Carel AL021

What does Carel AL021 mean?

AL021 activates on the Carel FLSTDmSCHE screw chiller when the condensing temperature on Circuit 1 exceeds the configured alarm threshold. Elevated condensing temperature indicates the condenser cannot reject sufficient heat, which raises discharge pressure and risks damage to the compressor. The controller shuts down Compressor 1 in a controlled sequence. This alarm has automatic reset after the condensing temperature drops below the threshold.

Symptoms

• AL021 displayed on pCO5 terminal with Compressor 1 in controlled shutdown
• Condensing temperature probe reading above the alarm setpoint on the controller display
• Condenser fans running at maximum speed attempting to reduce head pressure

Common causes

• Condenser coil fouled with dirt, debris, or biological growth reducing airflow and heat transfer
• Condenser fan failure reducing airflow across the coil on Circuit 1
• Ambient temperature exceeding the chiller's rated design conditions
• Refrigerant overcharge causing excessively high condensing pressure and temperature
• Condenser airflow short-cycling — hot discharge air recirculating back into condenser intake

Diagnostic steps

1. 01

   Check condenser fan operation on Circuit 1

   Confirm all fans assigned to Circuit 1's condenser are running at full speed. A failed fan motor or seized bearing drastically reduces condenser capacity.

2. 02

   Inspect and clean the condenser coil

   With fans off, inspect the condenser fins for dirt, lint, or debris. Clean with low-pressure water spray from inside out, or use an approved coil cleaner. Even 10–15% fin blockage raises condensing temperature significantly.

3. 03

   Check for airflow short-cycling

   Verify the condenser discharge air has a clear path away from the unit and is not recirculating back into the condenser intake. Remove any obstructions around the unit within the recommended clearances.

4. 04

   Verify refrigerant charge

   With the chiller running and all compressors loaded, compare suction and discharge pressures against the refrigerant pressure-temperature chart. Overcharge produces abnormally high discharge temperature with normal suction superheat.

5. 05

   Allow auto-reset and monitor

   AL021 resets automatically once condensing temperature drops below the alarm threshold. After the root cause is resolved, monitor condensing temperature on the next operating cycle to confirm it stays within the setpoint.

When to call a professional

Refrigerant charge adjustment requires EPA 608 certification. If condenser cleaning and fan verification do not resolve AL021, a licensed HVAC/R technician with manifold gauges should perform a full system diagnostic. Repeated high condensing temperature alarms that correlate with ambient conditions may require engineering review of the installed condenser capacity relative to site-specific thermal loads.