AA-00 on a Daikin Altherma LT monobloc: Back up heater safety error
Short answer
On a Daikin heat pump, AA-00 means: Back up heater safety error. 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: Back up heater safety error.
Part involved: backup heater / thermal protector.
When it shows: Thermal protector Q1L is activated (open circuit). Water temp > 90°C.
Likely causes:
- Air in water circuit / backup heater.; Faulty thermal protector Q1L.; Faulty outlet backup heater thermistor.; Faulty outlet water heat exchanger thermistor.; Faulty backup heater assy.; Faulty wiring between outdoor unit Control box DHW tank.; Faulty backup heater.; Water pressure is too low.; Water flow is too low.; Faulty extension PCB A4P.
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 + reset via user interface + pressing the button on the Q1L (if activated).
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).
What the engineer will check
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
2 owner and engineer reports mention this.
- No heating on the Altherma LT monobloc: 1 report
- No hot water on the Altherma LT monobloc: 1 report
Sources
- Service manual Daikin Altherma Monobloc CA EBLQ/EDLQ05-07CAV3 (ESIE15-08B) ↗ · ESIE15-08B · page 19 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026