The flame sensor exists to prove a flame across the burners. Without this flame sensor, there’s a risk of releasing gas without flame and risking an explosion upon ignition. The flame sensor is positioned on the opposite end of the burners from the igniter to ensure all burners ignite correctly.
Upon a call for heat, before the burners ignite, the furnace control board will send AC voltage to the flame sensor. This amount of voltage varies from manufacturer to manufacturer, but it’s usually between 80v – 100v. This voltage goes through a wire from the control board, to the wire that connects to the flame rod. You can think of it like the electrons are building up at the tip of the flame rod just waiting for a path so they can keep moving.
Since there is no flame upon a call for heat, there’s no path between the flame rod and the burners. The board needs to confirm the safety is in working condition and reading no flame before gas is released.
If you do sense flame before ignition, you may get an error similar to, “flame sensed without gas valve energized.”
Once flames are present, the board has seconds to recognize it. You have 80 – 100v AC from the board to the flame sensor. This current can travel from the flame rod through the flame due to flame ionization. The flame bridges the gap between the burners and the flame rod.
When we burned fuel, it releases electrons from the molecules in the fuel, and this lets the flame act as a path for current to move from the rod, through the flame, and to the burners grounding surface. Since the furnace burners have a much larger surface area than the flame rod, the current cannot backtrack to the rod and is rectified (hence the name flame rectification) to DC (direct current) which only travels in the one direction.
You measure the flow of current passing through the flame sensor in DC microamps. The control board will monitor the DC microamp through a ground wire usually located on the burners, back panel, or on the gas valve. You’re commonly looking for a reading of between 1 – 4 microamps.
Even without a call for heat, the board is still checking the flame sensor for safety at all times, so you will always have voltage present at this sensor.
If your appliance is failing on flame signal, here’s a few things to check:
- Clean any buildup on the flame sensor. As this flame sensor sits in the flame, it becomes dirty with oxidation or carbon buildup. You can clean this with a piece of sandcloth but take it easy on it. The flame sensor doesn’t need to sparkle
- Ensure your ceramic insulator is not cracked causing the AC voltage to bypass the flame sensor and go to ground without rectification
- Check to make sure your grounds are secured, clean, and you have a proper 120v dedicated ground
- Check to make sure you have the correct polarity of 120v hot and neutral
- Check to see if the flame sensor is sitting in the flame correctly
- Check to make sure gas pressure is suitable for the unit
- Check to see if you have AC voltage going to the flame sensor or that is correct. You may need to call technical support for this
How to measure flame rectification
- With the appliance off, connect your multimeter in series with the flame sensor. In series means you are making a connection point in line with the flame rod. Disconnect the wiring lead from the flame sensor. Connect one side of your meter leads with an alligator clip to the flame sensor. Connect your other multimeter lead to the wire removed from the flame sensor. The other end of that wire connects to the control board or ignition board
- Make sure your multimeter is set to microamps