Diagnosing A/C Systems : AC Troubleshooting Summary

 Key Takeaway: "Normal as a Problem"

• Scenario: A customer complains that the air conditioning unit isn't cooling adequately, but upon inspection, all indicators (evaporator temperature, temperature difference (TD), superheat, condensing temperature, and subcooling) are normal.
• Possible Issues:
  • Ductwork or Air Distribution Problems: If the refrigeration system is functioning normally, issues like poor duct design, unbalanced air distribution, or airflow obstructions could be causing inadequate cooling.
  • System Sizing: The system may not be appropriately sized for the space, especially if the load calculation was not accurate. Manual J load calculations are essential for verifying correct system size.
  • Unrealistic Customer Expectations: Customers may expect rapid cooling, but removing heat and humidity from a space takes time, particularly in extreme ambient conditions.

Example Diagnostic Form Review:

• Symptoms:

  • Return air temperature: 85°F
  • Supply air temperature: 55°F
  • Low side pressure: 43 PSIG → 20°F evaporating temperature
  • High side pressure: 243 PSIG → 115°F condensing temperature
  • Liquid line temperature: 105°F

• Analysis:

  • Evaporator Delta T: Only 10°F (should be closer to 20°F)
  • Evaporator TD: 65°F (normal is around 35°F)
  • Superheat: 40°F (indicates a starved coil)
  • Condenser Split: 20°F (normal is around 30°F)
  • Subcooling: Normal at 10°F
  • Findings: The evaporator coil is likely starved, which could be due to a variety of issues, such as a restriction, low charge, or airflow problems.

Key Diagnostic Indicators:

1. Evaporator Temperature Difference (TD):

   • Normal TD: 30-40°F below the return air temperature.
   • High TD: More than 40°F, indicating a possible undercharge or restriction.
   • Low TD: Less than 30°F, potentially due to high load or an oversized system.

2. Superheat and Subcooling:

   • Superheat:
     • High superheat indicates a starved evaporator.
     • Low or zero superheat indicates potential flooding.
   • Subcooling:
     • Normal subcooling suggests proper charge.
     • High subcooling may indicate an overcharge or restricted flow.
     • Low subcooling can suggest a low charge.

3. Condenser Split and Condensing Temperatures:

   • Condenser Split:
     • Normal: 25-35°F difference between condensing temperature and ambient air temperature.
     • High Split: Indicates high condensing temperature, potentially due to a dirty coil or restricted airflow.
     • Low Split: Indicates low condensing temperature, possibly due to low ambient conditions or undercharging.
   • Condensing Temperature:
     • High condensing temperature suggests the system is working harder to reject heat, possibly due to a dirty condenser or high ambient temperatures.
     • Low condensing temperature might indicate low ambient conditions or low refrigerant levels.

System Performance Context:

• Manual J Load Calculation: Essential for verifying correct system size, especially in cases of persistent cooling issues.
• Customer Communication: Educate customers on realistic expectations for system performance, particularly during high ambient conditions or when the system has just been turned on.

Additional Tips:

• Sight Glass Usage: Be cautious when using a sight glass for diagnostics, as it can be misleading. Bubbles in the sight glass can be caused by low load conditions or high-temperature glide in newer refrigerants, not necessarily by a low charge.
• Evaporator Icing: Always investigate airflow issues first if the evaporator is iced over. Low airflow is a common cause of icing, and correcting airflow can often resolve the problem.