F.735 on a Vaillant heat pump: Evaporation temperature too high
Short answer
On a Vaillant 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 · 3 sources
What it means
On the aroTHERM plus (VWL ../6 A)
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.
On the flexoTHERM exclusive (VWF ../4)
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.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
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.
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
On the aroTHERM plus (VWL ../6 A)
- 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). The operating instructions allow the reset button on the indoor unit (or a reset via the system control) to be pressed once; if the code returns, do not keep resetting - contact your installer or Vaillant. 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.
On the flexoTHERM exclusive (VWF ../4)
- 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). Do not keep resetting the unit; report the code to your installer (the operating instructions for this product were not checked for a homeowner reset procedure). 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.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
- 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). Do not keep resetting the unit; report the code to your installer (the operating instructions for this product were not checked for a homeowner reset procedure). 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
On the aroTHERM plus (VWL ../6 A)
- Check the system temperatures; Check the refrigerant filling volume for overfilling; Check the EEV (does the EEV move to the limit stop? Use sensor/actuator test); Check the sensor for the evaporation temperature (depending on the position of the 4-port diverter valve); Check the volume flow in cooling mode; 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.
On the flexoTHERM exclusive (VWF ../4)
- Reduce or stop the external heat that is entering; Check the de-icer (does it heat up even though it isOff in the sensor/actuator test?); Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test); Check the compressor inlet sensor and low-pressure sensor.
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.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
- Reduce or stop the external heat that is entering; Check the de-icer (does it heat up even though it is "Off" in the sensor/actuator test?); Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test); Check the compressor inlet sensor and low-pressure sensor.
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
What owners and engineers report
14 owner and engineer reports mention this.
- No heating on the aroTHERM plus (VWL ../6 A): 8 reports
- Keeps stopping / cutting out on the aroTHERM plus (VWL ../6 A): 6 reports
Sources
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.35; uniTOWER VIH QW 190/6 E 0020291525_05 p.61; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.58 · Reviewed 23 September 2026
- flexoTHERM exclusive VWF 57/4 - 197/4 400V installation and maintenance instructions ↗ · 0020213394_05 · page flexoTHERM 400 V 0020213394_05 p.53; flexoTHERM 230 V 0020213395_04 p.48 · Reviewed 23 September 2026
- geoTHERM mini VWS 36/4.1 230 V installation manual ↗ · 0020250181_00 · page geoTHERM mini 0020250181_00 p.39 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026