UF on a Daikin heat pump: Code shown on the remote controller for the installation error(s): Stop valve left closed; incorrect interconnections between units (piping and wiring inconsistent); refrigerant overcharge (Flex outdoor installation manual, test-operation table)
Short answer
On a Daikin heat pump, UF means: Code shown on the remote controller for the installation error(s): Stop valve left closed; incorrect interconnections between units (piping and wiring inconsistent); refrigerant overcharge (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. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the Altherma Flex Type
The manual says: Code shown on the remote controller for the installation error(s): Stop valve left closed; incorrect interconnections between units (piping and wiring inconsistent); refrigerant overcharge (Flex outdoor installation manual, test-operation table).
Part involved: inter-unit communication / PCB combination.
When it shows: Detected during test operation / normal operation after installation errors listed.
Likely causes:
- Stop valve left closed; incorrect interconnections between units (piping and wiring inconsistent); refrigerant overcharge.
On the Altherma HT (high temperature split)
The manual says: System is not set yet.
Part involved: inter-unit communication / PCB combination.
When it shows: The malfunction is determined as soon as the abnormality afor ementioned is detected through checking the system for any erroneous connection of unit.
Likely causes:
- Improper connection of transmission wiring between indoor-outdoor unit; QA wiring problem; Defect of indoor unit PC board.
Is it safe?
On the Altherma Flex Type
| 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).
On the Altherma HT (high temperature split)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
What you can check
- Read the full error code shown on the user interface and note it, with the date/time, for your installer.
What the engineer will check
On the Altherma Flex Type
- 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 installer (Daikin-trained).
Certification: None specific; MCS-certified installer recommended.
On the Altherma HT (high temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check improper connection of transmission wiring between indoor-outdoor unit; check qA wiring problem; check defect of indoor unit PC board.
Who to ask for: Heat pump installer (Daikin-trained).
Certification: None specific; MCS-certified installer recommended.
What you might notice
Sources
- Installation manual Daikin Altherma Flex EMRQ8-16AAY1 (4PWEN61262-1B) ↗ · 4PWEN61262-1B · page 32 (printed 29) · Reviewed 23 September 2026
- Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 221 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026