What Senses That the Flames Are Lit on a Furnace?

Sensor Types

• Gas furnaces use a long and skinny thermocouple or rectifier diode as their flame sensor. A thermocouple emits a weak electric current when heated and keeps up the current flow for as long as it stays hot. A rectified diode flame sensor carries a weak potential electric charge. Hydrocarbon-fueled fire is somewhat electrically conductive because of ionized carbon in the flames. When gas or oil flames bathe a flame rectification sensor, they complete an electric circuit between the sensor and metal burner and a weak current flows between the sensor and burner for as long as the flames stay lit. Oil furnaces use an optical detector that emits a weak electric current when struck by the light emitted by the oil flame.

Pilot Detection

• In gas furnaces with pilot light ignition, the sensor rests in the pilot flame and emits a weak current so long as the pilot flame stays lit. The furnace gas valve controller will open and close the main gas valve in response to signals from the thermostat so long as the pilot flame sensor tells it the pilot flame is lit. If the sensor emits no current because the pilot light has gone out, the controller shuts off the gas to the pilot light and locks down the main burner, requiring a manual relighting of the pilot and a furnace reset.

Electronic Ignition

• In gas and oil furnaces with electronic ignition, the furnace goes through its ignition sequence. If the flame catches, a weak current from the flame sensor tells the furnace controller that a flame is present so it can keep feeding fuel to the burner until the thermostat calls for shutoff. If there’s no confirmation of flame from the sensor after a few seconds, the controller shut off the fuel, waits a bit and tries ignition a second or third time. But if there’s still no flame, the controller shuts off the fuel and locks down the furnace, requiring a manual reset.

Sensor Failure

• When a flame sensor fails to signal the presence of flame, your furnace won’t start. In gas furnaces, the flame sensor can collect combustion deposits that insulate the sensor so it cannot detect flame even when flames are present. In oil burners, soot and dirt can collect on the sensor window, blocking the light. A reasonably experienced home handyman can clean the flame sensor of deposits, and this may restore normal performance. If cleaning the sensor doesn’t work, it will have to be replaced. If flame sensor replacement doesn’t restore furnace operation, you have other problems.