AA-02 on a Daikin Altherma: External backup heater overheated
Short answer
On a Daikin heat pump, AA-02 means: External backup heater overheated. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: External backup heater overheated.
Part involved: backup heater / thermal protector.
When it shows: Thermal protector is activated. Measured water temperature too high.
Likely causes:
- Water pressure is too low.; Water flow is too low.; Air in the water circuit.; Increased water temperature due to an external heat source.; Faulty backup heater thermal protector.; Faulty outlet water after backup heater thermistor.; Faulty backup heater.; Faulty hydro PCB.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no.
Manual reset: Power reset via outdoor unit.
What you can check
- Read the full error code and sub-code shown on the user interface (Malfunctioning screen; press the help button for Daikin's long description) and note it, with the date/time, for your installer.
- Check the heating-system water pressure reading on the gauge/display: Daikin operation manuals ask users to check it is above 1 bar. If it is below that, report it to your installer (do not open the unit).
- Do NOT bleed radiators while an error/malfunction symbol is displayed: Daikin operation manuals warn refrigerant may have leaked into the water circuit - ventilate and call your installer.
- If heating/hot water has stopped, the operation manual lets the user confirm on the Malfunctioning screen whether the backup/booster heater may take over as emergency heating (much higher running cost) until the installer attends.
What the engineer will check
- Check the water pressure. ; Check the water flow. ; Purge the water circuit. ; Check the water circuit for an external heat source. ; Perform a check of the backup heater thermal protector. ; Perform a check of the outlet water after backup heater thermistor. ; Perform a check of the backup heater. ; Perform a check of the hydro PCB.
Who to ask for: Heat pump installer with electrical competence.
Certification: Electrical competence to BS 7671 for heater/thermal cut-out work.
What you might notice
What owners and engineers report
12 owner and engineer reports mention this.
- No heating on the Altherma: 7 reports
- Unexpected backup / auxiliary heater use on the Altherma: 3 reports
- No hot water on the Altherma: 2 reports
Sources
- Service manual Daikin Altherma 3 M EBLA/EDLA09~16DA (ESIE20-06A) ↗ · ESIE20-06A · page 22 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026