How to Light a Furnace–A Step-by-Step Guide

Do you know how to light a furnace pilot light if it goes out? A pilot light is an essential component of some gas furnaces. It is responsible for igniting the burners at the beginning of heating cycles. Standing pilot lights are supposed to stay lit but occasionally go out due to airflow or gas supply issues. The furnace won’t heat up if the pilot goes out. Luckily, relighting the pilot is a simple process for homeowners to perform.

Most modern furnaces don’t include standing pilot lights. Instead, new furnaces use electronic ignitors to light the burners and initiate heating cycles. Like an extinguished pilot, a faulty ignitor prevents a furnace from heating up. Unlike pilots, electronic ignitors cannot be manually lit when they fail. However, a homeowner can take troubleshooting measures to determine if the ignitor is bad or if some other issue is causing the furnace not to blow hot air.

Standing pilot lights were standard in older furnaces but have been phased out because they waste fuel and are less safe and reliable than electronic ignition systems. Modern furnaces use electronic ignitors, which are more energy efficient and require less maintenance.

Furnaces manufactured before 2010 typically have standing pilot lights. A standing pilot consumes a steady fuel supply as the flame constantly burns, even when the furnace isn’t running. When the thermostat calls for heat, the pilot’s flame ignites the gas in the burners.

A pilot light system consists of a small burner, a gas valve, and a thermocouple. A thermocouple is a safety device that detects the presence of the pilot’s flame. The flame heats the thermocouple to produce an electrical current. The thermocouple transmits the current to the solenoid to keep the gas valve open. The current ceases if the flame goes out and the gas valve closes.

Most furnaces manufactured after 2010 have electronically controlled ignition systems. Unlike pilot lights that burn constantly, electronic ignitions turn on at the start of the heating cycle. When the thermostat initiates a heating cycle, a current flows through the ignitor to light the gas in the burners.

The two main types of electronic ignition systems are hot surface ignitors and intermittent pilots.

Intermittent pilots use a high-voltage spark to ignite the pilot. An intermittent pilot burns fuel and lights the burners much like a standing pilot, but an intermittent pilot only burns when lighting the burners at the beginning of the heating cycle.

A hot surface ignitor is a metal resistance heating element, similar to a lightbulb filament. Unlike an intermittent pilot, a hot surface ignitor doesn’t consume fuel. An electric current runs through the ignitor’s metal until it is hot enough to light the gas in the burners. The ignitor shuts off when the furnace flame sensor detects the burner’s flames. Some hot surface ignitors also act as the flame sensor.

After the power has been shut off, remove the access panel. Locate the main gas line, typically a dark metal pipe running into the furnace’s side. Follow the gas line to the furnace gas valve, a boxy component with a knob labeled “on,” “off,” and “pilot.” A knob or switch with only “on” and “off” labels indicates the furnace has an electronic ignition. See the section on electronic ignitions below for further instructions.

Locate a silver metal tube extending from the gas valve. This is the pilot tube, and it leads to the burners. Follow the line with your flashlight to the pilot nozzle at the end. Under regular operation, the pilot’s flame extends from the nozzle. If no flame is present, the pilot needs to be relit.

Step 2: Turn off the gas valve and wait five minutes.

Turn the gas valve’s knob to the “off” position. Wait five minutes for any residual gas to dissipate and for the furnace to cool before lighting the pilot. Accumulated gas is an explosion risk.

Step 3: Press or Hold Down the Reset Button

Automatic pilots automatically ignite by pressing the red reset button. Other pilots need to be manually ignited with a lighter or match. If the pilot doesn’t light automatically, hold the reset button or depress the valve knob, depending on the furnace model.

Step 4: Relight the pilot light

While pushing the reset button, bring the lighter or match to the end of the pilot nozzle to light it. Continue pressing the reset button for thirty seconds after the pilot lights to maintain gas flow and ensure the thermocouple heats up enough to open the main gas valve. When the pilot stays lit, gently release the reset button and turn the gas valve knob to “on.”

Step 5: Turn on your furnace and see if it works

Replace the access panel, and restore power to the furnace. If necessary, increase the thermostat’s temperature to get the furnace to kick on. If the burners don’t ignite, the thermocouple might not have heated up enough. Repeat the lighting procedure, holding the reset button for forty-five to sixty seconds after lighting the pilot.

The ignitor is located on this side behind the burners and looks like a thin, flat, rectangular metal. Lower the thermostat’s temperature to initiate a heating cycle when you restore power to the furnace. Switch the furnace on, and watch the ignitor. After the inducer motor starts, you will hear the gas valve click open and gas hiss out. At this point, the ignitor should begin to glow red. If it doesn’t, there is an issue with it.

Inspect the Hot Surface Ignitor

Cut power to the furnace and let it cool for several minutes. Disconnect the ignitor's wire, carefully remove it, and don’t touch the filament. Ignitors are incredibly fragile and can break under a minimal amount of force. Check if the ignitor is singed or cracked. Even a microscopic crack will cause an ignitor to fail.

Test the Hot Surface Ignitor

If you own a multimeter, you can test the voltage or continuity of the hot surface ignitor. Test the voltage while the furnace is running. Touch the multimeter’s probes to the ignitor’s wires. The reading should be 120 volts. The ignitor is bad if there is voltage, but the furnace isn't igniting. If there’s no voltage, another component is bad.

Check the ignitor’s resistance with the furnace's power turned off. Set the multimeter to the lowest ohms of resistance. Unplug the ignitor and touch probes to each terminal. The multimeter will read between 30 and 200 ohms if there is continuity. If there is no reading or only a change, the ignitor is bad.

Check the Control Board Code

Most electronic ignition furnaces have control boards that indicate the causes of furnace failures with LED lights. The lights on the control board blink to reveal failure codes that can be deciphered with a chart located in the owner’s manual, on the access panel, or inside the furnace. The control board is typically located in the lower blower compartment.

Try these additional troubleshooting tips if you can’t determine why your furnace isn’t igniting.

Furnaces use standing pilot lights or electronic ignition systems to light the burners. The furnace won't heat up if the pilot goes out or the ignitor fails. If an electronic ignition fails, it must be replaced to make the furnace operational. Several methods help you determine if the ignitor or something else is faulty, and the additional troubleshooting measures might be enough to get your furnace running again.