Diagnosing A/C Systems : Low Ambient Control

 Overview:

• A low ambient control is needed when the outdoor temperature drops below 60°F during air conditioning operation, which is common in light commercial and commercial systems but rare in residential units. Without this control, the system may face several performance issues.

Effects of Operating Without Low Ambient Control:

1. Evaporator Temperature:

   • Condition: Decreases below normal levels.
   • Reason: Lower outdoor temperatures reduce the pressure entering the thermostatic expansion valve (TEV), leading to a low evaporating temperature. This could also cause icing on the evaporator coil.

2. Superheat:

   • Condition: Remains relatively normal but may increase slightly if the TEV begins to starve due to insufficient refrigerant flow under cold conditions.

3. Condensing Temperature:

   • Condition: Decreases significantly.
   • Reason: Condensing temperature is directly tied to outdoor ambient temperature. As the outdoor temperature drops, the high side pressure and condensing temperature decrease, potentially causing issues with system performance.

4. Subcooling:

   • Condition: Remains relatively normal but may slightly increase.
   • Reason: Colder ambient temperatures can provide additional sensible cooling to the refrigerant in the condenser, but this change is usually minor.

Example: Low Ambient Conditions on a 10-SEER Air Conditioning System with R22:

• 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.

• Low Ambient Conditions (55°F Ambient Temperature):

  • Condensing Side:

    • High side pressure decreases to 133 PSIG → 75°F condensing temperature.
    • Condenser split decreases to 20°F (75°F condensing temperature - 55°F ambient air).
    • Subcooling remains relatively normal but may increase slightly due to the colder ambient temperatures.

  • Evaporator Side:

    • Low side pressure decreases to 55 PSIG → 30°F evaporating temperature.
    • Evaporator temperature difference (TD) increases to 50°F (80°F return air - 30°F evaporator temperature), which is higher than normal.
    • Superheat increases to 20°F due to lower liquid line pressure, causing the TEV to starve the evaporator slightly.
    • Possible sight glass bubbling, which should not be mistaken for a low charge.

Key Considerations:

• Symptoms of Low Ambient Issues: Low evaporator temperature, increased superheat, and bubbling in the sight glass can mimic symptoms of a low refrigerant charge. However, these are often due to low ambient conditions, not an actual low charge.
• Misdiagnosis: Adding refrigerant to address bubbling in the sight glass under low ambient conditions can lead to overcharging the system. Always confirm if a low ambient control is needed before adding refrigerant.

Low Ambient Control Solutions:

• Fan Cycling Control:

  • Function: Turns the condenser fan off when the head pressure drops too low, reducing heat rejection and stabilizing or increasing the high side pressure.
  • Advantages: Simple and cost-effective.
  • Disadvantages: May not be as precise as other methods.

• Head Pressure Regulating Valve:

  • Function: Regulates refrigerant flow to maintain proper head pressure, even under low ambient conditions.
  • Advantages: More precise control over head pressure.
  • Disadvantages: More complex and expensive, requires modifications to the piping.

Diagnostic Steps:

• Check for Low Ambient Control: Determine if the system needs a low ambient control, especially if it operates in temperatures below 60°F.
• Assess System Performance: Look for signs of low evaporator temperature, increased superheat, and low condensing temperature, which indicate the need for a low ambient control.
• Avoid Overcharging: Confirm the presence of low ambient issues before adding refrigerant to prevent overcharging.