Direct Spark Ignition
Direct spark ignitors have no standing pilot lights. When the thermostat calls for heat, a pilot flame lights up and a sensor detects the flame, which opens a gas valve to release the gas needed to ignite the spark from the ignitor. If the sensor doesn’t detect a pilot flame, the gas valve shuts off. If the sensor doesn’t detect a flame, it signals the gas valve to shut off. If you try this three times without the furnace turning on, the direct spark ignition locks out the ignition for at least an hour to prevent the buildup of gas.
Intermittent Pilot Ignition
The Intermittent pilot ignition has a pilot light that lights only when the thermostat calls for heat. After the pilot light is lit, the intermittent pilot ignition circuit board detects the flame and opens the gas valve so main burners in the furnace can ignite and produce heat. If the intermittent pilot ignitor doesn’t detect a pilot flame, it tries four times to light the furnace burner before locking out the ignitor for a period of time.
Hot Surface Ignition
Hot surface ignition sources glow red hot when the thermostat calls for heat. After the call for heat, the circuit board sends voltage to the ignitor so it can turn on and glow. After the ignitor gets hot enough, the sensor calls for the gas valve to open and supply the gas, which ignites the furnace. There is no pilot flame associated with a hot surface ignition.
The mercury bulb system is not used much today because of the environmental dangers associated with mercury. If your furnace has a mercury bulb ignitor, the switch on the mercury bulb senses the pilot light when the thermostat calls for heat and sparks to ignite the burner.