U2-44 on a Daikin Altherma 4 H: Supply voltage error

What it means

The manual says: Supply voltage error.

Part involved: power supply / inverter electrical.

When it shows: Undervoltage during operation.

Is it safe?

The heat pump stops until the fault clears or is reset.

Resetting itself: no.

Manual reset: Manual.

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 heat pump isolator switch(es) and the consumer-unit breaker/RCD for the heat pump are ON. If a breaker or RCD has tripped, do not keep resetting it - leave it and call an engineer.
• 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 engineer / qualified electrician.

Certification: Electrical competence to BS 7671 (Part P where applicable) for mains supply work.

What you might notice

• Heat pump will not start
• Keeps stopping / cutting out
• No heating
• Tripping RCD or breaker

Sources

• Installer reference guide Daikin Altherma 4 H W EPSK/EPSKS + EPBX(U) (4P773390-1C) ↗ · 4P773390-1C · page 219 · Reviewed 23 September 2026

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026