F6 on a Daikin heat pump: Code shown on the remote controller for the installation error(s): Stop valve left closed; refrigerant overcharge (Flex outdoor installation manual, test-operation table)
Short answer
On a Daikin heat pump, F6 means: Code shown on the remote controller for the installation error(s): Stop valve left closed; 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; refrigerant overcharge (Flex outdoor installation manual, test-operation table).
Part involved: refrigerant circuit / pressure protection.
When it shows: Detected during test operation / normal operation after installation errors listed.
Likely causes:
- Stop valve left closed; refrigerant overcharge.
On the Altherma LT (low temperature split)
The manual says: Too high condensing pressure.
Part involved: refrigerant circuit / pressure protection.
When it shows: Activated when the temperature being sensed by the heat exchanger thermistor rises above 60°C. (Deactivated when the said temperature drops below 50°C) | Activated when the temperature being sensed by the heat exchanger thermistor rises above 60°C. (Deactivated when the said temperature drops below 50°C) 3 3.
Likely causes:
- The installation space is not large enough; Faulty outdoor unit fan; Faulty electronic expansion valve; Faulty defrost thermistor; Faulty outdoor unit PCB; Faulty stop valve; Dirty heat exchanger.
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 LT (low 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.
Manual reset: Reset via the remote controller (Inspection/Test button) after the cause is fixed; some codes need a manual reset of the remote-controller memory (service manual section 1.4).
What you can check
- Read the full error code shown on the user interface and note it, with the date/time, for your installer.
- Look at the outdoor unit from outside: check the air inlet/outlet and fan grille are not blocked by leaves, debris, snow or heavy ice build-up (do not remove panels or touch the fan).
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: F-gas certified heat pump engineer (Daikin-trained installer / Stand By Me partner).
Certification: UK F-gas certification (Category I) for refrigerant circuit work; R-290 (Altherma 4) also needs flammable-refrigerant/hydrocarbon competence.
On the Altherma LT (low temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check the installation space is not large enough; check faulty outdoor unit fan; check faulty electronic expansion valve; check faulty defrost thermistor; check faulty outdoor unit PCB; check faulty stop valve.
Who to ask for: F-gas certified heat pump engineer (Daikin-trained installer / Stand By Me partner).
Certification: UK F-gas certification (Category I) for refrigerant circuit work; R-290 (Altherma 4) also needs flammable-refrigerant/hydrocarbon competence.
What you might notice
What owners and engineers report
1 owner and engineer report mentions this.
- No heating on the Altherma LT (low temperature split): 1 report
Sources
- Installation manual Daikin Altherma Flex EMRQ8-16AAY1 (4PWEN61262-1B) ↗ · 4PWEN61262-1B · page 32 (printed 29) · Reviewed 23 September 2026
- Service Manual ALTHERMA ERHQ006~008BAV3 / ERLQ006~008BAV3(9) / EKHBH(X)008BA (ESIE10-02) ↗ · ESIE10-02 · page 169 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026