Furnace Pilot Light: How It Works, Why It Goes Out, and How to Relight It
A furnace pilot light is a small, continuously burning flame that ignites the main burner in older gas furnaces. If your pilot light goes out, your furnace will not produce heat. In most cases, relighting it is a safe, straightforward process that homeowners can do themselves in under 10 minutes.
What Is a Furnace Pilot Light?
A furnace pilot light is a small flame, typically blue, that serves as the ignition source for the main gas burner in a standing-pilot gas furnace. When your thermostat calls for heat, the pilot light ignites the burner, which then heats the air distributed throughout your home.
Standing pilot lights are found almost exclusively in older furnaces (generally manufactured before the mid-1990s). Modern furnaces use electronic ignition systems—either intermittent pilot igniters or hot surface igniters—that only activate when heat is needed, making them more energy-efficient.
How Does a Furnace Pilot Light Work?
The pilot light works in tandem with a safety device called a thermocouple. Here's how the system functions:
- The pilot light burns continuously, keeping the thermocouple warm.
- The thermocouple generates a small electrical signal as long as it detects heat from the pilot flame.
- This signal holds the gas valve open, allowing gas to flow to the main burner when the furnace calls for heat.
- If the pilot light goes out, the thermocouple cools, the signal stops, and the gas valve closes automatically—preventing unburned gas from accumulating in your home.
This safety mechanism is why you must hold the reset button while relighting: it manually overrides the valve until the thermocouple warms up enough to take over.
Why Does a Furnace Pilot Light Keep Going Out?
A pilot light that won't stay lit is usually caused by one of several common issues:
- Dirty or faulty thermocouple: The most frequent cause. A buildup of grime or a worn thermocouple may fail to detect the flame reliably.
- Weak or misaligned pilot flame: If the flame is too small or positioned away from the thermocouple tip, the thermocouple won't generate a strong enough signal.
- Draft or air current: Vents, gaps in the furnace cabinet, or a nearby door can blow out the flame.
- Low gas pressure: Insufficient gas supply can cause the flame to sputter and extinguish.
- Blocked pilot orifice: Dust or debris can partially clog the small opening that feeds gas to the pilot.
When to call a professional: If your pilot light goes out repeatedly—even after relighting and cleaning—the thermocouple likely needs replacement. This is an inexpensive furnace repair typically costing $80–$150 when performed by a licensed HVAC technician1.
How to Relight a Furnace Pilot Light: Step-by-Step
Before you begin: Locate your furnace's instruction label, usually printed on the inside of the furnace door or cabinet. Always follow manufacturer-specific instructions where available. If you smell a strong gas odor at any point, do not attempt to relight the pilot—evacuate your home and call your gas utility provider immediately.
What you'll need:
- A long lighter or fireplace matches
- A flashlight (optional)
Steps to relight your furnace pilot light:
- Turn the thermostat down to its lowest setting to prevent the furnace from calling for heat during the process.
- Locate the pilot light assembly near the base of the furnace burner. There will be a control knob with settings typically labeled "On," "Off," and "Pilot."
- Turn the control knob to "Off" and wait at least 5 minutes. This allows any residual gas to dissipate before you introduce a flame.
- Turn the knob to "Pilot."
- Press and hold the reset button (or press the control knob inward, depending on your model). This manually opens the gas valve to the pilot.
- While holding the reset button, bring your lit lighter or match to the pilot opening and ignite the flame.
- Continue holding the reset button for 30–60 seconds after the flame ignites. This gives the thermocouple time to heat up and generate the signal needed to keep the gas valve open.
- Slowly release the button. If the flame stays lit, the pilot light has been successfully relit.
- Turn the control knob to "On" and restore your thermostat to your desired temperature.
If the flame goes out when you release the button, repeat the process, holding the reset button for a full 60 seconds on the next attempt. If it fails after two or three tries, the thermocouple may need cleaning or replacement.
What Color Should a Furnace Pilot Light Be?
A healthy furnace pilot light should burn with a steady blue flame, sometimes with a small yellow tip. A strong, stable blue flame indicates proper gas combustion and correct positioning against the thermocouple.
- Yellow or orange flame: May indicate incomplete combustion or a dirty pilot orifice. This can sometimes produce low levels of carbon monoxide—have your furnace inspected if the flame is consistently yellow.
- Flickering or weak flame: Often caused by a partially blocked orifice or low gas pressure.
- No flame: The pilot light is out and needs to be relit.
Furnace Pilot Light vs. Electronic Ignition: What's the Difference?
| Feature | Standing Pilot Light | Electronic Ignition |
|---|---|---|
| Common in | Furnaces pre-1995 | Furnaces post-1995 |
| Flame burns | Continuously | Only when heating |
| Energy use | Higher (constant gas burn) | Only when heating |
| Reliability | Higher (constant gas burn) | More efficient but more parts |
| DIY relight | Yes | No (requires professional diagnosis) |
Standing pilot furnaces are simpler mechanically, but their continuous gas consumption makes them less energy-efficient than modern electronic ignition systems. If your furnace is more than 20–25 years old, replacing it with a high-efficiency model may reduce your heating costs significantly.
1https://www.hvac.com/expert-advice/furnace-thermocouple-replacement/About The Author: Travis Baugh is a Digital Brand Marketing Manager for Bryant, where he creates clear, helpful content to guide homeowners through heating, cooling, and indoor air quality decisions. His goal is to empower readers with the knowledge they need to choose the right comfort solutions for their home—confidently and comfortably.
