A5-00 on a Daikin Altherma: OU: High pressure peak cut / freeze protection problem
Short answer
On a Daikin heat pump, A5-00 means: OU: High pressure peak cut / freeze protection problem. 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: OU: High pressure peak cut / freeze protection problem.
Part involved: plate heat exchanger freeze protection.
When it shows: Pressure is too high in heating / domestic hot water, too low in cooling.
Likely causes:
- Water pressure is too low.; Water flow is too low.; Closed stop valve in the water circuit.; Air in the water circuit.; Faulty ambient air thermistor.; Faulty expansion valve.; Closed stop valve in the refrigerant circuit.; Clogged refrigerant circuit.; Refrigerant overcharge or shortage.; Non‑condensables and/or humidity in the refrigerant circuit.
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).
- 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).
- 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
- Check the water pressure. . ; Check the water flow. . ; Check that all stop valves of the water circuit are open. . ; Purge the water circuit. . ; Perform a check of the outdoor air thermistor. . ; Perform a check of the expansion valve. . ; Check that all stop valves of the refrigerant circuit are open. . ; Check if the refrigerant circuit is clogged. . ; Check if the refrigerant circuit is correctly charged. . ; Check for the presence of non‑condensables and/or humidity in the refrigerant circuit. . ; | Troubleshooting.
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
17 owner and engineer reports mention this.
- No heating on the Altherma: 7 reports
- Pressure keeps dropping on the Altherma: 3 reports
- Keeps stopping / cutting out on the Altherma: 2 reports
- No hot water on the Altherma: 2 reports
- Stops working in very cold weather on the Altherma: 2 reports
- Frequent defrosting on the Altherma: 1 report
Sources
- Service manual Daikin Altherma 3 M EBLA/EDLA04~08E (ESIE22-09) ↗ · ESIE22-09 · page 33 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026