Daikin Chiller Error Code AA

Full Diagnosis

The auxiliary or crankcase/water heater's thermal protector (relay 26WH) has opened due to excessive heater temperature, and the chiller controller latches an alarm that disables that heater until the fault is reset. This is a heater-circuit safety trip rather than a compressor or refrigerant-circuit fault, so compressor operation is not directly stopped, though heating functions tied to that heater — such as crankcase heat or freeze protection — become unavailable. It differs from A4 (freezing protection malfunction), which trips on low water temperature or flow rather than a heater over-temperature condition.

All Recorded Causes

Heater element shorted or breaking down internally. Failed or drifted 26WH thermal protector. Heater control relay stuck closed, energizing the heater continuously. Restricted airflow or fouling around the heater preventing heat dissipation. Incorrect heater sizing or wiring for the application. Defective heater control PCB.

Verify Before Replacing Parts

1. Confirm the exact Daikin model family and chiller context before acting on a generic code description.
2. Verify airflow first, then confirm operating pressures and temperature relationship before adjusting charge, since heater element shorted or breaking down internally.
3. Look for restrictions, iced coils, or dirty heat-transfer surfaces that can mimic a charge problem, and separately confirm failed or drifted 26WH thermal protector.
4. After the correction, run the system long enough to confirm the chiller fault does not return under normal load.