What Does a Gas Furnace Igniter Do?

Igniter Systems

• Most gas furnaces employ hot surface ignition. In this system, a ceramic igniter is heated to yellow-hot incandescence by an electric current supplied by the control board. When the igniter reaches the correct temperature, the control board opens the gas valve to the main burner. The blazing hot igniter element lights the gas. Some furnaces use spark ignition where the main burner is ignited by a high-voltage electric spark that jumps from the igniter element directly to the main burner or to a pilot burner that then lights the main burner. In both systems, a flame sensor probe confirms that the burner flame has caught on.

Igniter Doesn't Glow

• Hot surface igniters heat to 1,800 degrees Fahrenheit and cool off again repeatedly throughout the day. The thermal stress can cause tiny cracks in the ceramic heating element that can interrupt the flow of electricity. This means that the igniter won’t glow. Sometimes a white area will develop around the crack. If your visual inspection of the igniter doesn’t reveal any cracks, you can use an ohmmeter to check your igniter. Turn off the power to the furnace and apply the ohmmeter test leads to the igniter’s wires. If the meter reads 45 to 95 ohms, the igniter is good and you have trouble elsewhere. If the reading is more than 110 ohms, the igniter is bad and you replace it.

Igniter Doesn't Spark

• If your furnace is equipped with a spark igniter that fails to spark, there are several things to check. A spark gap that’s too large or too small may not produce a spark hot enough to ignite the gas. The spark gap should be set to manufacturer specifications, typically between 1/8 inch and 1/4 inch. A spark igniter electrode fouled with combustion products won’t produce a hot spark, so you will need to clean the electrode with a bristle brush.

Cracked Insulator

• If the ceramic insulator holding the spark igniter electrode is cracked, you must replace the electrode. Check the wiring to the electrode for fraying or terminal corrosion. Replace the wire or clean the connections. Make sure there’s power to the furnace and that all access panels are closed. Check to see if the spark generator is getting the 24 volts that it needs. If there’s power to the spark generator and everything else checks out, the spark generator probably has gone bad and should be replaced.