8H-08 on a Daikin Altherma 3 H HT: Overheating water circuit
Short answer
On a Daikin heat pump, 8H-08 means: Overheating water circuit. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Overheating water circuit.
Part involved: water circuit temperature protection.
When it shows: Mixed leaving water thermistor detects too high temperature after tank in the space heating circuit. In heating mode: Thermistor >Set temp + 7°C In cooling mode: Thermistor >Set temp + 30°C.
Likely causes:
- Faulty mixed leaving water thermistor.; Faulty 3‑way valve.; Faulty backup heater contactor(s).; Faulty hydro extension PCB. 34 Daikin Altherma 3 H HT ECH₂O.
Is it safe?
| 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.
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
- Perform a check of the mixed leaving water thermistor. ; Perform a check of the domestic hot water/space heating 3-way valve. ; Perform a check of the backup heater contactor(s). ; Perform a check of the hydro extension PCB.
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
2 owner and engineer reports mention this.
- No heating on the Altherma 3 H HT (R-32 high temperature split): 1 report
- No hot water on the Altherma 3 H HT (R-32 high temperature split): 1 report
Sources
- Service manual Daikin Altherma 3 H HT ECH2O EPRA14-18D + ETSH/ETSX(B)16P30/50E (ESIE22-11A) ↗ · ESIE22-11A · page 34 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026