Heat pumps · Error codes · Daikin
AJ-03Daikin · Altherma
AJ-03 on a Daikin Altherma: Too long DHW heat-up time required
Short answer
On a Daikin heat pump, AJ-03 means: Too long DHW heat-up time required. The heat pump keeps running.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Too long DHW heat-up time required.
Part involved: DHW heat-up.
When it shows: Domestic hot water heat‑up time >6 hours.
Likely causes:
- Faulty backup heater.; ▪ Faulty or disturbance of the power supply (power supply MUST be within range of nominal operating voltage ±4%), ▪ Power drop, ▪ Short circuit.; Faulty hydro PCB.; Faulty 3‑way valve.; Leaking field installed domestic hot water tap.; Backup heater NOT allowed.; Mismatch between the software ID and EEPROM on the PCB or user interface.; Domestic hot water consumption too large.
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
The heat pump keeps running.
Resetting itself: yes.
Manual reset: Automatic reset after a domestic hot water heat‑up time <6 hours.
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 domestic hot water / programme schedule and settings on the user interface (Daikin notes AH-00 can be caused by large hot-water draws or stopping the tank during disinfection).
- Daikin documents this code as resetting automatically once the condition clears; if it keeps coming back, contact your installer.
- 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.
What the engineer will check
- hours. Automatic reset after a domestic hot water heat‑up time <6 hours. ; Perform a check of the backup heater. . ; Check if the power supply is compliant with the regulations. . ; Perform a check of the hydro PCB. . ; Perform a check of the 3-way valve. . ; Check the installation for a leaking field installed domestic hot water tap. . ; Check the settings of the backup heater [4‑00]. . ; Check the software and EEPROM version on the user interface and PCB. . ; Check that the domestic hot water consumption is NOT too large. Lower if needed. ; | Troubleshooting.
Who to ask for: Heat pump installer / heating engineer.
Certification: G3 unvented hot water competence for cylinder safety components.
What you might notice
What owners and engineers report
7 owner and engineer reports mention this.
- Hot water not hot enough on the Altherma: 5 reports
- Hot water runs out quickly on the Altherma: 2 reports
Sources
- Service manual Daikin Altherma 3 M EBLA/EDLA04~08E (ESIE22-09) ↗ · ESIE22-09 · page 36 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026