A5 on a Daikin Altherma LT: Freeze-up protection or High pressure control
Short answer
On a Daikin heat pump, A5 means: Freeze-up protection or High pressure control. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Freeze-up protection or High pressure control.
Part involved: plate heat exchanger freeze protection.
When it shows: ■ High pressure control During heating operations, the temperature detected by the hydro-box heat exchanger thermistor is above 65°C ■ Freeze-up protection When the hydro-box heat exchanger temperature is below 0°C during cooling operation | High pressure control During heating operations, the temperature detected by the hydro-box heat exchanger thermistor is above 65°C Freeze-up protection When the hydro-box heat exchanger temperature is below 0°C during cooling operation.
Likely causes:
- Detection error due to faulty hydro-box heat exchanger thermistor; Detection error due to faulty hydro-box PCB.
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: Reset via the remote controller (Inspection/Test button) after the cause is fixed; some codes need a manual reset of the remote-controller memory (service manual section 1.4).
What you can check
- Read the full error code shown on the user interface 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).
- Look at the outdoor unit from outside: check the air inlet/outlet and fan grille are not blocked by leaves, debris, snow or heavy ice build-up (do not remove panels or touch the fan).
What the engineer will check
- Follow the service-manual troubleshooting flow chart for this code: check detection error due to faulty hydro-box heat exchanger thermistor; check detection error due to faulty hydro-box 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
1 owner and engineer report mentions this.
- No heating on the Altherma LT (low temperature split): 1 report
Sources
- Service Manual ALTHERMA ERHQ006~008BAV3 / ERLQ006~008BAV3(9) / EKHBH(X)008BA (ESIE10-02) ↗ · ESIE10-02 · page 145 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026