AC-00 on a Daikin Altherma: Booster heater overheated
Short answer
On a Daikin heat pump, AC-00 means: Booster heater overheated. The heat pump keeps running. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Booster heater overheated.
Part involved: booster heater / thermal protector.
When it shows: Bridge connection over booster heater contact is NOT made.
Likely causes:
- Open jumper on X21A on hydro PCB.; Faulty hydro PCB.
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: no.
Manual reset: Manual reset via user interface when bridge connection is made.
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 that the bridge connector is correctly connected to X21A of the hydro PCB. . ; 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/EDLA04~08E (ESIE22-09) ↗ · ESIE22-09 · page 35 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026