cfm Tech Tips – Troubleshooting Furnace Control Boards — Faults & Flashes

Topics

  • Furnace Control Boards – Faults 7 & 8 – Ignition Failure Fault Codes
  • Furnace Control Boards – Rapid Amber Flash – Low Flame Current

Models

  • All Furnaces with an Integrated Furnace Control Board
Beginning January 1, 1992, all furnaces were manufactured with an integrated furnace control board, often referred to as FCB (Furnace Control Board). The FCB manages all operation requirements of the furnace. Fault and Information codes are displayed by one or more LEDs on the board. The FCBs used by York/JCI have the ability to store and retain the last 5 codes in ‘non-volatile memory’ – meaning that loss of power to the control doesn’t erase the stored codes – a very helpful tool when troubleshooting furnaces. This month’s topic is Ignition Failure Faults and Troubleshooting Tips.

Ignition Failure Fault Codes:

7 Red Flashes – Flame could not be sensed after 5 tries for ignition

8 Red Flashes – Flame has been sensed, then lost 5 times during a call for heat

Rapid Amber Flash – Flame Sense Current is low

All York/JCI residential furnaces now utilize Flame Rectification as a means of determining if all burners have ignited and the flame is burning correctly. Furnaces also utilize ‘Hot Surface Ignition’, to ignite the gas entering the furnace through the burners.

Sequence of Operation for Proving Flame

  1. Thermostat calls for heat
  2. Inducer starts and FCB confirms operation with pressure switch
  3. Hot Surface Igniter (HSI) is energized by FCB
  4. There is a 30 second ‘pre-purge’ period before HSI is energized
  5. There is a 17 second ‘igniter warm-up’ period before Gas Valve is energized
  6. Gas Valve is energized by FCB
    1. There is a 7 second ‘trial for ignition’ period
  7. Gas is ignited by HSI
  8. FCB senses flame
    1. There is a 2 second ‘flame stabilization’ period to ensure flames stay lit
  9. 30 seconds after ‘proof of flame’, blower starts on heating speed

Flame Rectification

Flame rectification is a very accurate method of proving flame and ensuring that burner operation is safe and complete. Below is a description of how it works.

  • The FCB has a flame sense circuit that is connected to the Flame Sense Rod.
  • The Flame Sensor is a Kanthal (Aluminized Metal) Rod inserted into the dark blue part of the flame. It is insulated from the case of the furnace with a ceramic insulator. Attached to the ceramic insulator is a mounting bracket to hold the flame sensor in the correct location.
  • 120 Volts AC is sent to the flame rod by the FCB through a high impedance circuit.
  • A good flame acts as a conductor.
  • When flame is present and in contact with both the flame sense rod and the burners, DC current flows from the flame sensor through the burners to the case of the furnace; then through the case of the furnace back to the FCB. There must be a ground wire connecting the FCB to the case of the furnace. The FCB reads the DC Current – in microamps – and determines that flames are present.
  • After a trial for ignition period (about 7 seconds) and a flame stabilization period after ignition (about 2 seconds), flame sense is certified by the FCB.
  • 30 seconds after flame sense, the FCB energizes the blower and the heat cycle is continued until the thermostat is satisfied or flame is lost.
  • The process of converting AC Volts into DC Current – the result of circuit design and the presence of flames – is called Flame Rectification.

Important Notes

  1. The FCB will not accept AC Current as a method of sensing flame.
    1. So, a jumper can’t be used to fool the FCB into thinking flames are present when they’re not.
  2. The high impedance circuit in the FCB protects a technician from electric shock if he/she touches the flame sensor when its energized.
  3. The Flame Sense Circuit is energized anytime there is power to the FCB. This is so the FCB can detect flames if they are present without a call for heat.
  4. Touching the flame rod while flames are being sensed by the FCB will create a pathway to ground through your hand to the case of the furnace. Although you may not feel it, you have just caused a short circuit and flame sense will be lost (if flame is present), resulting in the FCB shutting down the burners.
  5. The Hot Surface Igniter is located on the First Burner and the Flame Sensor is located on the Last Burner – so if the Flame Sense Circuit can prove flame, it indicates that all burners are lit, and flames have carried over completely from first to last burner.
  6. A Digital Volt Meter can be used to check for AC Volts on the flame sense rod.
    1. Digital Meters are typically ‘high impedance’ devices, but still draw some current. This current draw will result in a voltage drop on the flame sensor. Thus, the 120 volts applied will display between 60 and 85 volts on the meter.
  7. Current York FCBs have a ‘Flame Pad’ to check flame rectification current with a DC Voltmeter. 1 DC Volt is equal to 1 Microamp, thus checking flame current is very simple.
  8. Current York FCBs will display a Rapid Amber Flash if flame sense current falls below 1.5 microamps – a warning that the Flame Sensor may need to be cleaned.

Red Code 7

If the FCB can’t sense the presence of flame after the trial for ignition period, it shuts off the gas valve and records a ‘no flame present’ in the processor. The inducer continues to operate for a period of 60 seconds to purge unburned gasses from the heat exchanger and a ‘retry’ for ignition is initiated. If flame cannot be sensed after 4 retries (5 total trials for ignition), the FCB shuts down the entire process and flashes a Red Code 7, indicating that flame could not be sensed.

Troubleshooting Red Code 7

Requirements for ignition and flame sense to occur:

  • There must be AC Volts on the Flame Sense Rod
    • Check voltage on Flame Sensor with a Digital Voltmeter
  • There must be gas/air mixture in contact with the igniter
    • Check Inlet Gas Pressure during the Trial for Ignition period
    • Check Manifold Gas Pressure during the Trial for Ignition period
    • Check Burners for cleanliness and/or corrosion
  • The igniter must be hot enough to ignite the gas/air mixture
    • Igniter information
      • Silicone Carbide = 45 to 400 Ohms
      • Silicone Nitride = 40 to 75 Ohms
  • Flames must carry over from First to Last burner within the Trial for Ignition period
    • Check burner alignment and cleanliness

Summary

  • Igniter glows, gas valve opens, gas/air mixture ignites, flames carry over to flame sensor, flame is proven. If this doesn’t occur after 5 tries – A lockout occurs, and a Red Code 7 is flashed.
  • Divide and Conquer to determine at what point in the process the problem is located.

Red Code 8

If flame sense is certified by the FCB then lost, a ‘loss of flame’ is recorded by the processor and a retry is initiated. If flame is lost after 4 retries (5 total loss of flame occurrences) the FCB shuts down the entire process and flashes a Red Code 8, indicating loss of flame after flame is sensed.

Troubleshooting Red Code 8

Reminder – For a Red Code 8 to be flashed, the Requirements for Ignition and Flame Sense have already taken place. This is very helpful for a technician – the FCB has already applied ‘divide and conquer’ between failure to prove flame and loss of flame after proof. We already know flame was sensed, we just need to discover why flame sense is being lost.

  • Check manifold pressure to determine if gas is being lost after ignition occurs.
  • Check inlet pressure to determine if low manifold pressure is related to low inlet pressure.
  • Check flame rectification current by using the Touch Pads on the control board.
    • If Touch Pads aren’t present – use the ‘Microamp’ setting on your meter and insert it in series with the flame sense circuit.
    • Typical flame sense current is above 3 microamps.
    • Flame sense below 1 microamp could result in loss of flame.

Rapid Amber Flash Code

Current FCBs keep track of flame current when flame is present and being sensed. If flame sense current falls below 1.5 microamps, the FCB will alert the technician to check the flame sense circuit. A Rapid Amber Flash Code is usually the result of a dirty flame sense rod.

Reasons

Combustion air entering the burners can contain chemicals, dust, and other unwanted components. When burned, these can become attached to the flame sense rod in the form of organo-silica. A buildup of this on the rod causes an insulating effect on the surface of the flame rod, which must be cleaned off periodically. Organo-silica is relatively soft and easy to clean. Cleaning the flame sensor should be a routine part of ‘clean and check’ procedures.

Correctly Cleaning a Flame Sensor

  • Do Not use Sandpaper or Emory Cloth
    • Sandpaper and Emory Cloth contain grit which will scratch the surface of the flame rod and leave fines of sand in the groves. When heated, this turns into hard silica which is very difficult to clean and is located down in the grove where it’s unreachable. Using Sandpaper or Emory Cloth can permanently damage a flame sensor.
  • Do use Steel Wool or something that won’t scratch the flame rod
    • Steel Wool is softer than the flame rod, but harder than the organo-silica. The coating is removed, and the flame sensor is not damaged.
    • Some Technicians prefer to use a dollar bill or the back side (cloth side) of the Emory Cloth which is fibrous and doesn’t scratch the flame rod.

Other Notes

  • Incorrect voltage can result in loss of flame sense or low flame sense current
    • A correctly grounded furnace is essential for successful flame sensing. A ground conductor must be present between the furnace and the load center.
    • Check voltage between Neutral and Ground with the blower operating. If voltage is greater than 1.5 volts, check the Neutral Conductor for loose connections.
    • Refer to Tech Tips 2 for more information on checking voltages.

Summary

  • Using good service practices will eliminate most Flame Sense Problems.
  • Using common sense and good troubleshooting practices will result in easy diagnosis and servicing of furnaces with Flame Sense Problems

Coming next month: Fault Codes 4 and 11 – Primary Limit Switch Faults.

 

Paul Flora
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Paul Flora

Service and Training Manager at cfm Distributors, Inc.
Paul has been with cfm Distributors, Inc. over 20 years and is currently the Service and Training Manager.His experience in the HVAC industry provides a high level of technical experience and knowledge for our customers.
Paul Flora
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