UA-16 on a Daikin Altherma: Extension/hydro communication problem
Short answer
On a Daikin heat pump, UA-16 means: Extension/hydro communication problem. The heat pump keeps running. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Extension/hydro communication problem.
Part involved: inter-unit communication / PCB combination.
When it shows: Communication abnormality between hydro PCB and hydro extension PCB.
Likely causes:
- Faulty wiring between the hydro PCB and hydro extension PCB.; ▪ Faulty or disturbance of the power supply (power supply MUST be within range of nominal operating voltage ±4%), ▪ Power drop, ▪ Short circuit.; Faulty hydro PCB.; Faulty hydro extension PCB.
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: no.
Manual reset: Manual reset via user interface. EPRA14~18D + ETSH/X(B)16P30+50E Daikin Altherma 3 H HT ECH₂O 3 | Troubleshooting.
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.
- 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 wiring between the hydro PCB and the hydro extension PCB. ; Check if the power supply is compliant with the regulations. ; Perform a check of the hydro PCB. ; Perform a check of the hydro extension PCB.
Who to ask for: Heat pump installer (Daikin-trained).
Certification: None specific; MCS-certified installer recommended.
What you might notice
What owners and engineers report
13 owner and engineer reports mention this.
- No heating on the Altherma: 7 reports
- Controller / thermostat problem on the Altherma: 6 reports
Sources
- Service manual Daikin Altherma 3 H HT ECH2O EPRA14-18D + ETSH/ETSX(B)16P30/50E (ESIE22-11A) ↗ · ESIE22-11A · page 86 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026