EVCO AL04

What does EVCO AL04 mean?

AL04 on the Evco SAVE AHU controller (144SAVEU0E01) is the hot water coil anti-freeze alarm. This alarm activates when the controller detects that the temperature of the hot water heating coil or the air on the coil's leaving side has dropped to an unsafe level risking freeze damage to the coil. The controller responds automatically by fully closing the fresh air damper to stop cold outside air from reaching the coil. The alarm has a selectable delay period.

Symptoms

• AL04 displayed on the SAVE AHU controller.
• Fresh air damper drives to the fully closed position automatically.
• Anti-freeze temperature sensor is reading below the configured minimum threshold.
• Heating coil water temperature or leaving-air temperature is dangerously low.
• Normal AHU ventilation is interrupted until the coil temperature recovers.

Common causes

• Hot water supply to the heating coil has been shut off or the control valve has failed closed.
• Building heating system is off or underperforming, delivering water below minimum temperature.
• Very cold outdoor air is flooding the AHU faster than the heating coil can warm it.
• Hot water circulation pump has failed, stopping flow through the coil.
• Anti-freeze sensor is faulty or has been sited incorrectly.

Diagnostic steps

1. 01

   Verify hot water supply temperature and flow

   Check the hot water supply temperature at the coil inlet using a contact thermometer or the building management system. Confirm the heating system is operational and water is circulating through the coil.

2. 02

   Inspect the hot water control valve

   Check that the heating coil control valve is opening on demand from the AHU controller. A valve stuck closed stops hot water flow. Manually test valve operation if an actuator override is available.

3. 03

   Check the hot water circulation pump

   Confirm the zone pump or secondary pump serving the AHU heating coil is running. A stopped pump causes the coil to receive no heat even if the boiler is firing.

4. 04

   Inspect the anti-freeze sensor and wiring

   Verify the anti-freeze sensor is positioned correctly in the leaving air stream or on the coil return pipe per the design specification. Check sensor wiring for loose connections. An incorrectly positioned sensor triggers false AL04 events.

5. 05

   Restore heating and allow the coil to warm before reopening damper

   Once hot water supply is confirmed and flowing, allow the coil temperature to rise above the AL04 threshold. The fresh air damper will reopen automatically and AL04 will clear once the temperature recovers.

When to call a professional

Contact a licensed HVAC technician if AL04 activates repeatedly or if the hot water coil temperature will not recover despite confirmed water supply. A technician should inspect for coil fouling (scale buildup), control valve actuator failure, or hydronic balance issues. If the coil has already frozen, it must be inspected for burst tubes before returning to service — a burst coil requires immediate qualified repair.