AA on a Daikin Altherma LT: Back-up heater thermal protector is open
Short answer
On a Daikin heat pump, AA means: Back-up heater thermal protector is open. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Back-up heater thermal protector is open.
Part involved: backup heater / thermal protector.
When it shows: The error is generated in case the temperature of the backup heater rises above specific temperature.
Likely causes:
- Malfunction of the backup heater relays [K1M, K2M]; Incorrect parameter of the solar pump station (maximum tank temperature is too high, etc.); Fuse FU2 blown ESIE10-02 Error Codes: Hydro-box Part 3 –.
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: Reset via the remote controller (Inspection/Test button) after the cause is fixed; some codes need a manual reset of the remote-controller memory (service manual section 1.4).
What you can check
- Read the full error code shown on the user interface 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
- Follow the service-manual troubleshooting flow chart for this code: check malfunction of the backup heater relays [K1M, K2M]; check incorrect parameter of the solar pump station (maximum tank temperature is too high, etc.); check fuse FU2 blown ESIE10-02 Error Codes: Hydro-box Part 3 –.
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
1 owner and engineer report mentions this.
- No heating on the Altherma LT (low temperature split): 1 report
Sources
- Service Manual ALTHERMA ERHQ006~008BAV3 / ERLQ006~008BAV3(9) / EKHBH(X)008BA (ESIE10-02) ↗ · ESIE10-02 · page 146 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026