of course, keeping a flame burning all the time is inefficient and potentially dangerous. Intermittent pilot ignition systems address these issues by briefly running the pilot light long enough to ignite the main burners. These systems typically use a small, electronically controlled spark rod for ignition. Once the flame rectification process proves ignition, the pilot light shuts off.

These systems consist of four essential components:

  • Pilot nozzle
  • Igniter
  • Thermocouple
  • Control module

Common Intermittent Pilot Problems

An intermittent pilot system overcomes many downsides of a standing pilot light but does so by adding complexity. As with a standing pilot light, problems can develop with the nozzle, gas supply, or thermocouple. Clogs or contamination may prevent the nozzle from supplying adequate gas, while a dirty or damaged thermocouple may kill the gas supply after ignition.

However, intermittent systems add a few additional points of failure. For example, the spark igniter can become dirty, wear out, or fail, preventing it from producing the arc necessary to ignite the pilot light. More commonly, the ignition control module can suffer from an internal failure. This module supplies a spark and provides flame rectification so a failure will stop your furnace from lighting.

Diagnosing an intermittent pilot system isn't hard, but it requires knowledge of how they work and the tools and experience to test electrical connections. If your intermittent pilot furnace refuses to light, you shouldn't hesitate to contact a trained HVAC technician. An expert with experience working on older furnaces will be able to locate the problem quickly and restore your home's heat. 

For more info about furnace repair, contact a local professional.