Diagnosing A/C Systems : Condenser Operation and Subcooling

 Condenser Operation:

• Heat Rejection: The condenser rejects heat absorbed in the evaporator.

  • More Evaporator Heat: Higher condensing temperature.
  • Less Evaporator Heat: Lower condensing temperature.

• Additional Heat Sources:

  • Sensible Heat: From superheat in the evaporator and suction line.
  • Motor Heat: From the compressor.
  • Heat of Compression: Additional heat added by the compression process.

• Normal Condensing Temperatures:

  • Condenser Split: Equal to refrigerant condensing temperature minus ambient temperature.
  • Example: For a 10-SEER unit with R22 refrigerant:
    • Return air: 75°F.
    • Evaporating temperature: 40°F (from 69 PSIG).
    • Condenser split: 30°F (ambient air 95°F, condensing temperature 125°F).

Subcooling:

• Definition: The amount of cooling of the liquid refrigerant below its condensing temperature.

• Normal Range: 5 to 15°F, depending on the manufacturer.

• High Subcooling: Above 15°F.

  • Causes: Overcharge or non-condensables in the condenser coil.

• Low Subcooling: Below 5°F.

  • Causes: Low charge or undercharge in the system.

Diagnosing with Subcooling:

• High Subcooling:

  • Overcharge or presence of non-condensables.

• Low Subcooling:

  • Low refrigerant charge results in low pressure and low condensing temperatures.
  • Low load on the evaporator (e.g., low ambient temperature below 70°F without ambient control).

• Normal Subcooling:

  • Indicative of proper refrigerant charge and normal system operation.

Examples:

1. Low Charge:

   • Normal Conditions: 40°F evaporator temperature, 125°F condensing temperature.
   • Low Charge Symptoms:
     • Suction pressure: 43 PSIG (20°F evaporating temperature).
     • High side pressure: 210 PSIG (105°F condensing temperature).
     • Ambient air: 95°F, resulting in a 10°F condenser split (should be 20-30°F).
     • Subcooling: Zero or significantly lower than normal, indicating a low charge.

2. Iced Evaporator Coil:

   • Symptoms:
     • Suction pressure: 43 PSIG.
     • High side pressure: 210 PSIG.
     • Ambient air: 95°F, resulting in a 10°F condenser split.
     • Subcooling: Normal (10°F), distinguishing it from a low charge condition.

Key Points:

• Proper Airflow: Ensure filters, evaporators, and blower wheels are clean to maintain proper airflow.
• Condenser Split and Subcooling: Critical for diagnosing system performance.
• Comparing Subcooling and Superheat: Both measurements are useful, but subcooling is more effective with a TEV metering device.

Causes of High Condensing Temperatures:

1. Overcharge of Refrigerant:

   • Leads to increased pressures throughout the system.
   • Results in higher condensing and evaporating temperatures.

2. Non-Condensables in the System:

   • Air or other non-condensable gases take up space in the condenser, reducing its efficiency in rejecting heat.
   • Proper evacuation and dehydration of the system before charging are critical to avoid this issue.
   • Purge hoses carefully when adding refrigerant to prevent introducing air into the system.

3. Low Condenser Airflow:

   • Can be due to an inoperative condenser fan or a dirty condenser coil.
   • Obstructions around the condenser, such as hedges, shrubs, or poor installation locations (e.g., under decks), can also limit airflow.

4. High Evaporator Load:

   • Increased heat absorption in the evaporator leads to more heat that must be rejected by the condenser, raising the condensing temperature.

Diagnosing with Subcooling:

• High Subcooling: Often indicates an overcharge of refrigerant or the presence of non-condensables.
• Normal Subcooling: Generally seen with low condenser airflow or high evaporator loads, although a slight increase in subcooling (a few degrees) may occur.

Examples of High Condensing Temperatures:

1. Overcharge Example:

   • Normal Conditions:
     • Low side pressure: 69 PSIG → 40°F evaporating temperature.
     • High side pressure: 278 PSIG → 125°F condensing temperature.
   • Overcharge Conditions:
     • Low side pressure: 84 PSIG → 50°F evaporating temperature.
     • High side pressure: 360 PSIG → 145°F condensing temperature.
   • Condenser Split: 50°F (95°F ambient air, 145°F condensing temperature), which is well above the normal 35°F split.
   • Subcooling: 30°F (145°F condensing temperature - 115°F liquid line temperature), which is excessive compared to the normal range of 10-15°F.

2. Dirty Condenser Coil/Inoperative Condenser Fan Example:

   • Pressure and Temperature Increases:
     • Low side pressure: 84 PSIG → 50°F evaporating temperature.
     • High side pressure: 360 PSIG → 145°F condensing temperature.
   • Condenser Split: 50°F (similar to the overcharge scenario).
   • Subcooling: Approximately 15°F, which is within the normal range despite the high condensing temperature. Subcooling may slightly increase but remains close to normal.

Summary of Condensing Temperature Changes:

• Ambient Temperature Drop: Condensing temperature drops.
• Evaporator Load Drop: Condensing temperature drops (less heat absorbed in the evaporator, less heat to reject).
• Ambient Temperature Increase: Condensing temperature increases (split is maintained).
• Evaporator Load Increase: Condensing temperature increases (more heat absorbed, more heat to reject).
• Low Condenser Airflow: Condensing temperature increases.
• Overcharge of Refrigerant: Condensing temperature increases.

Key Takeaways:

• High condensing temperatures are often caused by overcharge, non-condensables, or airflow restrictions.
• Subcooling is a useful diagnostic tool, particularly in distinguishing between overcharge and airflow issues.
• Understanding the relationship between system conditions and condensing temperatures is critical for accurate diagnosis and effective troubleshooting.