F.735 on a Glow-worm GeniaAir: Evaporation temperature too high
Short answer
On a Glow-worm heat pump, F.735 means: Evaporation temperature too high. 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: Evaporation temperature too high.
Part involved: Evaporation temperature too high.
Likely causes:
- Temperature in the environment circuit (heating mode) or building circuit (cooling mode) too high for compressor operation. Feed-in of external heat into the environment circuit too high due to increased fan speed.
- Glow-worm trade page: Evaporation temperature too high Poor air flow through the evaporator, poor heat transfer into the heating circuit (see F.733), electronic expansion valve fault, fault in the electrical connection of the evaporator temperature sensor, evaporator temperature sensor fault, high refrigerant volume.
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 system temperatures. 2. Check the refrigerant filling volume for overfilling. 3. Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test). 4. Check the sensor for the evaporation temperature (depending on the position of the 4-port diverter valve). 5. Check the volume flow in cooling mode. 6. Check the air volume flow in heating mode.
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