Diagnosing A/C Systems : Non-Condensables

 Overview:

• Non-condensables, primarily air, can enter the system due to poor service practices. These gases do not condense and therefore take up space in the condenser, leading to increased pressures and inefficiencies.

Effects of Non-Condensables in Systems with a Fixed Bore Metering Device:

1. Condensing Temperature:

   • Condition: Increases significantly.
   • Reason: Air in the condenser reduces the effective surface area for heat rejection, causing condensing pressure and temperature to rise.

2. Subcooling:

   • Condition: Increases.
   • Reason: The significant difference between the high condensing temperature and the temperature at the liquid line outlet results in increased subcooling.

3. Evaporator Temperature:

   • Condition: Increases.
   • Reason: The higher condensing pressure forces more refrigerant into the evaporator, raising the evaporating temperature.

4. Superheat:

   • Condition: Decreases, possibly to zero.
   • Reason: The excess refrigerant entering the evaporator leads to insufficient heat absorption, causing liquid refrigerant to leave the evaporator and reducing superheat, potentially leading to liquid floodback.

Example: Non-Condensables in 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.

• Non-Condensables Conditions:

  • High side pressure increases to 360 PSIG → 145°F condensing temperature.
  • Condenser split increases to 50°F (145°F condensing temperature - 95°F ambient air).
  • Subcooling increases dramatically to 35°F (145°F condensing temperature - 110°F liquid line temperature).
  • Low side pressure increases to 102 PSIG → 60°F evaporating temperature.
  • Evaporator temperature difference (TD) decreases to 20°F (80°F return air - 60°F evaporator temperature).
  • Superheat drops to 0°F, indicating liquid refrigerant entering the suction line, risking compressor damage.

Effects of Non-Condensables in Systems with a Thermostatic Expansion Valve (TEV):

1. Condensing Temperature:

   • Condition: Increases, similar to fixed bore systems.
   • Reason: Air in the condenser reduces the effective surface area for heat rejection.

2. Subcooling:

   • Condition: Increases, similar to fixed bore systems.
   • Reason: The significant difference between the high condensing temperature and the temperature at the liquid line outlet leads to increased subcooling.

3. Evaporator Temperature:

   • Condition: Remains normal.
   • Reason: The TEV regulates the amount of refrigerant entering the evaporator, maintaining a consistent evaporating temperature.

4. Superheat:

   • Condition: Remains normal.
   • Reason: The TEV maintains consistent superheat by modulating refrigerant flow based on the evaporator load.

Example: Non-Condensables in 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.

• Non-Condensables Conditions:

  • High side pressure increases to 360 PSIG → 145°F condensing temperature.
  • Condenser split increases to 50°F.
  • Subcooling increases dramatically to 35°F.
  • Low side pressure remains relatively normal at 76 PSIG → 45°F evaporating temperature.
  • Evaporator TD remains normal at 35°F (80°F return air - 45°F evaporator temperature).
  • Superheat remains normal at 10°F (55°F suction line temperature - 45°F evaporating temperature).

Key Takeaways:

• High Condensing Temperature and Subcooling: Key indicators of non-condensables in the system.
• Fixed Bore Metering Device: Non-condensables cause higher evaporator temperatures, lower superheat, and potential compressor damage.
• TEV Systems: While the evaporator side remains stable, the increased high side pressure still leads to higher compressor amp draw and reduced system efficiency.
• Prevention: Use proper service practices, including using a micron gauge to achieve a minimum of 500 microns during evacuation, and always purge refrigerant hoses before charging the system.