Today’s units react to unsafe conditions in less than a second. This is accomplished through the use of “computerized control modules” and a flame sensing control system known as flame rectification.
The safety used in today’s furnaces that accomplishes flame rectification is the flame sensor. A flame sensor is nothing more than a stainless steel rod partially encased in porcelain. By itself, it does nothing. There are no moving parts, no switches.
When attached to a control module, the module sends out an A/C voltage to the sensor. This voltage is always present at the sensor. The reason voltage is always present is because the sensor is always looking for flame, even if there is no call for heat.
If it sensed flame without a call for heat (ie. – gas valve stuck open), the control would see this and make sure the blower and inducer turned on or stayed on to prevent the furnace from becoming a little Chernobyl (complete melt down) in the home.
When there is a call for heat and the burners ignite, flame hits this sensor creating a “path to ground through the flame” for the A/C voltage. When this happens, a pulsating D/C signal, in the form of micro amps goes back to the control and the module decides if the signal is strong enough to allow the unit to continue operating. Because this system is “electronic”, it reacts to unsafe ignition problems in fractions of seconds instead of minutes.
As stated above, the flame sensor does not contain any moving or functional parts. The flame sensor is energized with AC voltage, which is rectified to DC in the presence of flame. The DC current flow is measurable in micro amps (uA). A minimum micro amp measurement is required to continue the heating cycle. On gas furnaces, 3.7 uA is a typical measurement and measurements less than .5 uA result in interruption of the heating cycle.
The micro amp measurement must be read by placing a meter set to DC Amps in series with the flame sensor and the control board.
One of the most common causes of flame failure is the sensor. Keep in mind that the control is sending out an A/C voltage signal. If the sensor becomes dirty or corroded, it is like covering that stainless steel probe with insulation and the current can not pass to the flame (ground) and create the D/C signal back to the board. Or, if the wire from the board to the sensor is cracked or the porcelain is cracked, the voltage “bleeds” off to ground at the point of the “leak” changing the micro amp signal to the board and not allowing it to lock in.
If the flame signal is not adequate for flame proof, check the following:
- A dedicated ground wire must be connected to the furnace
- All wiring connections must be tight
- Proper line voltage polarity.
- Manifold pressure must be correct per the furnace rating plate
- The flame sensor must be in the flame.
- The flame sensor must be clean and free of oxidation. If necessary to clean, use extra fine steel wool. Do not use sand paper or anything course as this can create “grooves” on the sensor which will cause it to become contaminated quicker. If corrosion or contamination is severe enough that cleaning with steel wool will not improve the sensors performance, the flame sensor must be replaced.
- Porcelain on the flame sensor must be intact. If the ceramic insulator is cracked, the flame sensor must be replaced.
- Check to see if there is voltage at the sensor. Without a call for heat, there should be voltage (approximately 90 to 110 volts) between the sensor and ground. The sensor is always looking for flame and is always energized. If no voltage output is found, check the wiring to the board or replace the board if no voltage output is present.
- Flame sensors replaced due to contaminants in the combustion air or cracked ceramic are not covered under most manufacturer’s warranties.
Since the flame sensor is the primary safety for ignition, the signal should be check every time a unit is cleaned and serviced. If the signal is getting weak, the sensor should be cleaned. Cleaning it is a fix if the signal does not reach the nominal micro amps required by the control. Keep in mind that this is the primary safety in the furnace. It is important to make sure that it functions properly.