Carel AL10

What does Carel AL10 mean?

AL10 on Carel µChiller and pCO chiller controllers signals that the low-pressure (LP) safety switch on compressor 1's heating circuit has tripped. In heat-pump mode, the LP switch monitors suction pressure on the heating cycle; if it drops too low, compressor 1 is shut down to prevent operation in a vapor-starved condition during low outdoor temperature heating operation.

Symptoms

• AL10 displayed on the Carel controller.
• Compressor 1 is shut down during heating operation.
• Suction pressure on compressor 1's circuit is at or below the LP cutout in heating mode.
• Alarm requires manual reset after diagnosis.

Common causes

• Outdoor ambient temperature too low for the heat pump's rated operating range.
• Refrigerant undercharge causing low suction pressure even in heating mode.
• Outdoor coil (acting as evaporator in heating mode) heavily iced over, blocking airflow.
• Outdoor fan motor failed, reducing airflow over the outdoor coil.
• LP switch cutout pressure set too high relative to low-ambient heating conditions.

Diagnostic steps

1. 01

   Record the outdoor ambient temperature

   Check the current outdoor temperature against the unit's rated minimum heating ambient. Operating below the minimum rating causes LP trips in heating mode.

2. 02

   Inspect the outdoor coil for ice

   Check the outdoor coil for ice bridging or heavy frost that blocks airflow. Initiate a defrost cycle if icing is present and investigate why defrost did not clear it automatically.

3. 03

   Verify outdoor fan operation

   Confirm the outdoor fan is running at correct speed. A failed fan motor starves the outdoor coil of airflow, dropping evaporating pressure in heating mode.

4. 04

   Connect refrigerant gauges and check suction pressure

   Measure suction pressure on compressor 1's circuit in heating mode. Compare to expected saturation pressure for the current outdoor temperature.

5. 05

   Reset the alarm after correction

   After addressing the identified cause, reset AL10 at the controller and monitor the system through a full heating cycle.

When to call a professional

Call a licensed refrigeration/heat pump engineer for refrigerant handling, recharge, LP switch replacement, or investigation of LP trips occurring at ambient temperatures within the unit's rated operating range. These indicate refrigerant circuit or control faults that are unsafe to diagnose without proper tools and F-Gas certification.