U8-26 on a Daikin Altherma 4 H: Room thermostat software version compatibility error
Short answer
On a Daikin heat pump, U8-26 means: Room thermostat software version compatibility error. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Room thermostat software version compatibility error.
Part involved: user interface / room thermostat.
When it shows: Incompatible room thermostat is detected on the bus. Please update the device to the latest version using the Madoka Assisstant app.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: yes.
Manual reset: Automatic.
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 any room thermostat/controller display is lit and its batteries (if battery-powered) are not flat; check thermostat and schedule settings.
- 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
- See Daikin service manual for this generation (not public).
Who to ask for: Heat pump installer (Daikin-trained).
Certification: None specific; MCS-certified installer recommended.
What you might notice
Sources
- Installer reference guide Daikin Altherma 4 H W EPSK/EPSKS + EPBX(U) (4P773390-1C) ↗ · 4P773390-1C · page 221 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026