What it means
Daikin's guides list it as "Too long DHW heat-up time required". Its service manual gives the trigger as more than 6 hours to heat the tank, after which the unit switches to space heating or cooling for 3 hours.
What to check, in order
- Check for heavy hot water use
Daikin's service manual lists very high hot water use and a leaking hot water tap among the causes. Make sure no hot tap is running or dripping.
- Let it reset
Daikin's installer reference guide says AJ-03 resets automatically as soon as the tank heats up normally again.
When to call your installer
If it keeps happening with normal hot water use, call your installer. Daikin's service manual lists a faulty backup heater, the backup heater not being allowed, power supply problems, a faulty 3-way valve, a faulty hydro PCB, or a software mismatch between the board and the user interface.
Quick answers
What does the Daikin Altherma AJ-03 error code mean?
Heating the hot water tank has taken too long, so the heat pump has switched to space heating for a while. Daikin's guides list it as "Too long DHW heat-up time required". Its service manual gives the trigger as more than 6 hours to heat the tank, after which the unit switches to space heating or cooling for 3 hours.
Can I fix Daikin Altherma AJ-03 myself?
Very easy (A few minutes). Start with: check for heavy hot water use; let it reset. If it keeps happening with normal hot water use, call your installer. Daikin's service manual lists a faulty backup heater, the backup heater not being allowed, power supply problems, a faulty 3-way valve, a faulty hydro PCB, or a software mismatch between the board and the user interface.
Is Daikin Altherma AJ-03 worth repairing?
Usually free. After unusually heavy hot water use it clears itself. If it keeps coming back with normal use, a heater, valve or power fault needs checking.
Checked against
- Daikin Altherma 3 R ECH2O installer reference guide, error codes overview, p. 275 (ManualsLib copy)
- Daikin Altherma 3 R ECH2O installer reference guide, error code notes, p. 277 (ManualsLib copy)
- Daikin Altherma 3 R W installer reference guide 4P496758-1B, error codes, p. 90 (ManualsLib copy)
- Daikin service manual ESIE20-06A, Daikin Altherma 3 M (EBLA/EDLA), PDF (copy hosted on the OpenEnergyMonitor forum)
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.