Diagnosing A/C Systems : Superheat

 Superheat Overview:

• Definition: Superheat is the amount of sensible heat added to the vapor after it is fully vaporized within the evaporator coil.
• Normal Range: Typically, evaporator superheat ranges from 5 to 15 degrees, with an average of around 10 degrees.
• Measurement: Measure the suction line temperature at the evaporator outlet and subtract the evaporator temperature from that suction line temperature.

High Superheat:

• Indication: High superheat indicates a starved evaporator coil (superheat over 15 degrees).
• Causes:
  • High heat load causing refrigerant to boil too quickly.
  • Undercharge or leak in the system.
  • Restriction at the metering device or filter dryer.
• Example: A 10 SEER unit with a high heat load may show:
  • Low side pressure: 84 PSIG.
  • Evaporator temperature: 50°F.
  • Suction line temperature: 80°F, resulting in 30°F of superheat.

Low Superheat:

• Indication: Low superheat indicates a flooded evaporator (superheat under 5 degrees).
• Causes:
  • Low evaporator airflow (e.g., dirty filter, dirty blower wheel).
  • High condenser pressure pushing more refrigerant into the evaporator.
  • Overcharge of refrigerant or non-condensables in the system.
• Example: An overcharged system with a piston-type metering device may show:
  • Evaporator temperature: 60°F.
  • Suction line temperature: 60°F, resulting in no superheat, indicating liquid floodback.

Fixed Bore Metering Devices:

• Characteristics:
  • Cannot regulate refrigerant flow.
  • Fixed orifice designed for a specific pressure drop based on system application.
• Impact of Head Pressure Changes: As head pressure rises, evaporator pressure and temperature also rise, and vice versa.

Flooded Evaporator Example:

• Overcharge Scenario:
  • Low side pressure: 102 PSIG.
  • Evaporator temperature: 60°F.
  • Superheat: 0°F, indicating liquid refrigerant returning to the compressor.

Iced Evaporator Coil:

• Symptoms:
  • Low evaporator temperature and low superheat.
  • Example: Suction line temperature at 30°F with no superheat, confirming liquid floodback.
• Causes: Inadequate airflow across the evaporator (e.g., dirty filter).

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Key Points to Remember:

• Always compare system readings to normal operating conditions.
• Check with the manufacturer for specific ratings and guidelines.
• Pay attention to evaporator temperature differences (TD) and superheat readings for accurate diagnosis.