Carel AL05

What does Carel AL05 mean?

AL05 is the No Air Flow alarm on the Carel Standard Air-conditioner (pCO controller, manual +030221426). The air flow switch has opened, indicating that supply airflow through the precision air conditioning unit has dropped below the minimum threshold. All loads — compressors, heaters, and the valve — shut down immediately to prevent overheating and evaporator icing without adequate airflow.

Symptoms

• All compressors and the cooling valve shut off immediately.
• AL05 displayed on the pCO controller with alarm notification active.
• Air flow switch in the supply air path has opened its contact.
• Unit may show no supply air at the discharge grilles.

Common causes

• Supply fan motor failed or tripped its thermal overload.
• Drive belt broken or slipped on belt-driven fan assemblies.
• Dirty air filter blocking airflow to below the flow switch threshold.
• Blocked or damaged ductwork restricting air delivery.
• Air flow switch set point adjusted too high or the switch has failed.

Diagnostic steps

1. 01

   Verify supply fan operation

   Check whether the fan motor is energized and rotating. A motor tripped on thermal overload requires 15 minutes to cool before manual reset at the overload relay.

2. 02

   Inspect and replace air filters

   Check the filter condition. A heavily loaded filter causes a severe pressure drop that reduces airflow below the switch threshold. Replace disposable filters or clean permanent ones.

3. 03

   Check for duct blockages

   Inspect supply and return air paths for obstructions such as collapsed flexible duct, closed dampers, or furniture blocking return grilles. Restore unobstructed airflow paths.

4. 04

   Test the air flow switch

   With the fan confirmed running, check continuity across the flow switch terminals. If the switch remains open with confirmed adequate airflow, calibrate or replace the switch.

5. 05

   Check the fan belt condition

   On belt-driven units, inspect the belt for cracks, glazing, or slack. A broken or slipping belt allows the motor to run while the fan wheel stops, triggering the flow alarm.

When to call a professional

Call a qualified technician if the fan motor has failed, if bearing replacement is required, or if the variable speed drive (if fitted) shows a fault code. Electrical motor repairs, drive diagnostics, and duct system modifications require licensed electricians or HVAC engineers.