F.755 on a Glow-worm GeniaAir: The 4-port diverter valve is not in the expected position
Short answer
On a Glow-worm heat pump, F.755 means: The 4-port diverter valve is not in the expected position. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: The 4-port diverter valve is not in the expected position.
Part involved: The 4-port diverter valve is not in the expected position.
Likely causes:
- Incorrect position of the 4-port diverter valve. If, in heating mode, the flow temperature is lower than the return temperature in the building circuit. The temperature sensor in the EEV environment circuit displays an incorrect temperature.
- Glow-worm trade page: Incorrect position of the 4-port valve 4-ort valve stuck in the incorrect position, 4-port valve coil incorrectly fitted or faulty, incorrectly positioned flow or return temperature sensor, pipework incorrectly connected (flow and return pipework crossed), fault in the electrical connection of the flow or return ….
Is it safe?
| 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 - the fault remains displayed until the cause is eliminated.
Manual reset: yes - after the cause is eliminated, press the reset button (indoor unit) or reset via the system control; if it recurs after several resets contact customer service.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). Check the outdoor unit's air inlet and outlet are not blocked by leaves, debris, snow or ice build-up and clear any loose debris from around it (do not open the casing, poke the fins or use tools). Glow-worm's MiPro Sense operating instructions say always contact your competent person in the event of a fault message; until they arrive you may select a limp-home (back-up heater) setting on the control, which is expensive to run. If you smell gas/solvent near the outdoor unit or hear hissing, switch the heat pump off at the isolator, keep ignition sources away and call an engineer.
What the engineer will check
- Check the 4-port diverter valve (is an audible switching available? Use the sensor/actuator test). 2. Check that the coil is positioned correctly on the 4-port diverter valve. 3. Check the wiring harness and plug connections. 4. Check the temperature sensor in the EEV environment circuit.
Who to ask for: F-gas certified refrigeration engineer with heat pump experience (R290 flammable-refrigerant competence required for aroTHERM plus / GeniaAir).
Certification: F-gas Category I (Regulation (EU) 517/2014 as retained in GB) certificate; flammable (R290) refrigerant training for R290 units.
What you might notice
Sources
- GeniaAir Appliance Interface installation and maintenance instructions ↗ · 8000043014_01 · page HPIM 7 8000043014_01 p.33 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026