Flame Rectification

The Process of Flame Rectification

1.  The board puts out 40VAC to 120VAC (manufacturer dependent) on the flame rod wire. The flame rod is always energized when the furnace board is powered.
   • Honeywell Smartvalves are ~170 to 180VAC
   • Honeywell S8610U3009 are ~170VAC that will produce ~1 to 2 µADC
2. The fuel ignites and flame is present.
3. The 40VAC to 120VAC passes current through the flame to complete the circuit by grounding to the burners
   • This "grounding" of the circuit makes a good ground essential!
   • The ions in the flame only allow half (+ ions) of the AC sine wave to pass through the flame. This is called "Rectification". This process is the changing of the AC signal into a rectified DC signal. 
4. Once the flame is present and this rectification circuit completes, then power flows through the circuit and is measured in Microamps DC (µADC)
5. The furnace board will constantly monitor this µADC signal to verify a safe operation during the heating run cycle.
   • A typical flame rod circuit will carry between 1 to 6 µADC
   • Most furnace boards need a minimum of 0.5 µADC for the furnace to stay running

Measuring Flame Rectification

A µADC measurement can be made during furnace operation by....

• ALL Brands

Placing your multi-meter in the µADC setting and putting the meter leads in series with the flame rod circuit. Look for a stable 1 to 6 µADC reading 

• JCI Products

JCI offers a quick way of testing the flame rectification. On their boards, they have 2 small test pads (labeled "+" and "-") where you can put your meter leads. The board circuitry does a conversion for you and changes the µADC signal into a VDC signal. This is helpful for those multi-meters that don't have the capability to measure µADC. Its is a 1 to 1 conversion.

1 µADC = 1 VDC 

2.1 µADC = 2.1 VDC

3.8 µADC = 3.8 VDC

Common Flame Rectification Issues

Due to such a small electrical amp flow, there are multiple different reasons why the board fails to prove flame. Here are some of the most common things to look for...

• Must have a dedicated ground wire all the way back to the electrical panel - (good earth ground)
• All ground wires tight with good connections (very low ohm reading between the burners back to neutral)
• Dedicated ground wire from the burner box to the board must be tight with good connections
• Burners tight and making good connections to the sheet metal
• Rust on the face of the burners can be an insulator (high resistance) stopping the path of electrons
• 120VAC to the board with correct polarity. (see article on polarity)
  • Neutral (white) to Hot (black) = 120VAC
  • Ground (green or bare wire) to Hot (black) = 120VAC
  • Neutral (white) to Ground (green or bare wire) = less than 1.0 VAC
  • Dedicated circuit back to panel - (no electrical interference for other appliances)
  • 120VAC circuit not running next to florescence ballasts, etc... (EMF issues)
• Manifold gas pressure at the correct pressure per the manufacture - (Fuel/Air mixture verification on negative venturi systems)
• The flame must be impinging (hitting and engulfing) the flame sensor
• Board output to the flame sensor must be 40VAC to 120VAC (manufacturer dependent)
• The flame sensor must be clean and free of any oxidation, silica or carbon (insulates and causes high resistance). Clean flame sensor with something that won't leave a residue, like steel wool.
  • Cleaning of the flame sensor is a regular maintenance procedure
• The porcelain on the flame sensor must not be cracked (provides an incorrect path to ground)
• Poor burner alignment not allowing flame to transfer across all the burners
• Plugged or dirty crossover channels on burners 
• The flame sensor must have low resistance and can be tested with a multi-meter in the ohms setting like in this illustration...

• If you have tested and are certain that all-the-above are correct, then by process of elimination, a bad board is left.
  • NOTE: make sure you are using your tools correctly, they are functioning properly and that they are correctly calibrated - Misdiagnoses' can be costly!

Environmental Issues

It is also very noteworthy... If you are cleaning a flame sensor more than just during regular annual maintenance, then there could be some environmental issues going on in the mechanical room (CAZ).

• 1 Pipe systems (exhaust only) can draw in chemicals or particles into the combustion process and be coating a flame sensor. It is recommended to run a dedicated intake pipe (2 pipe system)
• Check for chemicals in the combustion area:
  • Paint/Thinner
  • Hair spray/Aerosols
  • Water softener salts
  • Dryer sheets
  • Baby powder
  • Drywall dust
  • Chlorine bleach
  • Etc...