Flame sensors, whether they are used in a home furnace or on a large industrial boiler, indicate the presence of a flame and are part of the safety circuit. If the flames go out and the furnace or boiler continue to feed the unit with fuel, a catastrophic explosion could occur. The flame sensor sends an electrical signal to a central controller. This central controller operates the gas or fuel valves for releasing the combustible materials into the burning chamber of the furnace or boiler. Upon an initial signal, the flame sensor is placed on a timer. During this "ignition period" the reading from the flame sensor is ignored until a large flame can be "seen" by the sensor. Once the timer has elapsed, the signal from the flame sensor is then constantly read as to monitor the burning of the combustible materials.
Metal Strips or Optical
There are two basic types of flame sensors: a bi metallic strip and an optical reader. Both of these sensors will output a small voltage in the presence of a flame. The bi metallic strip will output a very small voltage in the form of a mille volt, read as ".001 volts" or "1mv." When the tube enclosed strips come in contact with a heat source a voltage is generated. The greater the heat, the more voltage will be generated by these two electrically opposing pieces of metal. Generally the bi metallic strip type of flame sensors is the most common flame sensor used on home furnaces. These flame sensors are relatively inexpensive and extremely accurate for detecting the flame. Since bi metallic strips output such a small voltage they must be interconnected with some type of amplifier to enhance the voltage so the controller can read the flame. Optical readers are generally used on large industrial furnaces and boilers. These type of flame sensors can "see" the flame and generate a much larger voltage. This voltage is in the range of 2 to 5 volts direct current (DC). Optical readers use a photocell, that when exposed to ultraviolet light will actually generate a voltage from the light source. The greater the intensity of the light, the larger the voltage generated. Optical flame sensors are very sensitive to any dust or debris that can accumulate in the burning chamber. The sensor window must have a full exposure to the flame or else it cannot "see" the intensity and thus will not generate a voltage to send to the controller.
Most home furnaces also employ a pilot flame sensor as well. Designed and used in much the same fashion as the larger flame sensor, the pilot sensor monitors the presence of the pre-ignition pilot flame. The pilot flame is used to ignite the larger furnace main flame for heating. Older furnace units may employ a constant pilot flame for the ignition process. Newer furnace units now use a real time pilot mechanism which generates a spark to ignite the furnace flame. In these types of units, a pilot sensor is no longer employed and the main flame is the only one recorded with the bi metallic sensor.