F.739 on a Glow-worm GeniaAir: Refrigerant mass too low
Short answer
On a Glow-worm heat pump, F.739 means: Refrigerant mass too low. 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: Refrigerant mass too low.
Part involved: Refrigerant mass too low.
Likely causes:
- Leakage in the refrigerant circuit. Filling with the wrong refrigerant volume (e.g. after maintenance or during initial filling).
- Glow-worm trade page: Low refrigerant volume Low refrigerant volume, fault in the electrical connection of the compressor inlet or outlet temperature sensor, compressor inlet or outlet temperature sensor fault, fault in the electrical connection of the refrigerant low- or high-pressure sensor, refrigerant low- or high-pressure sensor fault.
Is it safe?
Switch the heat pump off at its isolator and call an engineer.
| 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). Low refrigerant mass may indicate a leak of R290 (flammable). Switch the heat pump off at its isolator, keep ignition sources away from the outdoor unit, and call an F-gas/R290-competent engineer.
What the engineer will check
- Check the compressor’s inlet temperature sensor and, if required, replace it. 2. Check the refrigerant low-pressure temperature sensor and, if required, replace it. 3. Check the refrigerant circuit for leaks and, if required, eliminate these. 4. Check the refrigerant volume (too low) and, if required, top up. 5. Check the refrigerant high-pressure temperature sensor and, if required, replace it. 6. Check the condenser outlet temperature sensor (cooling) and, if required, replace it.
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.32 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026