# Flame Rectification Explained: The Truth About How Your Furnace Proves Flame
Key Takeaways
- Flame Acts as Conductor: The flame doesn’t generate the signal – it conducts AC voltage from the flame sensor to ground, creating a DC microamp signal through rectification
- 10:1 Ground Reference Rule: The grounded burner surface area must be at least 10 times larger than the flame sensor for proper rectification to occur
- Microamp Testing: Use a meter with microamp (μA) function connected in series between flame sensor and control board to verify proper flame signal
- Common Failures: Most flame failures stem from poor grounding, loose connections, or faulty ignition controls – not the flame sensor itself
The Detective Story of Flame Rectification
A jittery pilot flame sits in a damp, poorly lit mechanical room. The only source of light is a low-hanging fixture swaying back and forth due to a noisy ventilation system pulling in air from a filthy return air grill.
The room smells of cigarette smoke, stale coffee, and deceit.
In walks a gruff-looking character who appears to be visibly upset with the situation.
“So,” he says… “I heard you’ve been running your mouth again, telling all the good HVACers of the world that you are solely responsible for creating the signal required to prove flame in the heating cycle?”
The flame develops a facade of anger and blurts out, “You can’t prove a thing. Go eat another donut, pig!”
“Okay, have it your way… Hey Sarge, bring up the hose, would yah!”
The Common Misconception About Flame Sensors
There’s a widespread misconception in our industry that the flame itself generates the signal needed to prove flame and energize the main valve of the gas valve. While the flame does play an integral role, it doesn’t actually generate the signal – it merely changes an AC current to a DC current through the process of flame rectification.
Back when I was in class for Gas Tech 1 training, my instructor would repeat often that the ground reference used for flame rectification must be, at minimum, ten times greater than the flame sensor itself for the process to work. While I’m not sure how scientifically precise that number is, in general terms, it does play a significant role in proper flame sensing operation.
Understanding the Flame Rectification Process
Let’s examine the process of flame rectification as it relates to gas-fired appliances, which is fundamental knowledge whether you’re working on residential furnaces or [troubleshooting commercial heating systems](https://hvacknowitall.com/blog/the-september-sweet-spot-commercial-hvac-maintenance).
The Ignition Sequence
On a call for heat, providing that all safety switches and interlocks are in the correct position, the ignition control will receive power. The controller will then send a spark to the burner assembly and energize the gas valve (pilot valve).
Upon ignition, many technicians believe the flame generates a DC microamp signal and sends it back to the ignition control. This is wrong – well, kind of.
The Science Behind Flame Conduction
Before we continue, we must understand that the flame acts as a conductor, just like a wire. But how does this happen?
Within the flame, there are free-moving ions. These ions in the flame allow current to pass through the flame just like current passes through a wire. This is the key to understanding flame rectification.
The Complete Circuit Path
So let’s rewind and look at what actually happens:
- After the spark has been created, the ignition control sends an AC voltage to the flame sensor (usually less than 100 volts)
- That voltage passes through the flame and makes its way to ground
- The burners, providing they are grounded properly, have a much larger surface area than the flame sensor
- Due to this size difference, the current moves in one direction toward the grounded burners
- During this process, there is a small pulsating DC-offset current that results (flame rectification)
- That small DC current, also known as microamps, is what the ignition control uses to verify flame presence
If the flame drops out, the path to ground disappears, and so does the microamp signal.
Rectification – The process in which AC current is converted to DC current.
Troubleshooting Flame Failures
Most flame failure issues have nothing to do with a faulty flame sensor. Remember, it’s just a wire acting as a probe. The real culprits are typically:
- Poor grounding of the burner assembly
- Loose connections at the flame sensor or control board
- Faulty ignition control board
- Contaminated flame sensor (carbon buildup)
Keep in mind that while a dirty flame sensor will cause ignition failure issues and should be kept clean, it’s rarely the root cause of chronic flame failures. Just as with [proper heat load calculations](https://hvacknowitall.com/blog/hvac-design-heat-loads-precision-versus-accuracy), accurate diagnosis requires understanding the complete system.
Testing Microamp Signal
To check for a microamp signal, you must use a meter that has a microamp function. It will be labeled μA on the meter selector switch, similar to the Testo 770-3 shown in our header image.
Testing Procedure:
- Set your meter to microamps (μA)
- Attach one meter lead to the flame sensor
- Attach the other meter lead to the flame sensor terminal on the ignition control
- Your meter leads should be connected in series with the circuit
- During ignition, the microamp signal will be displayed on your meter screen
The correct microamp reading will differ depending on the manufacturer – typically ranging from 0.5 to 10 microamps. I strongly suggest you contact the manufacturer for the correct specifications for your specific model.
Check out this quick video on how to check a flame signal:
For further explanation, check out instructor12b’s YouTube video below and subscribe to his channel:
Quick Recap and Best Practices
To quickly recap the flame rectification process:
- The ignition control board provides an AC voltage to the flame sensor
- The flame becomes a conductor allowing current to flow toward the burners
- During the process, the result is a pulsating DC-offset current (microamps)
- This current can be read with a meter connected in series with the circuit
- This process is known as flame rectification
Maintenance Best Practices
- Keep flame sensors clean using fine steel wool or emery cloth
- Ensure burners are properly grounded
- Check all electrical connections during routine maintenance
- Verify microamp readings match manufacturer specifications
Understanding flame rectification is just one aspect of comprehensive heating system diagnostics. Similar attention to detail is required when working with [heat pump reversing valves](https://hvacknowitall.com/blog/heat-pump-reversing-valves-explained-how-they-work-in-hvac-systems) or [building management systems](https://hvacknowitall.com/blog/bms-network-architecture-communication).
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Resources and Further Learning
Check out my [YouTube channel](https://www.youtube.com/channel/UC-MsPg9zbyneDX2qurAqoNQ?view_as=subscriber) for more tips, tricks, and troubleshooting videos, and check out The HVAC Know It All [podcast here](https://hvacknowitall.com/podcasts) or on your favorite podcast app.
Remember, mastering flame rectification troubleshooting is essential whether you’re maintaining [residential heating systems](https://hvacknowitall.com/blog/the-september-sweet-residential-spot-hvac-maintenance) or working on complex commercial equipment. Keep learning, stay curious, and always prioritize safety when working with gas-fired appliances.
