U1 on a Daikin Altherma Flex Type: Code shown on the remote controller for the installation error(s): Phases of the outdoor power supply reversed; no power / phase interruption (Flex outdoor installation manual, test-operation table)
Short answer
On a Daikin heat pump, U1 means: Code shown on the remote controller for the installation error(s): Phases of the outdoor power supply reversed; no power / phase interruption (Flex outdoor installation manual, test-operation table). 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: Code shown on the remote controller for the installation error(s): Phases of the outdoor power supply reversed; no power / phase interruption (Flex outdoor installation manual, test-operation table).
Part involved: power supply / inverter electrical.
When it shows: Detected during test operation / normal operation after installation errors listed.
Likely causes:
- Phases of the outdoor power supply reversed; no power / phase interruption.
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: After correcting the abnormality press BS3 RETURN on the outdoor PCB to reset and repeat the test operation (installer only).
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 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.
What the engineer will check
- Installer remedial action per manual: open stop valves; correct phase sequence; check power wiring; check piping/wiring consistency; correct refrigerant charge by recalculation; repeat test operation within -20~35 C.
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
Sources
- Installation manual Daikin Altherma Flex EMRQ8-16AAY1 (4PWEN61262-1B) ↗ · 4PWEN61262-1B · page 32 (printed 29) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026