Carel AL28

What does Carel AL28 mean?

AL28 is a high-pressure alarm for Compressor Circuit 1, triggered by a pressure probe reading exceeding the configured threshold. Unlike a mechanical high-pressure switch (which triggers AL34), AL28 comes from the analogue pressure transducer connected to the pCO controller. The controller shuts down Compressor 1 and logs the event. This alarm has a manually-selectable reset depending on system configuration.

Symptoms

• AL28 displayed on pCO terminal; Compressor 1 stopped
• High-side pressure reading on controller display at or above the alarm setpoint
• Condenser may be running at full speed attempting to reduce head pressure

Common causes

• Condenser airflow blocked by dirty coil, failed fans, or obstructed discharge air path
• Refrigerant overcharge causing elevated condensing pressure
• Ambient temperature above design conditions overwhelming condenser capacity
• Condenser fan motor failure reducing heat rejection

Diagnostic steps

1. 01

   Check condenser fans

   Verify all condenser fans on Circuit 1 are spinning at the correct speed and direction. A failed fan motor or reversed phase will cut heat rejection sharply and spike discharge pressure.

2. 02

   Inspect condenser coil for blockage

   Check that the condenser coil fins are clean and free of debris. Even partial blockage on one side of the coil raises condensing temperature significantly.

3. 03

   Verify ambient temperature is within design range

   Record the ambient temperature and compare it to the unit's rated operating range. If ambient exceeds design, high-pressure conditions are expected and system capacity may need to be de-rated.

4. 04

   Review refrigerant charge

   With the unit running, check suction and discharge pressures and compare against the pressure-temperature chart for the refrigerant. Overcharge produces abnormally high discharge pressure with normal suction pressure.

5. 05

   Reset alarm and monitor

   After resolving the cause, reset AL28 from the pCO alarm screen. Monitor discharge pressure on the next run cycle and confirm it stays within the programmed setpoint.

When to call a professional

Refrigerant charge verification and adjustment requires EPA 608 certification and proper manifold gauges. Any work involving refrigerant recovery, addition, or leak detection must be performed by a licensed HVAC/R technician. If high-pressure events recur after fan and coil maintenance, a certified technician should perform a full refrigerant circuit audit.