EVCO AL03

What does EVCO AL03 mean?

AL03 on the Evco SAVE AHU controller (144SAVEU0E01) is a differential pressure switch alarm. The controller monitors the pressure difference across the supply fan or air filters via a differential pressure switch. When AL03 activates — indicating either no airflow (fan failure) or a severely blocked filter — the controller shuts down all AHU devices. This alarm requires manual reset by the operator once the cause is identified and corrected.

Symptoms

• AL03 displayed on the SAVE AHU controller.
• All AHU outputs switch off — fan, heating coils, and cooling coils are deactivated.
• Differential pressure switch contacts are open, indicating loss of airflow or extreme filter blockage.
• No air movement detectable from supply diffusers.
• Alarm requires manual operator intervention to reset.

Common causes

• Supply fan failed to start or has stopped running, eliminating airflow across the pressure switch.
• Air filters are severely blocked with dust and debris, generating zero measurable differential.
• Differential pressure switch has failed in the open position, producing a false alarm.
• Pressure switch tubing (sensing lines) is kinked, disconnected, or blocked with moisture.
• Fan belt has broken or slipped off the sheave on belt-drive units.

Diagnostic steps

1. 01

   Confirm supply fan operational status

   Check whether the supply fan motor is energized and running. If the fan is not running, investigate why — an AL01 thermal overload may have preceded AL03. Resolve fan issues before investigating the pressure switch.

2. 02

   Inspect air filters

   Check all AHU filters for blockage. Hold a filter up to light — if light cannot pass through, the filter is overloaded. Replace filters and note the filter change date; severely blocked filters can generate low differential that triggers the pressure switch in fail condition.

3. 03

   Check differential pressure switch sensing lines

   Trace the plastic tubing connecting the pressure switch to the fan plenum high and low pressure taps. Confirm tubing is not kinked, cracked, or disconnected. Blow through the tubes gently to verify they are clear.

4. 04

   Test the differential pressure switch

   With the fan running and filters clean, use a handheld manometer to measure actual differential pressure across the fan. If actual differential is within the switch's operating range but the switch reads open, replace the differential pressure switch.

5. 05

   Perform manual reset on the controller

   After addressing the root cause, access the controller alarm menu and perform the manual reset for AL03. Restart the AHU and verify the differential pressure switch closes (indicating airflow) within the fan start-up period.

When to call a professional

Engage a licensed HVAC technician if AL03 persists after filter replacement and pressure switch tubing inspection, or if the supply fan is running but the differential pressure across the fan is significantly below design specification — this indicates a fan or drive component issue. Replacing differential pressure switches and recalibrating airflow instrumentation requires qualified HVAC personnel.