Diagnosing A/C Systems : Inadequate Evaporator Airflow

 Overview:

• Inadequate airflow across the evaporator coil can be caused by a dirty or iced evaporator, a clogged air filter, an obstructed return register, or a dirty blower wheel. This issue affects the system’s ability to absorb heat, leading to various symptoms.

Effects of Inadequate Airflow in Systems with a Fixed Bore Metering Device:

1. Evaporator Temperature:

   • Condition: Decreases.
   • Reason: Reduced airflow leads to less heat being absorbed, lowering the pressure and temperature in the evaporator.

2. Superheat:

   • Condition: Decreases, potentially to zero.
   • Reason: With less heat absorbed, the refrigerant may not fully vaporize, causing liquid refrigerant to exit the evaporator and potentially flood back to the compressor.

3. Condensing Temperature:

   • Condition: Decreases.
   • Reason: Less heat is transferred to the refrigerant, leading to lower high side pressure and condensing temperature.

4. Subcooling:

   • Condition: Remains relatively normal.
   • Reason: The amount of refrigerant in the condenser remains adequate, so subcooling does not significantly change.

Example: Dirty or Iced Evaporator on a 10-SEER Air Conditioning System with R22 (Fixed Bore Metering Device):

• Normal Operating Conditions:

  • Low side pressure: 69 PSIG → 40°F evaporating temperature.
  • High side pressure: 278 PSIG → 125°F condensing temperature.
  • Ambient air: 95°F, resulting in a 30°F condenser split.
  • Subcooling: Approximately 10°F.

• Inadequate Airflow Conditions:

  • Evaporator Side:

    • Low side pressure decreases to 43 PSIG → 20°F evaporating temperature.
    • Evaporator temperature difference (TD) increases to 60°F (80°F return air - 20°F evaporator temperature).
    • Superheat decreases to 0°F, indicating liquid refrigerant leaving the evaporator and entering the suction line, potentially causing compressor damage.

  • Condensing Side:

    • High side pressure decreases to 242 PSIG → 115°F condensing temperature.
    • Condenser split decreases to 20°F (115°F condensing temperature - 95°F ambient air).
    • Subcooling remains relatively normal at around 10°F.

Effects of Inadequate Airflow in Systems with a Thermostatic Expansion Valve (TEV):

1. Evaporator Temperature:

   • Condition: Decreases.
   • Reason: Similar to fixed bore systems, reduced airflow lowers the heat absorbed, decreasing evaporator temperature.

2. Superheat:

   • Condition: Remains normal.
   • Reason: The TEV adjusts refrigerant flow to maintain consistent superheat, even when airflow is reduced.

3. Condensing Temperature:

   • Condition: Decreases.
   • Reason: Less heat is absorbed in the evaporator, leading to lower condensing temperature and pressure.

4. Subcooling:

   • Condition: Remains relatively normal.
   • Reason: The refrigerant in the condenser remains adequate, so subcooling does not significantly change.

Example: Dirty or Iced Evaporator on a 10-SEER Air Conditioning System with R22 (TEV System):

• Normal Operating Conditions:

  • Low side pressure: 69 PSIG → 40°F evaporating temperature.
  • High side pressure: 278 PSIG → 125°F condensing temperature.
  • Ambient air: 95°F, resulting in a 30°F condenser split.
  • Subcooling: Approximately 10°F.

• Inadequate Airflow Conditions:

  • Evaporator Side:

    • Low side pressure decreases to 43 PSIG → 20°F evaporating temperature.
    • Evaporator temperature difference (TD) increases to 60°F (80°F return air - 20°F evaporator temperature).
    • Superheat remains normal at 10°F, thanks to the TEV’s ability to modulate refrigerant flow.

  • Condensing Side:

    • High side pressure decreases to 242 PSIG → 115°F condensing temperature.
    • Condenser split decreases to 20°F.
    • Subcooling remains normal at around 10°F.

Key Takeaways:

• Low Evaporator Temperature and Superheat: Indicators of inadequate airflow, causing the evaporator to ice over or starve.
• Low Condensing Temperature: Results from reduced heat absorption, leading to lower high side pressure.
• Subcooling: Remains relatively unaffected, making it less useful for diagnosing airflow issues.
• TEV Systems: Maintain normal superheat even under reduced airflow, but other symptoms like low evaporator temperature and condensing temperature still apply.

Action Steps:

• Check for Airflow Obstructions: Start by inspecting and cleaning the air filter, checking for any obstructions in return registers, and ensuring the blower wheel is clean.
• Monitor for Ice: If the evaporator coil is iced over, allow it to thaw and check for underlying airflow issues before restarting the system.