Diagnosing A/C Systems : Refrigerant Overcharge

 Overview:

• Overcharging a refrigerant system leads to increased pressure and temperature, which can negatively impact system performance. The effects vary depending on whether the system uses a fixed bore metering device or a thermostatic expansion valve (TEV).

Effects of Refrigerant Overcharge in Systems with a Fixed Bore Metering Device:

1. Condensing Temperature:

   • Condition: Increases above normal.
   • Reason: Excess refrigerant takes up space in the condenser, raising pressure and condensing temperature.

2. Subcooling:

   • Condition: Increases above normal.
   • Reason: More liquid refrigerant is available for sensible cooling, increasing subcooling.

3. Evaporator Temperature:

   • Condition: Increases.
   • Reason: Higher liquid line pressure forces more refrigerant into the evaporator, raising evaporating temperature.

4. Superheat:

   • Condition: Decreases, possibly no superheat at all.
   • Reason: Excess refrigerant causes flooding in the evaporator, leading to little or no superheating of the refrigerant.

Example: Overcharge 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.

• Overcharge 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 to 25°F (145°F condensing temperature - 120°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 and potential compressor damage.

Effects of Refrigerant Overcharge in Systems with a Thermostatic Expansion Valve (TEV):

1. Condensing Temperature:

   • Condition: Increases.
   • Reason: Excess refrigerant takes up space in the condenser, raising pressure and condensing temperature.

2. Subcooling:

   • Condition: Increases.
   • Reason: TEV backs up refrigerant in the condenser, allowing more liquid to subcool.

3. Evaporator Temperature:

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

4. Superheat:

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

Example: Overcharge on a 10-SEER Air Conditioning System with R22 (TEV):

• 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 to 25°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:

• Increased Condensing Temperature and Subcooling: Common indicators of refrigerant overcharge.
• Fixed Bore Metering Device: Overcharge causes high evaporator temperatures and low or zero superheat, risking compressor damage.
• TEV Systems: Although the evaporator side may operate normally, overcharge still results in higher compressor amp draw and reduced system efficiency due to increased high side pressure.