UA-21 on a Daikin heat pump: Extension/hydro mismatch problem
Short answer
On a Daikin heat pump, UA-21 means: Extension/hydro mismatch problem. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the Altherma
The manual says: Extension/hydro mismatch problem.
Part involved: inter-unit communication / PCB combination.
On the Altherma LT monobloc
The manual says: PCB combination malfunction.
Part involved: inter-unit communication / PCB combination.
When it shows: System detects combination mismatch between extension PCB A4P and hydro PCB A1P after the connection was already made. Setting on hydro PCB A1P does not match with extension PCB A4P. 19/07/16.
Likely causes:
- See possible cause for UA error.; Faulty extension PCB A4P.
Is it safe?
On the Altherma
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
On the Altherma LT monobloc
| 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. Page 39.
What you can check
On the Altherma
- 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.
On the Altherma LT monobloc
- 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.
What the engineer will check
On the Altherma
- 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.
On the Altherma LT monobloc
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
14 owner and engineer reports mention this.
- No heating on the Altherma: 7 reports
- Controller / thermostat problem on the Altherma: 6 reports
- No heating on the Altherma LT monobloc: 1 report
Sources
- Installer reference guide Daikin Altherma 3 R W ERGA04-08E + EHBH/EHBX-E (4P629085-1E) ↗ · 4P629085-1E · page 260 · Reviewed 23 September 2026
- Service manual Daikin Altherma Monobloc CA EBLQ/EDLQ05-07CAV3 (ESIE15-08B) ↗ · ESIE15-08B · page 39 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026