8H-00 on a Daikin heat pump: Abnormal increase outlet water temperature
Short answer
On a Daikin heat pump, 8H-00 means: Abnormal increase outlet water temperature. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the Altherma
The manual says: Abnormal increase outlet water temperature.
Part involved: water circuit temperature protection.
When it shows: Outlet water after backup heater thermistor detects a too high temperature during space heating without electrical heater.
Likely causes:
- Water flow is too low.; Faulty outlet water after backup heater thermistor.; Clogged water circuit.; Faulty water pump.; Faulty backup heater contactor(s).
On the Altherma 4 H (R-290)
The manual says: Overheating of the space heating water circuit.
Part involved: water circuit temperature protection.
When it shows: The maximum water temperature in the space heating water circuit is above the overheating temperature.
Is it safe?
On the Altherma
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no.
Manual reset: Manual reset via user interface.
On the Altherma 4 H (R-290)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: yes.
Manual reset: Automatic.
What you can check
On the Altherma
- 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.
On the Altherma 4 H (R-290)
- 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).
- 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.
- 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
On the Altherma
- Check the water flow. . ; Perform a check of the outlet water after backup heater thermistor. . ; Check if the water circuit is clogged. . ; Perform a check of the water pump. . ; Perform a check of the backup heater contactor(s). . ; | Troubleshooting.
Who to ask for: Heat pump installer / heating engineer.
Certification: None specific for water-side checks; MCS-certified installer recommended.
On the Altherma 4 H (R-290)
- See Daikin service manual for this generation (not public).
Who to ask for: Heat pump installer / heating engineer.
Certification: None specific for water-side checks; MCS-certified installer recommended.
What you might notice
What owners and engineers report
11 owner and engineer reports mention this.
- No heating on the Altherma: 7 reports
- Keeps stopping / cutting out on the Altherma: 2 reports
- No hot water on the Altherma: 2 reports
Sources
- Service manual Daikin Altherma 3 M EBLA/EDLA04~08E (ESIE22-09) ↗ · ESIE22-09 · page 30 · Reviewed 23 September 2026
- Installer reference guide Daikin Altherma 4 H W EPSK/EPSKS + EPBX(U) (4P773390-1C) ↗ · 4P773390-1C · page 206 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026