F.733 on a Vaillant heat pump: Evaporation temperature too low
Short answer
On a Vaillant heat pump, F.733 means: Evaporation temperature 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 · 3 sources
What it means
On the aroTHERM plus (VWL ../6 A)
The manual says: Evaporation temperature too low.
Part involved: Evaporation temperature too low.
Likely causes:
- Insufficient air volume flow through the outdoor unit's heat exchanger (heating mode) leads to an insufficient energy input in the environment circuit (heating mode) or building circuit (cooling mode); Refrigerant volume too low; If the building circuit contains thermostatic valves, check that they are suitable for cooling mode (check volume flow in cooling mode).
On the flexoTHERM exclusive (VWF ../4)
The manual says: Evaporation temperature too low.
Part involved: Evaporation temperature too low.
Likely causes:
- No flow rate in the environment circuit (heating mode); Energy input too low in the environment circuit (heating mode) or building circuit (cooling mode); If the building circuit contains thermostatic valves, check that they are suitable for cooling mode (cooling mode).
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
The manual says: Evaporation temperature too low.
Part involved: Evaporation temperature too low.
Likely causes:
- No flow rate in the environment circuit (heating mode); Energy input too low in the environment circuit (heating mode) or building circuit (cooling mode); If the building circuit contains thermostatic valves, check that they are suitable for cooling mode (cooling mode).
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 fan unit for dirt; Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test); Check the compressor inlet sensor; Check the refrigerant volume.
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)
- Check the flow rate in the environment circuit; Check the dimensioning of the environment circuit (heating mode) for ground/brine and ground water/brine; VWL SA (heating mode); Check the fan unit for dirt; Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test); Check the low-pressure sensor and compressor inlet 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)
- Check the flow rate in the environment circuit; Check the dimensioning of the environment circuit (heating mode); Check the low-pressure sensor and compressor inlet 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.38 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026